Luminous Landscape Forum

Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: erpman on August 21, 2011, 09:31:04 pm

Title: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 21, 2011, 09:31:04 pm
So, in my somewhat particular form of landscape photography Im after maximum depth of field (1,5m-infinity) and maximum sharpness for making huge, wall-sized prints. Do note, these prints are printed at around 300ppi.

Since I cant afford MF backs, but also because of the DOF consideration, I use a panoramic bracket and stitching tools to generate large image files for print. This also means that I use longer lenses (50-100mm), hence the critical DoF factor.

Having seen the reviews of the Pentax k5 Im getting tempted to trade in my 5dmkII for it, in order to gain more DOF. Im tired of soft images from having to stop down to f/22 and even further. But something tells me that the gain in DoF is at the expence of image quality resulting from the smaller sensor of the k5, and also, the lower aperture at which diffraction comes into the mix on a smaller sensor. Correct me if Im wrong. 

Stitching gives me all the resolution that I want, so thats not an issue. The only issue is wether trading in a ff sensor for crop is gonna give the expected increase in dof/sharpness, or whether Im just trying to overcome the laws of physics.

Could an alternative be to get the pentax 645d and just stopping it down more on somewhat shorter focal lengths? After all, the image quality and larger circle of confusion could make for a grand total with higher IQ overall.

Input is very appreciated!
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Sheldon N on August 21, 2011, 10:55:36 pm
If you're already shooting pano and are that far into post production, why not consider focus stacking in addition to pano shooting?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on August 21, 2011, 11:53:47 pm
Hi,

DoF does not really exist anyway. If you make big print, viewed very close DoF will be essentially nil. That said a smaller format may have some advantage, about one stop. I absolutely agree on the recommendation to use focus stacking, although I never have done it on stitched images.

I have an article discussing DoF and diffrcation here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures

Some examples of extending DoF are given here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/29-handling-the-dof-trap

Best regards
Erik

If you're already shooting pano and are that far into post production, why not consider focus stacking in addition to pano shooting?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: stever on August 22, 2011, 12:34:07 am
there is no free lunch.  diffraction results in serious loss of sharpness beyond f16 in full frame, and f11 in crop frame - pretty much in direct porportion to the difference in DOF.  my personal experience is that some high quality simpler design lenses such as the Canon 90 TS hold up better at f19 or f22 than complex designs like the 100 L - but it's a marginal difference.  however, using a tilt-shift lens can seriously improve DOF in one plane and this can be used with stitching (search posts on tilt-shift panoramas)

focus stacking can be very effective (for motionless subjects) but there's a serious learning curve.  if you're doing panos, i think you're better off with a good MF lens as you need to duplicate the focus stacks for each slice of your pano - or shoot tethered with the Helicon software to duplicate the focus stack for each slice

going to a crop frame camera will reduce resolution without increasing DOF

Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 22, 2011, 06:58:25 am
As I feared, and kinda had calculated myself, there is indeed no free lunch with regard to the earlier onset of diffraction on crop sensors.

Focus stacking is not an option, as the process is slow enough as it is, but TS might be an area to look into.

But how about MF? Im wondering if going with an MF system like the pentax 645d, shorter focal lengths and smaller stitches (3 images instead of 12-15), and then allow for some more enlargement/resampling in PS facilitated by the IQ produced by the larger sensor/lens.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 22, 2011, 07:12:27 am
Focus stacking is not an option, as the process is slow enough as it is...
Perhaps focus stacking would allow you to reduce the number of stitched images (using a wider lense), keeping the total time spent in check (but altering the balance between DOF and maximum sharpness)?

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 22, 2011, 10:07:51 am
Just did a side by side comparison between the 5dmkII and the pentax 645d. The 5dmkII image was upscaled to 200% in ps, the pentax to 400%. Pentax still looks better. There is something about the tonality and smoothness of mf that seems to give it the upper hand. Look at the feathers; they look so much cruder on the 5d. This is definitely interesting.

Both images received the same sharpening and I just tweaked the contrast of the 645 for comparison.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 22, 2011, 11:41:30 am
there is no free lunch.  diffraction results in serious loss of sharpness beyond f16 in full frame, and f11 in crop frame - pretty much in direct porportion to the difference in DOF.  

(http://img808.imageshack.us/img808/4615/pl2514.jpg)

that is 2x crop @ f11 by the way...
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 22, 2011, 12:57:35 pm
Not bad, what system is this?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: stever on August 22, 2011, 01:27:44 pm
your comparison of the 645 and 5D2 is consistent with others i've read.  the 645 will have higher IQ.  whether it will get you to the combination of sharpness and DOF you're looking for may still be a problem - you can shoot the 645 at f22, but DOF will be about the same as f16 with FF, so the gain in DOF will come from being able to shoot wider lenses - you can do the math on how much wider and whether that will get the DOF you're looking for.

if you can go enough wider to do single row vertical panos with a tilt lens then there are potentially big gains in apparent DOF for suitable subjects

it might be possible to do multi-row with tilt on the bottom row only, but doesn't seem very practical

focus stacking is the only way to seriously increase depth of field but is very time consuming and can be quite frustrating

dee - i think you misunderstood me - diffraction starts to become seriously noticeable with crop frame beyond f11
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 22, 2011, 01:29:34 pm
Not bad, what system is this?

m43, SLRGear is using 12mp camera to test m43 lenses ... so f11 on 2x crop is like actually f22 on FF (24x36) and still PL25/1.4 delivers good results @ f11...

Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 22, 2011, 01:33:45 pm
dee - i think you misunderstood me - diffraction starts to become seriously noticeable with crop frame beyond f11
which crop you are talking about ? if 1.5x then as 2x @ 12mp sensor is still good @ f11 -> w/ proper lenses 1.5x might be OK (and not seriously noticeable) @ f11 + 1/2 stop down... it is another story that most lenses are too bad to deliver on 1.5x and above @ those apertures...
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Wayne Fox on August 22, 2011, 04:17:02 pm
there is no free lunch.  diffraction results in serious loss of sharpness beyond f16 in full frame, and f11 in crop frame - pretty much in direct porportion to the difference in DOF. 

While I do not disagree with your statement, one thing I've found is you can fix some of the softness from diffraction in post ... something you cannot do with out of focus from not enough "depth of field".  Shooting something at f/22 may leave the file soft, but a little work and prints look pretty good. You probably loose some of the micro detail, but it can still work pretty well.

Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 22, 2011, 07:01:29 pm
Quote
While I do not disagree with your statement, one thing I've found is you can fix some of the softness from diffraction in post ... something you cannot do with out of focus from not enough "depth of field".  Shooting something at f/22 may leave the file soft, but a little work and prints look pretty good. You probably loose some of the micro detail, but it can still work pretty well.

I do agree on this, I use RawDeveloper which has a R/L Deconvolution algorithm and it does a fairly good job at restoring diffraction softness. Actually, when stitching, this means that you can, if done properly, slightly alter focus between images as you move through the scene since the diffraction softness/extended depth of field covers up small differences in focus from frame to frame.

Now, I did an interesting comparison with regard to the Pentax 645D. Since its sensor is not "true" 645 but slightly smaller (33.0 x 44.0) it means that 55mm is "standard" lens on this camera (crop factor of 0,79 i think). This translates to 43mm on FF, a very handy focal length for the work that I do. When you put this into the equation, along with the increased resolution and IQ and upscaling possibilities of the files, it gets closer to my goal. Have a look at the calculations from the Cambridge Dof calclulator at the bottom.

This means that I theoretically get 1m closer to my subject when using hyperfocal focusing. Or is there something that Ive overlooked? Could there be an error with the calculator assuming full 6x45 sensor size and therefore a larger CoC? The pixel pitch on the 645D is 5.93, smaller than on the 5dmkII. Does this mean that comparing f16 on the 5d to f22 on the pentax would be unfair?




Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: stamper on August 23, 2011, 04:14:41 am
While I do not disagree with your statement, one thing I've found is you can fix some of the softness from diffraction in post ... something you cannot do with out of focus from not enough "depth of field".  Shooting something at f/22 may leave the file soft, but a little work and prints look pretty good. You probably loose some of the micro detail, but it can still work pretty well.



Brian Peterson in his latest book on Exposure brings attention to this "problem". Two images shot, one at f/8 and one at f/22. Blows both up to 200% and invites the reader to look at them. They are trees. He points out there is only a little softness in the one shot at f/22 and states it isn't worth bothering about it. I am paraphrasing him. It isn't a scientific appraisal but a practical one which I think matters the most. I have since rethought my beliefs on this subject which meant that I rarely went beyond f/13. Practical examples are more important to me than theoretical ones. :)
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on August 23, 2011, 06:38:11 am
Hi,

On the other hand it's like using a 6 MP camera instead of 24 MP camera. Half the resolution is lost, cutting usable pixels in one fourth. Now, I have made decent A2-prints from 6MP, but a significant amount of information is lost.

The solutions to infinite depth of field is "Scheimpflug" or focus stacking. On the other hand, selective focus is one of the expressions we have in photography. It may be better to have main subject in focus than having everything slightly unsharp.

Diffraction depends only on aperture, there are no lenses more tolerant of diffraction. It's a property of light. The form of the aperture may matter, but I presume that all lenses we discuss have circular or near circular apertures.

This page illustrates the effects of defocus and diffraction:
http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1

Sharpening is discussed here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=2

Best regards
Erik




Brian Peterson in his latest book on Exposure brings attention to this "problem". Two images shot, one at f/8 and one at f/22. Blows both up to 200% and invites the reader to look at them. They are trees. He points out there is only a little softness in the one shot at f/22 and states it isn't worth bothering about it. I am paraphrasing him. It isn't a scientific appraisal but a practical one which I think matters the most. I have since rethought my beliefs on this subject which meant that I rarely went beyond f/13. Practical examples are more important to me than theoretical ones. :)
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: stamper on August 23, 2011, 06:58:15 am
Are you saying diffraction results in a loss of resolution? A new one on me but I am willing here to learn. :)
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 23, 2011, 07:28:12 am
Are you saying diffraction results in a loss of resolution? A new one on me but I am willing here to learn. :)

That's not only what Erik is saying, it's a commonly known fact called physics, unfortunately. Deconvolution sharpening can restore some of the resolution lost due to diffraction blur, but some loss remains and noise may increase. The fact that you act surprised suggests that you don't see a difference between an actual image taken at e.g. f/8 and f/22. I'm puzzled by that. Are you saying that you can fully remove the blur caused by diffraction, or do you see no difference to begin with?

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Walter Schulz on August 23, 2011, 07:39:41 am
Diffraction depends on "effective aperture" and therefore depends on which magnification is used in a particuliar picture.
If you're focussing objects far, far away (like leaves on a tree) effective aperture and the aperture number are very, very close together.
If you're doing macro work (details of bank notes), you will see diffraction effects at lower aperture numbers.

k(eff) = k (1 + magnification)

k = aperture number of the lens
k (eff) = effective aperture number
magnification = sensor or film size / image size

I've done some work at magnification 1:1 to 3:1. When doing pixel peeping (all of us are weak at times ...) you will see serious detail loss with 3:1 and f/11 (lens setting) because "effective aperture" is f/44.

For people able to understand german language (and not afraid of maths): http://www.traxel.de/foto/drf/schaerfentiefe.pdf
Hope this helps to see the whole picture (sorry for the lame pun).

Ciao, Walter
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: stamper on August 23, 2011, 08:26:17 am
That's not only what Erik is saying, it's a commonly known fact called physics, unfortunately. Deconvolution sharpening can restore some of the resolution lost due to diffraction blur, but some loss remains and noise may increase. The fact that you act surprised suggests that you don't see a difference between an actual image taken at e.g. f/8 and f/22. I'm puzzled by that. Are you saying that you can fully remove the blur caused by diffraction, or do you see no difference to begin with?

As I stated in an earlier post I have very rarely shot smaller than f/13 because that was the advice that I read. I saw the examples in Brian Peterson's book but it didn't mention loss of resolution. Yes there was some softening at 200%. I have a lot of photography books and a lot of the photographers are happily shooting away at f/22 so it becomes confusing as to who are right and who are wrong. I have done a lot of shooting with ND filters and sometimes a slow shutter speed means going smaller than f/13 but mostly at sunset, so detail isn't critical. As I said still learning. :)

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: fike on August 23, 2011, 10:09:05 am
Two topics: focus stacking and tilt shift to increase DOF in panos.

FOCUS STACKING
In landscape panoramic photography, I find that focus stacking is very difficult to do well. Typically you are oustide and there is some air movement. Leaves move.  That's it.  Any movement is too much.  It is the same attribute that tends to make HDR problematic with woodland or wilderness photography. If your subject are hoodoos in the southwest, it may work because they don't move, ever.  With all the ways to mess up a panoramic (each photo is another opportunity for movement, lens flare, or an earthquake) I have given up on panoramic focus stacking--too many variables.

TILT SHIFT PANOS
I experimented rather extensively with the new 24mm TS-E II.  That is a fantastically sharp lens.  It can do flat stitches, though there is no gain in DOF with a flat stitch.  You can do some small amount of tilt in the first row of a multi-row pano and then stitch it all together, but it is tricky and error prone.  Stitching a pano containing images with different tilts tends to create problems because the shape of the projection changes.  I was unable to get reliable results...though sometimes it was stunning. 

The most reliable method I managed to create with a TS is the least obvious.  It involved mounting the camera in the portrait orientation on a pano head and tilting the TS-E lens sideways fairly aggressively. Effectively what I did was make an in focus plane running vertically from the back of the image area to very close to the camera.  This plane runs roughly perpendicular to the camera, but actually it angles away slightly to the left or right.  The in focus region would be about the middle fourth of the frame.  This wasn't really a problem because I was stitching. Now here is where it is annoyingly time-consuming, instead of making a new image every 20 degrees, or so, I made a new image every 4-5 degrees, and then in stitching I only used that narrow-in-focus slice of the image.  This was complicated, but the results were quite good.  A sample can be seen here: http://www.flickr.com/photos/trailpixie/5124820432/in/photostream/lightbox/ (http://www.flickr.com/photos/trailpixie/5124820432/in/photostream/lightbox/)
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 23, 2011, 10:14:24 am
Diffraction depends only on aperture, there are no lenses more tolerant of diffraction. It's a property of light. The form of the aperture may matter, but I presume that all lenses we discuss have circular or near circular apertures.

sure, however it is a fact that different lenses deliver quite different results (same focal length, same stopped down to big F numbers aperture) on the same body... so in some cases when you have a subpar lens you get much worse results that can be explained by diffraction alone.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 23, 2011, 10:17:36 am
Two topics: focus stacking and tilt shift to increase DOF in panos.

you forgot superresolution to fight w/ loss of resultion (while increasing DOF by stopping down further... albeit it might allow you to stop just 1-1.5 stops further down... not much, but still) from diffraction using programs like from http://photoacute.com (it can do focus stacking as well)
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 23, 2011, 02:00:59 pm
you forgot superresolution to fight w/ loss of resultion (while increasing DOF by stopping down further... albeit it might allow you to stop just 1-1.5 stops further down... not much, but still) from diffraction using programs like from http://photoacute.com (it can do focus stacking as well)
But superresolution will mainly combat limited sensor resolution, not severly limited optical resolution due to diffraction. If diffraction is the main factor limiting the system resolution, I would not expect SR to fix it.

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: deejjjaaaa on August 23, 2011, 04:36:48 pm
But superresolution will mainly combat limited sensor resolution, not severly limited optical resolution due to diffraction. If diffraction is the main factor limiting the system resolution, I would not expect SR to fix it.

-h

why again we are talking about "severely limited optical resolution" ? I wrote that it might help you to stop down a 1-1.5 stops more... not 10 stops... if you still can get a good resolution @ certain aperture F(n), then superresolution might help you @ F(n+1 stop)... not a magical bullet but less work than focus stacking...
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Wayne Fox on August 24, 2011, 12:03:44 am
The fact that you act surprised suggests that you don't see a difference between an actual image taken at e.g. f/8 and f/22. I'm puzzled by that.
I think it is image dependent ... depends on the micro detail and how critical it is to the overall image.  Gaining sharpness in important detail but perhaps losing some of the ultra fine detail can still result in a very good image. Certainly you can't completely overcome the loss of sharpness and restores all the detail once you start stopping down.

So curious what others think, which image would be sharper and have better detail printed at 40x60, one taken with a good lens at f/22 on a 60-80mp sensor (no AA filter) or a dSLR taken at an optimum of f/8 (with no diffraction but with blurring from an AA filter).

Focus stacking is great but can't always be used (things moving, things in front of other things can leave fringe or halo of "softness" since the background around the edge of front objects can't be captured sharp).  Scheimpflug is great, but can't always be applied well (if at all).  Sometimes your only hope of any depth of field is just stop the thing down and take what you can get, and with a high resolution back what you get is usually really good despite diffraction.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on August 24, 2011, 01:03:48 am
Hi,

I essentially tried this with one of the images from the great 2006 MFDB shootout. There was an aperture series with a P45+ back. I "reproduced" the subject (a one dollar bill) and shot it with my 24.5 MP DSLR. Large prints were made from both. At f/8 a blind man could see that P45+ was wastly superior. With the lens on the P45+ stopped down to f/22 the difference was essentially gone.

Best regards
Erik

I think it is image dependent ... depends on the micro detail and how critical it is to the overall image.  Gaining sharpness in important detail but perhaps losing some of the ultra fine detail can still result in a very good image. Certainly you can't completely overcome the loss of sharpness and restores all the detail once you start stopping down.

So curious what others think, which image would be sharper and have better detail printed at 40x60, one taken with a good lens at f/22 on a 60-80mp sensor (no AA filter) or a dSLR taken at an optimum of f/8 (with no diffraction but with blurring from an AA filter).

Focus stacking is great but can't always be used (things moving, things in front of other things can leave fringe or halo of "softness" since the background around the edge of front objects can't be captured sharp).  Scheimpflug is great, but can't always be applied well (if at all).  Sometimes your only hope of any depth of field is just stop the thing down and take what you can get, and with a high resolution back what you get is usually really good despite diffraction.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: torger on August 24, 2011, 04:28:05 am
For landscape when you strive after "sharpness in whole frame" (maximum DOF), sensor size actually has no meaning. It is about resolution.

If your sensor is small, you can use shorter focal length for the same view (larger DOF), but if your resolution on the sensor is the same as on a larger sensor you'll have to use larger aperture to minimize diffraction, and it exactly cancels out what you gain on focal length. In other words, 20 megapixel on a small sensor and 20 megapixel on a large sensor have the same ability to produce large DOF - the larger diffraction problem on the small sensor is compensated with the ability to use shorter focal lengths. The two properties cancels out precisely so maximum DOF ability becomes exactly the same regardless of format. That smaller formats may appear to have larger DOF is only because they usually have lower resolution and thus accept larger blurs, but if resolution is the same (and the lens can resolve it) there is no difference.

However, in practice a larger sensor is preferable, due to better noise properties (can gather more light) and that current lenses can produce sharper results. Larger sensors requires larger image circles from the lenses, and it is harder to make a lens that resolves say 5 microns on a medium format image cirle that 5 microns on for example APS-C. Actually the sharpest lenses in terms of microns on the sensor are in cheap compact cameras. However, unlike diffraction vs aperture this is not a you-gain-as-much-as-you-lose game, with increasing lens image circle size you gain more in total pixel count than you lose in individual pixel size, so while it is possible to resolve say 100 megapixel with the best medium format lenses today, you can only do say 20 megapixel with the best APS-C lenses.

Another aspect is that today the full frame 35mm (and even APS-C) produce as high resolution that you need to start thinking more like a large format photographer. In the film days and early digital, tilt lenses were rarely used to optimize DOF on 35mm, since the resolution of the format was so low that you did not gain much from doing so. With a modern digital 35 mm camera resolution is comparable to larger film formats, and thus you can indeed make use of tilt in landscape photography to optimize DOF, just as large format view camera photographers always have. In the Canon system, the TS-E 24mm version II is a fantastic landscape lens. Of course a view camera is more flexible in terms of tilt, but the price-performance equation and the flexibility to use DSLRs for other things than still life photography makes them the most practical alternative to most of us.

Also note that we will probably see further developments in deconvolution technique in the future that will to some extent be able to restore blur caused by diffraction and lens resolution limitations.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 24, 2011, 04:58:31 am
Hi,

I essentially tried this with one of the images from the great 2006 MFDB shootout. There was an aperture series with a P45+ back. I "reproduced" the subject (a one dollar bill) and shot it with my 24.5 MP DSLR. Large prints were made from both. At f/8 a blind man could see that P45+ was wastly superior. With the lens on the P45+ stopped down to f/22 the difference was essentially gone.

Best regards
Erik


Hey, Erik, that would be interesting to see, can you post the results for us?

BTW, comparing f/8 and f/22 is a bit unfair to the p45, since f/8 wont give sufficient dof on the FF anyway. How was the difference between f/16 on the FF and f/22 on the p45?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 24, 2011, 09:56:08 am
So curious what others think, which image would be sharper and have better detail printed at 40x60, one taken with a good lens at f/22 on a 60-80mp sensor (no AA filter) or a dSLR taken at an optimum of f/8 (with no diffraction but with blurring from an AA filter).

Hi Wayne,

Let's assume both cameras can resolve detail up to the Nyquist frequency. In my experience many cameras can have more than 10% luminance response at Nyquist, even if an OLPF (AA-filter) is used and despite demosaicing.

The Nyquist frequency of a Canon EOS-1Ds Mark III (6.4 micron sensel pitch) is 78.1 cycles/mm.
The Nyquist frequency of a PhaseOne IQ180 digital back (5.2 micron sensel pitch) is 96.2 cycles/mm.

Diffraction puts an absolute (unrecoverable by deconvolution sharpening) limit (MTF=0%) on resolution at 81.9 cycles/mm for f/22, and at 225.2 cycles/mm for f/8, when we look at 555 nanometer wavelength. It is clear that the IQ180's resolution suffers much more from the f/22 aperture than the 1Ds3 resolution does from f/8. Here is a graph showing the diffraction limited MTF (http://www.imatest.com/docs/sharpness.html#optimum_aper):

(http://bvdwolf.home.xs4all.nl/temp/DiffractionLimitedMTF.png)

If we assume perfect (only diffraction limited) optics on the IQ180, and want a minimum MTF response of 10%, we get a maximum resolution at f/22 of 66 cycles/mm. We will assume a maximum resolution of the 1Ds3, not limited by diffraction at f/8, at it's Nyquist frequency of 78.1 cycles/mm.

What remains is the output magnification of the on-sensor resolution. The IQ180 sensor (40.4 mm in the vertical dimension) compared to the 24mm of the 1Ds3, gives a factor of 1.68 benefit to the larger sensor which needs less magnification. That would scale the 1Ds3 maximum resolution to a 46.4 cy/mm relative to the IQ180's for output.

If our goal would be 5 lp/mm output resolution (~ 254 PPI), then the 1Ds3 could stand magnification by 9.28x, thus 223 mm for the short dimension, and the IQ180 could stand 13.2x magnification, thus 533mm for the short dimension while maintaining equal resolution. Of course this is when both systems use quality optics, perfect focus, and a rock steady shooting setup. The defocused parts of the DOF zones would make the MTF suffer gradually more at narrow apertures, but the exact effect depends on the specific MTFs (lens aberration and AA-filter effects).  

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on August 24, 2011, 10:58:11 am
Hi!

Check Bart's explanation above, a very good one.

I'm on vacation now, but will try to post some images from the comparison.

The "test" I have done was described here, at the end of the article: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/24-how-many-megapixels-do-we-need

Regarding the difference between f/8 and f/22 on the P45 it was huge at actual pixels.

I have done some studies on DoF and diffraction here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1

Regarding comparing f/22 on P45 and f/8 on full frame it was not really about being fair, more about seeing how much of the MF advantage would be lost by indiscriminate stopping down. I'm not in doubt about the advantages of MFD just that I want to understand why they are at advantage.

Best regards
Erik

Hey, Erik, that would be interesting to see, can you post the results for us?

BTW, comparing f/8 and f/22 is a bit unfair to the p45, since f/8 wont give sufficient dof on the FF anyway. How was the difference between f/16 on the FF and f/22 on the p45?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: aman74 on August 24, 2011, 11:39:03 am
Subscribed.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on August 24, 2011, 02:23:17 pm
Of course, the difference is huge, I do agree.

BTW, the comparison that I posted had f/11 for the 5d and f/18 for the 645d

I looked into photoacute, very interesting program. Maybe a way to go could be to use a 645d, wider lens (35mm), and wider apertures. Then use superresolution to do part of the upscaling, and avoid stitching altogether, making the whole shooting ten times easier. Then I guess you could blow it up to gigantic proportions with fairly good quality, and a lot less hassle when shooting.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: torger on August 25, 2011, 04:47:19 am
A comment on the test when combining CoC and diffraction blur. Adding blurs works in the way that if you add a small blur to a large blur the total blur becomes a bit larger.

That is, if CoC is 4 um and diffraction is 5 um the result will be less sharp than if CoC is 0 um and diffraction 5 um.

In other words if you improve something in the chain of blur factors that is not the worst factor you may still see an improvement.

So the assumption that as long as diffraction is less than CoC it does not have any effect on sharpness is actually incorrect. However, at such a small scale and when working with DOF that assumption is perfectly ok concerning the lack of preciseness in the general photographic workflow (one rarely measure distances to objects in millimeters etc).
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 26, 2011, 06:55:47 am
In other words if you improve something in the chain of blur factors that is not the worst factor you may still see an improvement.

Indeed, however the effect on image quality can be complex and hard to predict. Diffraction blur has quite a different effect on resolution loss than defocus (COC) has. In an attempt to demonstrate that, I've prepared 2 graphs that show the combined effect of diffraction and defocus on the MTF.

First the situation with f/8 diffraction (10.8m diameter), and different amounts of defocus blur added:
(http://bvdwolf.home.xs4all.nl/temp/DiffractionLimitedMTF_f08.png)

As expected, adding more defocus blur (a larger COC limit) to the diffraction blur will further reduce the limiting resolution progressively. The overall MTF is also reduced, resulting in a lower contrast image also for the spatial frequencies that are still resolved. In general we can conclude that at wider apertures image quality will suffer significantly from defocus blur. That is nice for portraits, but very unwelcome for deep DOF landscapes. There is a significant difference as we transition from the plane of best focus to the COC zone limit, and the quality at the plane of best focus is still high, compared to the theoretical 5.2 micron sensel pitch sensor potential (Nyquist frequency is 96.2 cy/mm).

Next the situation with f/22 diffraction (29.8m diameter), and the same amounts of defocus blur as above added:
(http://bvdwolf.home.xs4all.nl/temp/DiffractionLimitedMTF_f22.png)

Perhaps somewhat less expected, the added defocus has hardly any effect on the limiting resolution, but the medium spatial frequencies (for which human vision is most sensitive) still suffer a bit. Diffraction is clearly the limiting factor, and the blur is also seriously affecting the plane of best focus. There is relatively little difference between the COC zone limit and the plane of best focus.

An added consideration in the trade-off between the effects at different apertures is that diffraction blur potentially responds better to deconvolution sharpening than defocus blur (especially for smaller sensel pitch sensors), but that deconvolution will have no effect when the MTF becomes zero, there is no signal to deconvolve. It will help to boost the remaining medium spatial frequencies though, so there is still some benefit in using it.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 28, 2011, 03:38:14 pm
Diffraction puts an absolute (unrecoverable by deconvolution sharpening) limit (MTF=0%) on resolution at 81.9 cycles/mm for f/22, and at 225.2 cycles/mm for f/8, when we look at 555 nanometer wavelength.
Is there such a thing as a brick-wall diffraction limit above which mft == 0? Or are you talking about the limit above which you have experienced practical deconvolution algorithms to fail due to falling SNR or whatever?

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 28, 2011, 07:43:59 pm
Is there such a thing as a brick-wall diffraction limit above which mft == 0? Or are you talking about the limit above which you have experienced practical deconvolution algorithms to fail due to falling SNR or whatever?

It's not really a sudden diffraction brick wall, but rather a gradual spatial frequency slope of reduced contrast that ends in zero modulation. Even with only 10% modulation response, it would render a subject contrast of 10:1 to a barely perceptible 1% response. Raw converters performance becomes very important. Diffraction does set an upper limit to what can be resolved/restored, as does defocus.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 29, 2011, 01:40:30 am
It's not really a sudden diffraction brick wall, but rather a gradual spatial frequency slope of reduced contrast that ends in zero modulation. Even with only 10% modulation response, it would render a subject contrast of 10:1 to a barely perceptible 1% response. Raw converters performance becomes very important. Diffraction does set an upper limit to what can be resolved/restored, as does defocus.

Cheers,
Bart
So what you are saying is that in your experience, deconvolution algoritms are unable to recover 1% contrast into something meaningful, not that the contrast is identical to zero.

The reason that I am asking, is that there have been lots of discussions about the "diffraction limit" at dpreview. A lot of people seem to think that it is a theoretically perfect brick-wall, but I am sceptical as theoretically perfect brick-walls are very seldomly seen in nature. Nature seems to dislike 100000 tap sin(x)/x filters, rather going for low-order ones.

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on August 29, 2011, 04:06:27 am
Hi,

My understanding is that diffraction is not a disc (like defocus for an ideal thin lens) but more like a "bell curve". For peak shapes similar to "bell curves" most often FWHM (Full With Half Maximum) is used, but the effect of diffraction will be broader than FWHM. But some detail may also be resolved within the FWHM diameter as we still have some gradient.

My article here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1 demonstrates this with real world samples. Diffraction is red circles and defocus is green circles. For diffraction the conventional value is used. FWHM would be somewhat smaller.

When looking at the above article keep in mind that diffraction is constant for each row. Defocus is increasing from left to right.

Last page of the article shows examples of sharpening using "basic" deconvulution using Smart Sharpen in CS5 and Topaz inFocus.

Best regards
Erik



So what you are saying is that in your experience, deconvolution algoritms are unable to recover 1% contrast into something meaningful, not that the contrast is identical to zero.

The reason that I am asking, is that there have been lots of discussions about the "diffraction limit" at dpreview. A lot of people seem to think that it is a theoretically perfect brick-wall, but I am sceptical as theoretically perfect brick-walls are very seldomly seen in nature. Nature seems to dislike 100000 tap sin(x)/x filters, rather going for low-order ones.

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: torger on August 29, 2011, 04:32:04 am
Diffraction is often described as a hard limit, which is wrong. It is a soft onset, a kind of a bell curve filter with some ringing (varies with wave length too), and it can be reversed with deconvolution, theoretically all of it. In practice there'll be noise and there'll be too low resolution and too many unknown parameters (such as color filters in the sensor, color of incoming light etc) to make a perfect result. A deconvolved image has typically a bit grainy look when pixel-peeped, but can make very good results when printed.

As with nearly all aspects of image quality, the less you need to fix up in post-processing the better the end result will be, so if you can avoid a large diffraction offset that is better. And of course, diffraction is only relevant in high resolution photography, still at dpreview one can see photographers that shoot hand-held worry about it ;-).
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: David Eichler on August 29, 2011, 06:42:10 am
That's not only what Erik is saying, it's a commonly known fact called physics, unfortunately. Deconvolution sharpening can restore some of the resolution lost due to diffraction blur, but some loss remains and noise may increase. The fact that you act surprised suggests that you don't see a difference between an actual image taken at e.g. f/8 and f/22. I'm puzzled by that. Are you saying that you can fully remove the blur caused by diffraction, or do you see no difference to begin with?

Cheers,
Bart

Restore lost resolution? How can resolution be restored once it is lost? Do you really mean accutance, perhaps?
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 29, 2011, 06:54:02 am
Thank you.

The link to this image seems to be broken?
http://echophoto.dnsalias.net/ekr/images/DoF2/A55_100Macro_small1-16.jpg


So basically, you are looking at defocusing (PSF expansion) due to defocusing (moving the object of interest out of focus), and due to diffraction limiting (shrinking the aperture). I would expect the total defocusing to be something like:

large aperture
small aperture
Out of focus
Large PSF
Medium PSF
In focus
Small PSF
Medium PSF
Is that confirmed by your test?
Hi,

My understanding is that diffraction is not a disc (like defocus for an ideal thin lens) but more like a "bell curve". For peak shapes similar to "bell curves" most often FWHM (Full With Half Maximum) is used, but the effect of diffraction will be broader than FWHM. But some detail may also be resolved within the FWHM diameter as we still have some gradient.

My article here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1 demonstrates this with real world samples. Diffraction is red circles and defocus is green circles. For diffraction the conventional value is used. FWHM would be somewhat smaller.

When looking at the above article keep in mind that diffraction is constant for each row. Defocus is increasing from left to right.

Last page of the article shows examples of sharpening using "basic" deconvulution using Smart Sharpen in CS5 and Topaz inFocus.

Best regards
Erik

Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 29, 2011, 06:58:26 am
Restore lost resolution? How can resolution be restored once it is lost? Do you really mean accutance, perhaps?
I think this is only semantics. Deconvolution can (ideally) do a filtering operation that bring details that have had their contrast reduced to invisible levels back again to visible levels. I.e. true details that are visibly lost (and really hard to regain using blind sharpening) can be restored to their original value. Or to an approximation of their original value corrupted by noise and flaws in the characterization of the PSF.

So yes, you are right, the resolution is never truly "lost", but it is degraded/corrupted in such a way that it is really hard to bring back.

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 29, 2011, 08:24:41 am
Restore lost resolution? How can resolution be restored once it is lost? Do you really mean accutance, perhaps?

Hi David,

You are correct, once it's lost it's gone. However, what many consider lost is actually restorable to a significant extent. It's not done by edge contrast enhancement, but by deconvolution.

Diffraction especially is a good candidate for deconvolution sharpening/restoration, because it is not just an average over an area but rather a weighted average. So, as long as there is some microcontrast left (even when spread over multiple pixels), and we have or can synthesize a reasonably accurate model of the blur pattern (a point spread function or PSF), a lot of the seemingly lost resolution will prove to be restorable to real resolution. However, the lower the modulation transfer of the optical system is the higher the chance that noise will reduce our chances of successful recovery by generation of artifacts. And when the diffraction blur is very pronounced, there comes a limit beyond which there is no hope for restoration.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: torger on August 29, 2011, 08:26:29 am
Restore lost resolution? How can resolution be restored once it is lost? Do you really mean accutance, perhaps?

You cannot restore lost signal, but the thing is that with diffraction (and many other types of distortion) the signal is not lost, just distorted. If you know the shape of the function that distorted the signal you can make an inverse function and run the distorted signal through that and restore the original.

So even if the signal is totally blurry, if the blur function is well-defined you can "run it backwards" and restore the original -- that is what deconvolution is about. Deconvolution is a mathematical process that is used in many signal processing applications, not just in photography. It is often used in audio applications for example.

However, if the signal has been distorted with a function that totally cuts off parts of the signal (not just shuffles it around) the original cannot be restored. Fortunately diffraction is a relatively well-behaved type of distortion that keeps all signal and only blurs it (randomly though which is bad, but with a specific probability function so with enough signal it becomes well-defined).

To make a perfect inversion (deconvolution) you need perfect signal without noise and have the exact distortion function (point spread function, PSF), which you in practice won't have, so what you today can achieve with deconvolution is quite limited, but I find it useful in many images, not only for diffraction but also for example to enhance sharpness in long exposures when there's been some vibration problem.
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 29, 2011, 10:03:18 am
My understanding is that diffraction is not a disc (like defocus for an ideal thin lens) but more like a "bell curve". For peak shapes similar to "bell curves" most often FWHM (Full With Half Maximum) is used, but the effect of diffraction will be broader than FWHM. But some detail may also be resolved within the FWHM diameter as we still have some gradient.

Hi Erik,

Yes that's correct. This will also mean that with very high sensel densities and/or very narrow apertures (= large diffraction pattern diameter), the diffraction blur pattern will be oversampled, which in turn will make it easier to successfully deconvolve such an image. A defocus blur PSF will look like a disc of more or less uniform brightness, which will not provide as much help for practical restoration. In theory, in a perfect world, there is no mathematical difference as long as the PSF is decribed accurately.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 30, 2011, 01:54:04 pm
This will also mean that with very high sensel densities and/or very narrow apertures (= large diffraction pattern diameter), the diffraction blur pattern will be subsampled, which in turn will make it easier to successfully deconvolve such an image.
Did you mean supersampled?

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 30, 2011, 01:59:08 pm
Diffraction especially is a good candidate for deconvolution sharpening/restoration, because it is not just an average over an area but rather a weighted average.
I am scratching my head over this. Why is it necessarily harder to invert the response of a rectangular filter kernel than a general non-rectangular kernel (switching my brain over to 1-d operations for convenience)?

The DFT of a rectangular function is a sin(x)/x function with periodic zero crossings. In 2-d space I guess the equivalent would be circular flat (space) and some Bessel function (frequency). This means that for some frequency components the recorded SNR will be very low (tending towards 0). But other shapes may have deep zeros as well. The question is if the image will look better if we amplify only those components where the SNR is satisfactory?

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 30, 2011, 04:55:18 pm
Did you mean supersampled?

You are correct, I meant supersampled/oversampled. Thanks for spotting that, I've changed the text to avoid further confusion.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: Bart_van_der_Wolf on August 30, 2011, 05:29:21 pm
I am scratching my head over this. Why is it necessarily harder to invert the response of a rectangular filter kernel than a general non-rectangular kernel (switching my brain over to 1-d operations for convenience)?

As I said, mathematically it isn't. However, because we rarely have an exact PSF, and in the presence of (photon- and) read-noise, and the demosaicing of that noisy signal, the algorithms used are not necessarily straight forward classic deconvolution. They usually result in noise amplification although the signal is amplified more. Even the Richardson Lucy algorithm is based on maximum likelihood statistics of Poisson noise distributions.

Now, as for the difference between a defocus and a diffraction blur, and the deconvolution of it. Consider a large uniform area (free of noise to make things easy) in the spatial domain with a small signal in the middle. Now blur it with a uniform disc shaped filter that's several times larger in diameter than the signal. The small signal will become the average of that full disc's area, and thus very small, maybe even less than 1 quantization unit difference from the surrounding area. Now compare that to blurring with a Gaussian or an Airy disk shaped blur filter. The blurred image is more likely to still have some (Gaussian) shape with a slightly higher signal directly in the middle of the original signal, because the blur filter took a weighted average instead of an area average. Combining this slightly better signal with a deconvolution, offers a (slightly) better chance of restoration.

Add noise, and we can use all the small bits of help we can get.

Cheers,
Bart
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: hjulenissen on August 30, 2011, 05:57:16 pm
Now, as for the difference between a defocus and a diffraction blur, and the deconvolution of it. Consider a large uniform area (free of noise to make things easy) in the spatial domain with a small signal in the middle. Now blur it with a uniform disc shaped filter that's several times larger in diameter than the signal. The small signal will become the average of that full disc's area, and thus very small, maybe even less than 1 quantization unit difference from the surrounding area. Now compare that to blurring with a Gaussian or an Airy disk shaped blur filter. The blurred image is more likely to still have some (Gaussian) shape with a slightly higher signal directly in the middle of the original signal, because the blur filter took a weighted average instead of an area average. Combining this slightly better signal with a deconvolution, offers a (slightly) better chance of restoration.

Add noise, and we can use all the small bits of help we can get.

Cheers,
Bart
That may be the case for a small star or a hypothetical object. But is it the case for general, complex objects? How do we know that the shape of a part of a tree does not interact with the Gaussian when convolving to form a perfectly flat (hard-to-recover) end-result where using a flat kernel would have produced something for R-L to work on?
 
How does an image with wide, flat general MTF, but periodic deep high-frequency zeros look like? Assuming that is the optimal correction of a circular, flat PSF.

-h
Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: erpman on September 06, 2011, 10:02:43 am
So, after this weeks math-class, lets have a look at some real world examples:

A side by side comparison of canon 1dsIII and the pentax645d at different apertures. Although diffraction is clearly noticable, the MF holds detail much better than the FF, and the detail lost to diffraction seems to be recoverable with sharpening/deconvolution.

http://www.ephotozine.com/article/pentax-645d-canon-eos-1ds-mark-iii-comparison-digital-slr-review-15653 (http://www.ephotozine.com/article/pentax-645d-canon-eos-1ds-mark-iii-comparison-digital-slr-review-15653)


Title: Re: The DOF vs Diffraction challenge: FF or crop?
Post by: ErikKaffehr on September 06, 2011, 10:41:19 am
Hi,

It's a bigger sensor so any image would be around 30% magnified.  But both sensor would probably loose 75% of their megapixels when stopped down to f/22. It is know that diffraction can in part be corrected by deconvolution.

Best regards
Erik


So, after this weeks math-class, lets have a look at some real world examples:

A side by side comparison of canon 1dsIII and the pentax645d at different apertures. Although diffraction is clearly noticable, the MF holds detail much better than the FF, and the detail lost to diffraction seems to be recoverable with sharpening/deconvolution.

http://www.ephotozine.com/article/pentax-645d-canon-eos-1ds-mark-iii-comparison-digital-slr-review-15653 (http://www.ephotozine.com/article/pentax-645d-canon-eos-1ds-mark-iii-comparison-digital-slr-review-15653)