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Author Topic: Medium format redefined  (Read 65611 times)

fdisilvestro

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Re: Medium format redefined
« Reply #100 on: September 13, 2015, 08:24:54 am »

EricWhiss, I'm genuinely interested in finding what is going on. You are making a claim which does not agree with theory, which is fine, as long as you provide the evidence (if you have experience publishing scientific papers you should be used to that, at least I am)

Nobody is forcing you to read this thread Synn, If you and many more are not interested just skip it, I don't have any issue.

Since I agree that this thread is not going anywhere if we don't provide any evidence, I made a test, which I will explain in detail.
If my methodology is wrong I'm the first one interested in rectifying it.

Test: comparison of DOF of images at same OUTPUT MAGNIFICATION using the following lenses:

1.- Hasselblad C 80mm Planar f2.8 (medium format)
2.- Nikon AIS 50mm f1.4

The camera used was a nikon D800. (the test is based on a different crops for each lens, so that OUTPUT MAGNIFICATION of the crop is the same, which would be equivalent to have the same angle of view in both formats).

Camera and subject was constant
Focusing distance was the closest possible with the Hasselblad 80mm (aprox 95 cm) without extension tubes

All files processed with default settings in LR, only the White balance was adjusted

Here are the results:

For the first test, the image to the left is the Hassy 80 @ f5.6, the center is the nikon @ f2.8 (2 stops compared to the 80mm) and the right one is the nikon @ f2 (3 stops compared to the 80mm)



The second test is the Hassy 80 @ f4.0, then the nikon @ f2 (2 stops compared to the 80mm) and finally at the right is the nikon @ f1.4 (3 stops compared to the 80mm)



My observation is that the difference between the two lenses which translates to same output size with equivalent fields of view is 2 stops.

I would really like (seriously) if you can share the results that show 3 f stops difference

« Last Edit: September 13, 2015, 08:30:48 am by FranciscoDisilvestro »
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Theodoros

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Re: Medium format redefined
« Reply #101 on: September 13, 2015, 10:14:14 am »


Since I agree that this thread is not going anywhere if we don't provide any evidence, I made a test, which I will explain in detail.
If my methodology is wrong I'm the first one interested in rectifying it.

The camera used was a nikon D800. (the test is based on a different crops for each lens, so that OUTPUT MAGNIFICATION of the crop is the same, which would be equivalent to have the same angle of view in both formats).

My observation is that the difference between the two lenses which translates to same output size with equivalent fields of view is 2 stops.

I would really like (seriously) if you can share the results that show 3 f stops difference

If I may Francisco... Where the test suffers is that you used crop (and thus magnified the FF lens result which lead to a perception of shallower DOF)... To do a proper test, you should have either used two lenses of the same focal length (one MF and one 35mm) and keep the image area of the 35mm DSLR constant... (where the difference would be of about a stop - the stop that you are missing) or to use 6x6 film with the Hasselblad and aim for an AOV that would be the same on the diagonal of the square vs. the 3:2 ratios (put the scale on the diagonal)...
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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #102 on: September 13, 2015, 10:29:49 am »

Test: comparison of DOF of images at same OUTPUT MAGNIFICATION [...]

Correct, that's the only sensible thing to do. That will factor in the need for different output magnification due to physical sensor size differences.

Quote
using the following lenses:

1.- Hasselblad C 80mm Planar f2.8 (medium format)
2.- Nikon AIS 50mm f1.4

The camera used was a nikon D800. (the test is based on a different crops for each lens, so that OUTPUT MAGNIFICATION of the crop is the same, which would be equivalent to have the same angle of view in both formats).

Yes, both are relatively symmetrical designs, and the focal lengths roughly offer a comparable field of view (as far as the aspect ratio differences allow to compare). To satisfy my curiosity, and to run the DOF calculation numbers on, on which Hasselblad was that?

Quote
My observation is that the difference between the two lenses which translates to same output size with equivalent fields of view is 2 stops.

Yes, 2 stops results in  pretty close similarity, as expected.

Cheers,
Bart
« Last Edit: September 13, 2015, 10:45:38 am by BartvanderWolf »
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ErikKaffehr

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Re: Medium format redefined
« Reply #103 on: September 13, 2015, 10:30:38 am »

Hi,

Thanks for sharing. I can do a demo when I get home with a P45+ and a Macro Planar 120 and a Sony 90/2.8 macro on the A7rII. These systems are well matched regarding resolution.

Main main issues with Eric Hiss statement are that they contradict theory, which is a serious thing in science/egineering, and that he doesn't report on how the observations were made. If his observations were made at actual pixels, criteria would differ between systems having different resolutions.

Personally, I have been shooting MFD , P45+ and Sony DSLR, mostly 24MP. I have not seen any magic differences. But I have not done a lot of detailed tests. But all tests I have made the systems are quite similar, except the P45+ having much higher resolution.

Best regards
Erik


EricWhiss, I'm genuinely interested in finding what is going on. You are making a claim which does not agree with theory, which is fine, as long as you provide the evidence (if you have experience publishing scientific papers you should be used to that, at least I am)

Nobody is forcing you to read this thread Synn, If you and many more are not interested just skip it, I don't have any issue.

Since I agree that this thread is not going anywhere if we don't provide any evidence, I made a test, which I will explain in detail.
If my methodology is wrong I'm the first one interested in rectifying it.

Test: comparison of DOF of images at same OUTPUT MAGNIFICATION using the following lenses:

1.- Hasselblad C 80mm Planar f2.8 (medium format)
2.- Nikon AIS 50mm f1.4

The camera used was a nikon D800. (the test is based on a different crops for each lens, so that OUTPUT MAGNIFICATION of the crop is the same, which would be equivalent to have the same angle of view in both formats).

Camera and subject was constant
Focusing distance was the closest possible with the Hasselblad 80mm (aprox 95 cm) without extension tubes

All files processed with default settings in LR, only the White balance was adjusted

Here are the results:

For the first test, the image to the left is the Hassy 80 @ f5.6, the center is the nikon @ f2.8 (2 stops compared to the 80mm) and the right one is the nikon @ f2 (3 stops compared to the 80mm)



The second test is the Hassy 80 @ f4.0, then the nikon @ f2 (2 stops compared to the 80mm) and finally at the right is the nikon @ f1.4 (3 stops compared to the 80mm)



My observation is that the difference between the two lenses which translates to same output size with equivalent fields of view is 2 stops.

I would really like (seriously) if you can share the results that show 3 f stops difference


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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #104 on: September 13, 2015, 11:11:24 am »

Hi,

Thanks for sharing. I can do a demo when I get home with a P45+ and a Macro Planar 120 and a Sony 90/2.8 macro on the A7rII. These systems are well matched regarding resolution.

And more importantly for a DOF comparison, their Angles of View are quite closely comparable:
A7rII  &   90mm   (angle of view:  h:20.5 v:13.8, d:24.5) (with close-up shooting , they get wider at infinity)
P45+ & 120mm    (angle of view:  h:20.3 v:15.3, d:25.2) (with close-up shooting , they get wider at infinity)
So then only the output magnification becomes the variable for equal size output, and total DOF should be relatively easy to compare.  

Shooting close-up also allows to more accurately determine the magnification factor (on sensor), which allows to use the more accurate DOF calculation formula (attached) that applies at any distance, with the additional possibility to factor in the pupil factor variable (P in the formulas).

Cheers,
Bart

P.S. The attached formulas also allow to compensate for the change in apparent or effective aperture at non-infinity focus distances, if the magnification factor is known or calculated from shooting distance (from first principal plane, which is less accurately known at close distances). All variables in the same units, f=focal length, s=shooting or subject distance, M=magnification factor, N=(effective) aperture, P=pupil factor (the Exit/Entrance pupil diameter ratio at the given focus setting).
« Last Edit: September 13, 2015, 01:12:07 pm by BartvanderWolf »
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Theodoros

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Re: Medium format redefined
« Reply #105 on: September 13, 2015, 11:36:24 am »

Correct, that's the only sensible thing to do. That will factor in the need for different output magnification due to physical sensor size difference.

Cheers,
Bart
I don't see how the output magnification in the test that Fransisco did factors for the different image circles that the two lenses project... The 6x6 lens projects an image circle that is 4 times the area of the image circle that a 35mm lens projects... (assuming that both lenses are designed to "just" cover the respective light sensitive areas of their respective cameras).

EDIT: Also, in the test that Erik proposes, the P45+ requires an image circle of (about) 65% the area that the lenses of the V system are able to project... So in both cases, one compares DOF for different FOV than the one that the MF lens (really) projects... Surely, if one will use a "full frame" MF back, (like Eric did) results will be different (and even more so if one uses the full image area that a 6x6 camera has).
« Last Edit: September 13, 2015, 11:57:25 am by Theodoros »
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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #106 on: September 13, 2015, 11:57:02 am »

I don't see how the output magnification in the test that Fransisco did factors for the different image circles that the two lenses project...

The size of (the part of) the image circle (used for composition) only matters for composing the shot, it is irrelevant for DOF. The magnification to (equal sized) output is relevant for the COC assumption, because the (acceptable) optical blur gets magnified when resizing for output.

Quote
The 6x6 lens projects an image circle that is 4 times the area of the image circle that a 35mm lens projects... (assuming that both lenses are designed to "just" cover the respective light sensitive areas of their respective cameras).

Because the physically smaller sensor needs more magnification than the larger sensor to achieve the same output size, it needs a different COC assumption, or a different aperture, for the DOF calculation. If the output size is equalized (which dictates the observable COC in output), then only aperture remains to make a difference.

Cheers,
Bart
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ErikKaffehr

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Re: Medium format redefined
« Reply #107 on: September 13, 2015, 12:03:15 pm »

Hi Bart,

Modern lenses often use internal focusing and floating lens groups, both affecting focal length. So how do we determine effective aperture?

Testing at medium magnifications may be a workaround for that problem?

Best regards
Erik




And more importantly for a DOF comparison, their Angles of View are quite closely comparable:
A7rII  &   90mm   (angle of view:  h:20.5 v:13.8, d:24.5) (with close-up shooting , they get wider at infinity)
P45+ & 120mm    (angle of view:  h:20.3 v:15.3, d:25.2) (with close-up shooting , they get wider at infinity)
So then only the output magnification becomes the variable for equal size output, and total DOF should be relatively easy to compare.  

Shooting close-up also allows to more accurately determine the magnification factor (on sensor), which allows to use the more accurate DOF calculation formula (attached) that applies at any distance, with the additional possibility to factor in the pupil factor variable (P in the formulas).

Cheers,
Bart

P.S. The attached formulas also allow to compensate for the change in apparent or effective aperture at non-infinity focus distances, if the magnification factor is known or calculated from shooting distance (from first principal plane, which is less accurately known at close distances). All variables in the same units, f=focal length, s=shooting distance, M=magnification factor, N=(effective) aperture, P=pupil factor (the Exit/Entrance pupil diameter ratio at the given focus setting).
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Theodoros

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Re: Medium format redefined
« Reply #108 on: September 13, 2015, 12:31:46 pm »

The size of (the part of) the image circle (used for composition) only matters for composing the shot, it is irrelevant for DOF. The magnification to (equal sized) output is relevant for the COC assumption, because the (acceptable) optical blur gets magnified when resizing for output.

Because the physically smaller sensor needs more magnification than the larger sensor to achieve the same output size, it needs a different COC assumption, or a different aperture, for the DOF calculation. If the output size is equalized (which dictates the observable COC in output), then only aperture remains to make a difference.

Cheers,
Bart
I believe that for theory (and the formulas) to be applicable, one must adjust the AOV (in the diagonal of the image circle) for the whole image area that the lens has been designed for...  The other (and easier) way to do an accurate test (for certain lenses) is to use two lenses of the same focal lengths at the same aperture setting, shoot both on the same sensor with the scale on the diagonal and then magnify the larger format lens image by the respective difference of its larger image circle area... (four times in this case)... IMO, that would give a (pretty) accurate approach...
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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #109 on: September 13, 2015, 01:10:34 pm »

Hi Bart,

Modern lenses often use internal focusing and floating lens groups, both affecting focal length. So how do we determine effective aperture?

Hi Erik,

You are right that focal length will remain a somewhat uncertain (limited precision) parameter in all DOF calculations.

Because M is determined by focal length and shooting distance (my first formula attachment earlier), the exact focal length is an important factor. However, it is also not obvious where the first principal plane of a lens is situated. So there we have to deal with two uncertainties/imprecisions.

That's why it's IMHO easier to use the effective result of both, the magnification factor itself, and calculate with that. A closer shooting distances we can quite accurately measure the magnification factor. Just shoot a ruler or other object of known size, and measure the size it produces on the sensor (pixels x pitch, or percentage of sensor width). Then divide that sensor projection size by the original subject size, and we have out magnification factor.
That will only leave us with a single uncertainty, the real focal length.

We do have the pupil factor as an additional correction though, and it can be approximated by looking/measuring the apparent entrance and exit pupil of the lens when looking though it.

Quote
Testing at medium magnifications may be a workaround for that problem?

Yes, up tot a point. With the magnification factor as a known certainty, the effective aperture can be factored in (which helps accuracy), and the focal length 'only' adds an uncertainty to the total DOF that's roughly twice the uncertainty of the focal length. IOW, a 1% change in assumed focal length, will result in a ~2% change in DOF. When we shoot from a closer distance, the magnification factor becomes larger and weighs more in the overall calculation, so the effect of the focal length uncertainty gets slightly smaller (but hardly significantly so).

I just prefer to have only one uncertain parameter (focal length) instead of two (focal length and distance). It also becomes harder to set the distance to an exact position (or measure it without a laser distance finder) as the distance increases, and it becomes harder to eyeball the visual DOF difference in output with increased distance.

So a medium distance (if that's what we normally shoot), or a bit closer (say 1 - 2 metres), seems good for testing to me. Of course DOF becomes more critical at even closer distances (and easier to see the acceptable limits), so it helps if the formula is more accurate for those distances as well.

Cheers,
Bart
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EricWHiss

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Re: Medium format redefined
« Reply #110 on: September 13, 2015, 02:05:45 pm »

EricWhiss, I'm genuinely interested in finding what is going on. You are making a claim which does not agree with theory, which is fine, as long as you provide the evidence (if you have experience publishing scientific papers you should be used to that, at least I am)



First off, your Hasselblad probably has a smaller sensor than my AFi-ii 12 - if you wrote what sensor I didn't see it, secondly I'm not making a claim, rather I shared an observation, and theres a difference between observation and claim.  Lastly, a lot of this DOF is arbitrary and depends on usage and interpretation which is subjective.  Results will likely depend on a number of other factors too such as equipment used, technique, and so on.  Trying to pin down the difference between your two stops and my three stops will be difficult.



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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #111 on: September 13, 2015, 02:38:12 pm »

I believe that for theory (and the formulas) to be applicable, one must adjust the AOV (in the diagonal of the image circle) for the whole image area that the lens has been designed for... 

If I understand what you are suggesting, then no. The only thing that matters is the diameter of the cone of rays that is intersected by the sensor, the cone that is focused slightly in front or behind of the sensor. The intersection is a sort of circular disc, depending on the shape of the aperture, and possible vignetting (by the lens barrel) in the shape of an oval on one or two sides, and it is the blurred image of our subject. The diameter of it is the COC, at the limit of acceptable blur. So it's the exit pupil, the size of which is determined by the aperture / the entrance pupil, that sets the angular limits of the cone of rays, not the designed image circle of the lens. A narrower cone can be intersected further away from the plane of best focus, before it exceeds the COC limit we need for sharp enough output after magnification.

Cheers,
Bart
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fdisilvestro

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Re: Medium format redefined
« Reply #112 on: September 14, 2015, 08:42:38 am »

Hi,

Just my closing (maybe) comment since I don't want to leave unanswered questions.
To satisfy my curiosity, and to run the DOF calculation numbers on, on which Hasselblad was that?

First off, your Hasselblad probably has a smaller sensor than my AFi-ii 12

It seems that I was not specific enough in my first post. Both lenses were mounted on a Nikon D800. The Hassy (zeiss) 80mm was mounted using a fotodiox adapter. The rationale here is that If I enlarge the image taken with the 50mm 1.6X with respect to the image taken with the 80mm (the relation between the lenses), I get the same output magnification, which would be the same as having used a medium format sensor (1.6x larger than 24x36) and producing an output image of the same size.

In case it is not clear, I used crops that show the same scene object. These crops have different size in pixels but show the same scene object, when you enlarge the crop of the 50mm lens by1.6 you get the same physical size as the one taken from the 80mm.

I guess that Theodoros and EricWHiss will not agree with this, maybe Bart and Erik Kafehr will, but that is my understanding on how to compare DOF across different formats, so I will not continue arguing about it.

I'm not making a claim, rather I shared an observation, and theres a difference between observation and claim.  Lastly, a lot of this DOF is arbitrary and depends on usage and interpretation which is subjective.  Results will likely depend on a number of other factors too such as equipment used, technique, and so on.  Trying to pin down the difference between your two stops and my three stops will be difficult.

All I was asking is if you can share some of the images / evidence from which you based your observations

------------------------------------------------------------------------------------------

Anyway, Playing with the detailed equations and doing the tests I learned many things that affect DOF at close ranges or less than 10X the focal length, especially the pupil magnification and the focal length. Another learning is that those variations are not symmetrical with respect the point of exact focus (based on CoC size).

Finally the following image is a comparison of a Hasselblad/Zeiss C 50mm F/4 Distagon (medium format, old type, no internal moving elements) and the Nikon 50mm f/1.4 using the same setup as before, at the closest possible distance without using extension tubes (ca. 45 cm.)

The left image is the Hassy @ 4.0, the middle is the Nikon 50 @ 4.0 and the right one is the nikon @ 2.8

My observation: DOF with the medium format lens is slightly less than the nikon 50 using the same aperture, with more difference in the near region than in the far region but not enough as 1 stop

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AreBee

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Re: Medium format redefined
« Reply #113 on: September 14, 2015, 10:17:17 am »

Francisco,

Quote
My observation: DOF with the medium format lens is slightly less than the nikon 50 using the same aperture, with more difference in the near region than in the far region...

From this article (refer also to the table approximately half way down):

"Even though the total depth of field is virtually constant, the fraction of the depth of field which is in front of and behind the focus distance does change with focal length".

Could this explain your real-world difference in the near- and far-region?
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Bart_van_der_Wolf

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Re: Medium format redefined
« Reply #114 on: September 14, 2015, 11:02:52 am »

Francisco,

From this article (refer also to the table approximately half way down):

"Even though the total depth of field is virtually constant, the fraction of the depth of field which is in front of and behind the focus distance does change with focal length".

And it changes with distance. At macro distances it's equal in front and to the rear, so a front:rear ratio of 1:1, while at the hyperfocal distance it's a ratio of 1:infinity. This and many more details can be found here (one of the, if not the, best resources for such matters).

Quote
Could this explain your real-world difference in the near- and far-region?

Yes, but to quantify one would need to do separate calculations for front DOF and rear DOF, instead of total DOF. A longer focal length has a hyperfocus point that is more distant, so it changes between 1:1 to a 1:infinity DOF ratio at a different pace.

The total DOF formula is also useful for close-up photography that uses Focus stacking with a focus rail, which keeps the magnification constant (but changes perspective), unlike refocusing which changes the total DOF for each shot because the magnification changes. One can use the sensel pitch as COC limit, and calculate how large the step distance for all shots must be. For refocusing one could assume the worst, and use the total DOF for the closest distance (largest magnification) and gradually have surplus DOF as one focuses further away.

Cheers,
Bart
« Last Edit: September 14, 2015, 11:26:43 am by BartvanderWolf »
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fdisilvestro

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Re: Medium format redefined
« Reply #115 on: September 14, 2015, 03:02:36 pm »

Hi Bart and Rob,

I haven't had the time to check in detail the information referred by Bart and so far my preliminary results are theoretical, using the formulas available here

The effect I am talking about is due to pupil magnification only. Given the same focal lenght and same subject distance, the greater the pupil magnification the lower the depth of field, but this change is different in the near and far regions (this is what I refer as non-symetrical). Anyway I need to review this subject further to be sure it is right.

landscapephoto

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Re: Medium format redefined
« Reply #116 on: September 14, 2015, 04:27:55 pm »

Anyway, Playing with the detailed equations and doing the tests

Just a warning: the equations you are playing with ignore all optical aberrations of the lenses (unless you are using ray tracing software). These aberrations are essential to characterise how "unsharp" the out of focus areas of a given lens are.
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fdisilvestro

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Re: Medium format redefined
« Reply #117 on: September 14, 2015, 05:00:08 pm »

Just a warning: the equations you are playing with ignore all optical aberrations of the lenses (unless you are using ray tracing software). These aberrations are essential to characterise how "unsharp" the out of focus areas of a given lens are.

Absolutely! The simplified models used are just to start to understand what is going on.

EricWHiss

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Re: Medium format redefined
« Reply #118 on: September 15, 2015, 01:45:41 am »

The perception of depth of field is influenced by many factors, for example the grain of the image or lack of grain, the resolution of the screen or print and so forth.  I wouldn't be surprised if different raw conversion and parameters used will also impacted a persons perception of DOF.  What looks like its within DOF on screen may not look like its still inside the DOF when printed. And of course there is the print size variable, and variation in peoples eye sight as well.   9274

If you have a very good lens and sensor, and a quantitative testing method, you may be able to observe the focus peak. I saw this with my Rolleiflex Schneider lenses using imatest and an 8ft chart shooting tethered.  I was shooting at 3-4 meters and could notice a jump in sharpness at the peak of focus. I don't think I would have seen this with normal focusing, but I had my camera mounted on a geared rail and was using the rail to tune the focus following the numbers on Imatest to see the max.  Taking steps smaller than 1mm you can see this jump in sharpness right at the peak - move 2 mm steps and you might miss it. This was observed with several lenses and with different aperture settings.

It would be interesting to graph the sharpness over distance to see how the sharpness rolls off from the focus peak in either direction.  I don't have time for this now, but I think it would be interesting to compare the roll off curve between formats.  My hunch is that the rate is faster with larger formats.  Using this method you could also make some better study of the differences in DOF and learn how much of the DOF falls behind the peak or in front.  If you wanted to measure the difference between say your Hasselblad and your DSLR then you could take lenses with equivalent lenses for each format and arbitrarily decide on a % of peak sharpness for the border of the DOF.  Then you could measure the DOF as you defined for the DSLR at say f/6.8 and then see what f/stop the MFDB needed to be set at to get the same DOF as defined by a drop from Peak sharpness to a % you chose.  If you had lots of time you could also repeat this with other different formats.  I guess if I were doing this test myself, I'd also want to process out the files the best way I knew how for each system - meaning using the best RAW converter for each camera. Imatest tells you whether you have over sharpened or not.




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Re: Medium format redefined
« Reply #119 on: September 15, 2015, 11:47:13 am »

Just a warning: the equations you are playing with ignore all optical aberrations of the lenses (unless you are using ray tracing software). These aberrations are essential to characterise how "unsharp" the out of focus areas of a given lens are.

you can use the same MF back with the same lens and just consider a crop from it matching the area of FF sensor... that shall be better.
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