Luminous Landscape Forum
Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: Michael Erlewine on March 24, 2015, 07:08:13 am
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I can read the various MTF and other lens-testing charts, but they are only as meaningful in my work as I can implement them in the studio or field. In other words, I am not much of a lens tester myself, except though actually using the lens for my own work. I am certain that any given lens reaches greatest resolution at a certain f/stop, just as the experts tell us. No doubt. However, what I really want to know is about what kind of curve the particular lens creates from its widest to its narrowest aperture and how does that curve affect my particular work. That’s the curve I actually use. In other words, is it “sharp” wide-open or does that sharpness start a couple of stops later, and how long is that sharpness maintained? What kind of curve do we have, sharp or gentle?
As someone who stacks focus, I don’t stack focus at the same aperture that I use for taking a traditional single-shot photo. With a one-shot photo I tend to, of course, push the aperture higher (narrower) to get as much depth-of-field as I feel I need for a particular shot, which often is as much as I can get without degradation of the image through diffraction. Yet when I stack focus, I don’t worry about using a narrower aperture to get my depth of field, but rather I use focus stacking to create the apparent depth of field.
So, for focus stacking I want a single aperture on the lens-curve that marks the point of greatest resolution for that lens. In summary, I don’t try to stack with narrow apertures, but almost always with a single aperture for the lens that is considered its peak-resolution, what commonly is called “sharpness,” although that is a rather nebulous term. That way every increment of the stacked layers has maximum resolution and therefore the resulting stacked images shares that too.
Not to be confusing, but sometimes I stack not at the point (aperture) of greatest resolution, but just a little higher (narrower) if I am trying to create a little additional faux micro-contrast for that image. I take advantage of the greater depth-of-field obtained at a narrower aperture and record the additional depth-of-field as if it were greater acutance – micro-contrast. I am still undecided whether this actually helps, but it is a concept I am playing with. Normally I stack at the aperture that the testers (or my eyes) tell me has the most resolution for that lens and leave it go at that. The point here is that I come up with my own idea of what aperture curve will work for the job at hand, i.e. what I can get away with.
All photographs IMO are impressions, our own mental and psychological impressions of what we see out in the world, given the caveat that much of what we see, our impressions, come not from the outside, but from our own mind and approach. Because focus stacking is a form of lossy sampling, a stacked photo is almost an impression of an impression, so to speak. I don’t easily fall into believing that what I am photographing out there in the world has a reality greater than my own impressions and approach. Let’s take the recent Zeiss 135mm APO as an example, and the following are just my thoughts on how I use this lens for close-up photography.
The Zeiss 135mm is sharp wide-open, so I don’t have to add a couple of f/stops to achieve better resolution. With this lens wide-open, I get a depth-of-field (DOF) that is razor sharp. With that ultra-thin slice of DOF, I can literally paint focus, layer by layer, until I create what we could call a block of focus that represents what I want in that image to be sharp and in-focus. Because the lens is fast and wide open, whatever I don’t layer-paint is automatically blurred or part of the bokeh of the image. Note that this is the opposite of much traditional advice for focus stackers, i.e. that we push the lens as high as we can without suffering too much diffraction and then stack. I am going against tradition here because I like the results better. Now, back to the Zeiss APO 135mm lens.
With traditional one-shot photos, when I am not stacking, I find that from the Zeiss 135 APO I can get usable resolution and acutance all the way to up to something like f/13, which is a long way. Yes, by then I am recording diffraction that bothers me (and way before that), but I often can get by with it. If I don’t need peak sharpness for the particular subject, I can shoot at f/16 and inject some little bit of needed clarity or contrast in post. Beyond f/16 I am getting too much diffraction and image-degradation to venture there.
Since I am primarily a close-up photographer (rather than a macro photographer), much less a micro-photographer, the lack of extreme detail at f/16 with the Zeiss 135mm APO is often acceptable, diffraction and all. In fact, I have an ongoing battle going on within me whether to do a lot less stacking and a lot more taking single-shot traditional photos.
I am also experimenting with what I call “short-stacks,” where I take two or three shots that capture the particular areas in a photo I want to be in high-focus and stack that. I find that with these new Zeiss APO lenses do actually work much better than I would have guessed for short stacks. Years ago, when I was first starting out with focus-stacking, I did short stacks because I was lazy, and the results were that I had way too many artifacts in the final images.
But with, as I have mentioned in many articles now, these three new Zeiss APO lenses (135mm, 55mm, 85mm), this short-stack technique seems to work out very well indeed. And I don’t even stack them in the ordinary way. Yes, I use Zerene Stacker with short stacks, but when retouching I have a different approach. Ordinarily, I retouch artifacts only, but with the short-stack approach I tend to just paint in from each of the layers just the main part that layer has in perfect focus, kind of in a whole-cloth sort of way. Most of us used to this in Photoshop. I do have to pay attention to where these layers overlap, but I have been surprised how successful that has been.
Here is a little tableau I have put together. I will have to show a larger view at another time, but I am focusing on the two-dollar bill, but have included some burlap (pleated) so that it rises up and we can see how much depth-of-field is available at the higher apertures. Perhaps some of you reading this will have suggestions for what kinds of objects I could additionally include.
These shots are not about color, but about resolution, diffraction, and depth-of-field. I notice that I can get away with f/11 (see the copper tacks), but with f/16 it is more iffy (but often still usable) for close-up, but not for macro. Lately my internal mantra seems to be “I always seem to go for high resolution,” but am interested more in acuity (micro-contrast) in post. And I only do all of this with APO lenses, for the most part.
Your thoughts? Are these kind of images useful to anyone by myself?
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Thank you, Michael. Your experience with the 135 @ f16 replicates mine with the 55. The only difference is I do landscape, but very little close-up.
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This Zeiss looks terrific wide open !
& a perfect flat field- beautiful
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The Zeiss 135mm is sharp wide-open, so I don’t have to add a couple of f/stops to achieve better resolution.
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With traditional one-shot photos, when I am not stacking, I find that from the Zeiss 135 APO I can get usable resolution and acutance all the way to up to something like f/13, which is a long way. Yes, by then I am recording diffraction that bothers me (and way before that), but I often can get by with it. If I don’t need peak sharpness for the particular subject, I can shoot at f/16 and inject some little bit of needed clarity or contrast in post. Beyond f/16 I am getting too much diffraction and image-degradation to venture there.
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Your thoughts? Are these kind of images useful to anyone by myself?
Hi Michael,
I have enjoyed your recent post, great images.
As already mentioned, in order to give you meaningful feedback one would need to better understand what it is that you mean when you speak of micro-contrast. Acuity as a term does not help because it includes psychovisual processing by the brain in its definition. We can make acuity whatever we want with just a little rough sharpening but the result certainly wouldn't look pretty to the trained eye. So I will assume that by micro-contrast you mean 'sharpness' in some part of the actual physical linear spatial resolution information captured by your raw files.
If so, the first thing to keep in mind is that it is very difficult to determine peak sharpness eyeballing the subject through the viewfinder. In fact it is difficult to determine it by pixel peeping images at 100%. The better way is to focus peak and measure it. Lenstip.com (http://www.lenstip.com/388.4-Lens_review-Carl_Zeiss_Apo_Sonnar_T*_135_mm_f_2.0_ZE_ZF.2_Image_resolution.html) is a site that does just that and they happen to have measured the Zeiss Apo Sonnar T*135f/2 on a D3X, which I believe you own. Unfortunately they test lenses at around 30X focal length as I recall, which may not be the way you use them, so take the following camera dependent comments with a grain of salt.
Take a look at their Apo T*135 measurements (http://www.lenstip.com/388.4-Lens_review-Carl_Zeiss_Apo_Sonnar_T*_135_mm_f_2.0_ZE_ZF.2_Image_resolution.html): f/4 results in best middle-of-the-road captured 'sharpness', about 10% better than what can be accomplished wide open (or again at around f/7). Although it is very difficult to see a difference of 5%, 10% would be definitely noticeable. By f/11 you are more than 20% off the peak; and by f/16 almost 1/3 off the peak, giving up a ton of 'micro-contrast'.
So leaving DOF considerations aside for a moment, if you were satisfied with your kit's performance at f/2 you may want to try it at f/4 or even f/7 to see how it does there with your kind of setup. If f/7 indeed proved as 'sharp' as f/2, it would most likely be preferable because it would sharpen up better in post, given the fewer aberrations and the better behaved PSF due in larger part to diffraction at that aperture.
Best,
Jack
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Thanks for commenting. I am not an expert in this testing, obviously. Here is what I understand, so please point out what I am missing.
What passes for “Sharpness” in common reference is not any kind of standard. On the contrary, it is made of several factors, perhaps more. There is sheer resolution, which the various test charts do a good job of defining. Then, as you mention, there is “acutance,” which has to do with the sharpening of edges, and this is what is sometimes called “micro-contrast.” Now, this term micro-contrast and by association acuity has, as you put it, “psychovisual processing,” with seems to be somewhat of a subjective term, personal to some degree. But so are we, subjective and personal beyond measuring.
So, there is resolution and acutance. Lastly, and something I am very interested in, is the degree of correction, what is called “apochromatic,” another term that has no standards such as resolution has. The above is what I understand, right or wrong.
We are used to assuming that peak “sharpness” is a couple stops up from wide-open, and this seems to be true for the new Zeiss APOs, but very much less than for most lenses. Then there are many industrial lenses (enlarge, scanners, etc.) that are best (and only best) wide open. Even though they have variable apertures, anything but wide open is degraded.
What I am trying to do is come up with a simple rule of thumb (I may have to write it down) for each lens that works for my close-up photography. I find that once I am satisfied that a lens is “sharp” enough and corrected well enough, then I set those considerations to the side and see what in general limits I can use the lens and not run into the kind of troubles we have with lenses in general.
In my case, after the technical stuff, I begin to consider composition and all of the more personal (and artistic) considerations. I find that I can often shoot f/16 (one-shot) with the Zeiss Otus 55mm and the images looks fine, at least good enough to bring out the “art” in the shot. I am not at all interested in forensics or copy-lenses, per se. All this technical stuff is just the base for artistic improvisation. That is why I do it, aside from (my wife would testify to this) that I just like fine lenses.
My particular take on close-up photography are shots like this, taken with one of the high-end Nikons bodies and the industrial CRT-Nikkor-O, a sharp, but not well-corrected lens that has quirks that I find creative.
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Thanks for commenting. I am not an expert in this testing, obviously. Here is what I understand, so please point out what I am missing.
What passes for “Sharpness” in common reference is not any kind of standard. On the contrary, it is made of several factors, perhaps more. There is sheer resolution, which the various test charts do a good job of defining. Then, as you mention, there is “acutance,” which has to do with the sharpening of edges, and this is what is sometimes called “micro-contrast.” Now, this term micro-contrast and by association acuity has, as you put it, “psychovisual processing,” with seems to be somewhat of a subjective term, personal to some degree. But so are we, subjective and personal beyond measuring.
Not missing anything Michael. I now understand that by micro-contrast you mean sharp edges, which relate to the highest spatial frequencies.
It may be helpful to separate the objective from the subjective in your endeavors, aka the hardware from the software. The idea is to capture the best spatial resolution information possible in the raw data (and measure that) comfortable in the knowledge that with better objective ‘sharpness’ information to start with we are going to achieve better final results after subjective processing.
Alternatively you could start from pleasingly (to you) sharp processed images and work backwards, but that would give you much more arbitrary indications.
So, there is resolution and acutance. Lastly, and something I am very interested in, is the degree of correction, what is called “apochromatic,” another term that has no standards such as resolution has. The above is what I understand, right or wrong.
Sounds pretty good. Another way to look at chromatic and spherical aberrations is that part of a scene's detail is out of focus. This may result in color fringes and reduced contrast. Measurements in the raw data will pick those up if present, resulting in lower performance than from an apochromat.
What I am trying to do is come up with a simple rule of thumb (I may have to write it down) for each lens that works for my close-up photography. I find that once I am satisfied that a lens is “sharp” enough and corrected well enough, then I set those considerations to the side and see what in general limits I can use the lens and not run into the kind of troubles we have with lenses in general.
The question is, how do you define 'sharp enough'? Say it's at least a contrast of 30% at 60 lp/mm on the sensor. You could measure contrast curves for every camera/lens combination of interest at various apertures and use that information to determine your ideal working range.
Lenstip.com did it for 50% contrast loss in the links above and the threshold there could have been, say, 45 lp/mm. That would have suggested that you use the ApoT*135 between f/2 and f/7, so maybe that's too stringent a criterion. Maybe it should be at 38 lp/mm, which would just include f/11. You see where I am going with this. You just need to customize the metric for your intended uses.
My particular take on close-up photography are shots like this, taken with one of the high-end Nikons bodies and the industrial CRT-Nikkor-O, a sharp, but not well-corrected lens that has quirks that I find creative.
Pretty!
Jack
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Hi Jack,
Something that is often stated that some lenses are more tolerant of diffraction than others. The same has been said about multishot, which achieves higher resolution by moving the sensor half a pixel pitch.
I have not really seen any good explanation. My impression is that most lenses are pretty comparable when stopped down to some medium aperture, say f/8. All of my lenses peaked between 1/5.6 and 1/8. I have a busload of lenses, but they are mostly "middle class".
Axial chroma, that I think Michael mentions several times, is a major issue with older lenses, and also on newer ones at large apertures.
Now I plan on getting a 50 MP camera from Sony when and if it may arrive and I am looking for a nice short telephoto without axial chroma at full aperture, so those Zeiss lenses seem quite attractive.
Best regards
Erik
Not missing anything Michael. I now understand that by micro-contrast you mean sharp edges, which relate to the highest spatial frequencies.
It may be helpful to separate the objective from the subjective in your endeavors, aka the hardware from the software. The idea is to capture the best spatial resolution information possible in the raw data (and measure that) comfortable in the knowledge that with better objective ‘sharpness’ information to start with we are going to achieve better final results after subjective processing.
Alternatively you could start from pleasingly (to you) sharp processed images and work backwards, but that would give you much more arbitrary indications.
Sounds pretty good. Another way to look at chromatic and spherical aberrations is that part of a scene's detail is out of focus. This may result in color fringes and reduced contrast. Measurements in the raw data will pick those up if present, resulting in lower performance than from an apochromat.
The question is, how do you define 'sharp enough'? Say it's at least a contrast of 30% at 60 lp/mm on the sensor. You could measure contrast curves for every camera/lens combination of interest at various apertures and use that information to determine your ideal working range.
Lenstip.com did it for 50% contrast loss in the links above and the threshold there could have been, say, 45 lp/mm. That would have suggested that you use the ApoT*135 between f/2 and f/7, so maybe that's too stringent a criterion. Maybe it should be at 38 lp/mm, which would just include f/11. You see where I am going with this. You just need to customize the metric for your intended uses.
Pretty!
Jack
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Some comments to the last two posts.
As for defining “sharp enough,” that means sharp enough for my purposes. There are no set standards I am aware of, except for resolution. With APO lenses, we can ask for correction for Red, Green, Blue (and all other colors), and some, like the El Nikkor APO 105mm enlarger lens that is corrected in the near UV and Near IR spectrum.
Lateral chroma is easier to fix in post than the axial variety.
Back to my original comments.
I have looked at about 4 score of lenses for close-up and macro work. Only with the advent of the new Zeiss APOs have I had any wiggle-room to experiment with not-stacking at all, due to their apparent (to me at least) high level of correction. These lenses open doors for me.
I am appreciative of those of you who are also experimenting with the new Zeiss APO. Please keep sharing what you are finding, especially any folks doing close-up work.
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Axial chroma is easier to fix in post than the lateral variety.
;)
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Something that is often stated that some lenses are more tolerant of diffraction than others. The same has been said about multishot, which achieves higher resolution by moving the sensor half a pixel pitch.
I have not really seen any good explanation.
There are none ;) , leaving multiscan/oversampling for a different discussion. Perhaps the more 'tolerant' lenses are simply those where diffraction, which is relatively easy to partly correct, dominates.
Axial chroma, that I think Michael mentions several times, is a major issue with older lenses, and also on newer ones at large apertures.
Hence their lower MTF values.
Cheers,
Jack
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However, what I really want to know is about what kind of curve the particular lens creates from its widest to its narrowest aperture and how does that curve affect my particular work. That’s the curve I actually use. In other words, is it “sharp” wide-open or does that sharpness start a couple of stops later, and how long is that sharpness maintained? What kind of curve do we have, sharp or gentle?
Hi Michael,
It's a gentle/smooth curve, with usually improving resolution from wide open towards one to two stops from wide open, and then it gets progressively worse due to diffraction as one stops down further. See attached chart for my EF 100mm f/2.8L Macro IS. The metric I used is the Blur radius, and thus also the Sharpening radius that's required for Capture sharpening.
The blur caused by defocus, or Depth of Field, is somewhat similar, but obviously different for each aperture, and for the focus distance. The quality of defocus blur (AKA Bokeh) also depends on the lens corrections, and usually differs between front defocus and rear defocus. I've also attached a chart of that for a different lens at its optimal aperture.
Cheers,
Bart
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Hi Jack,
I thought the main aberration reducing sharpness in the focusing plane on fast lenses used to be spherical aberration. I don't really know how axial chroma affects resolution but it gives a very bad bokeh, even with most modern lenses and I happen to be sensitive for that.
The colour bokeh and my inability to achieve exact focus made me loose interest in large aperture work, but now we have a few lenses with very good correction of all aberrations, including axial chroma, mostly due to new optical glass but also by going for complex construction. Then new Zeiss lenses are example of this.
My Minolta 80-200/2.8 APO lens is pretty sharp at 80mm but suffers from a lot of axial chroma. At longer lengths it also has lateral chroma but that handled well by most raw converters, and is therefore not a great problem normally.
(http://www.magezinepublishing.com/equipment/images/equipment/AF-80200mm-f28-APO-G-915/large/MINAFAPO8020028.gif)
The lens is very sharp at 80mm when stopped down to f/8. At 200 mm the tangential and sagittal lines split, and this is probably caused by lateral chromatic aberration.
There is a definition of apochromatic correction, the lens would be corrected for three wave lengths, but in practice it may mean that secondary spectral aberrations are kept low, or just a marketing term.
Best regards
Erik
There are none ;) , leaving multiscan/oversampling for a different discussion. Perhaps the more 'tolerant' lenses are simply those where diffraction, which is relatively easy to partly correct, dominates.
Hence their lower MTF values.
Cheers,
Jack
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I am looking (hoping) that the next great lens in the Zeiss Otus line will be a highly-corrected wide-angle lens, somewhere around 21mm-24mm or so. This would be a very difficult lens to produce I am told, but one that I very much could use.
Of course, I have written to the Zeiss lens designers a number of times asking for a very highly-corrected macro lens in the Otus line, but have been told that they would have to sell 10,000 of them to make any money. Perhaps the success of the Otus line will change their view. If I have an Otus wide-angle and macro lens, that is all I need. I don’t do much telephoto photo work. I could sell off scores of lenses sitting around here getting no use.
Since I that the tree Zeiss APOs, they (and the Voigtlander 125mm f/2/5 APO-Lanthar) are all I use, except for some lenses for family portraits.
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Hi Jack,
I thought the main aberration reducing sharpness in the focusing plane on fast lenses used to be spherical aberration. I don't really know how axial chroma affects resolution but it gives a very bad bokeh, even with most modern lenses and I happen to be sensitive for that.
The colour bokeh and my inability to achieve exact focus made me loose interest in large aperture work, but now we have a few lenses with very good correction of all aberrations, including axial chroma, mostly due to new optical glass but also by going for complex construction. Then new Zeiss lenses are example of this.
My Minolta 80-200/2.8 APO lens is pretty sharp at 80mm but suffers from a lot of axial chroma. At longer lengths it also has lateral chroma but that handled well by most raw converters, and is therefore not a great problem normally. The lens is very sharp at 80mm when stopped down to f/8. At 200 mm the tangential and sagittal lines split, and this is probably caused by lateral chromatic aberration.
There is a definition of apochromatic correction, the lens would be corrected for three wave lengths, but in practice it may mean that secondary spectral aberrations are kept low, or just a marketing term.
Best regards
Erik
Excellent, thank you Erik.
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It's a gentle/smooth curve, with usually improving resolution from wide open towards one to two stops from wide open, and then it gets progressively worse due to diffraction as one stops down further. See attached chart for my EF 100mm f/2.8L Macro IS. The metric I used is the Blur radius, and thus also the Sharpening radius that's required for Capture sharpening.
Nice chart, Bart. May I ask how you determined the blur radius readings? And did you do it off a raw single channel or demosaiced data?
The blur caused by defocus, or Depth of Field, is somewhat similar, but obviously different for each aperture, and for the focus distance.
Right. The main difference in my mind being that in theory one cannot undo defocus (= also axial spherical and chromatic aberrations in this context) in post while, always in theory, one can undo diffraction. Hence if one does not need the shallower DOF one would most likely be better off shooting the D3X+ApoT*135 at around f/7 and your 1DsIII+EF100 at f/8 instead of wide open: same linear spatial resolution, fewer aberrations, most likely 'sharper' result once properly rendered.
Jack
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There is a definition of apochromatic correction, the lens would be corrected for three wave lengths, but in practice it may mean that secondary spectral aberrations are kept low, or just a marketing term.
Quite true. The usual definition of apochromatic is that correction of longitudinal chromatic aberration is performed for three colors and spherical aberration is corrected for two colors, as discussed in this Zeiss article (http://www.dantestella.com/zeiss/achromat.html) for color correction.
However, the apochromat designation is often used for lenses that do not meet this designation, but merely keep secondary color "low", and the meaning of "low" is not defined.
The Zeiss article states, "In most cases, the third zero is not required in practice, but it is sufficient to reduce the secondary spectrum to meet the respective requirements. If this is successful, such a lens could be termed 'Apo lens'"
Sigma uses the Apo designation for many relatively inexpensive lenses, and I think this is mainly a marketing ploy. Acceptably low secondary color is subjective. Nikon uses what they call ED glass to help control chromatic aberration, but they do not designate these lenses as Apo.
Cheers,
Bill
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On an artistic note, I really like the blur and colors in Michael's plant (name of which escapes me) with the water drop a few posts above.
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Yes!
On an artistic note, I really like the blur and colors in Michael's plant (name of which escapes me) with the water drop a few posts above.
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It is just a standard Calla Lily. Here is another taken with the same lens, the industrial CRT Nikkor-O.
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your second lily shot is beautiful
it might even qualify for Michael's erotica
regards
Larry.
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Nice chart, Bart. May I ask how you determined the blur radius readings? And did you do it off a raw single channel or demosaiced data?
Hi Jack,
I produce such charts for my lenses with the procedure outlined here (http://bvdwolf.home.xs4all.nl/main/foto/psf/SlantedEdge.html).
For additional precision, I take 10 readings (evaluate, 1 line down and approx. 10 pixels to the right, repeat) along the slanted edge, in order to avoid phase errors that the approx. 10x oversampling may produce. This is especially important to eliminate the effects of aliasing. See the attached overview of such a data collection effort. I base the readings on the only White balanced and Raw converted (unsharpened) image (no geometric distortion correction, since that will blur parts of the image, but CA correction is allowed because it improves sharpness). That allows to incorporate the effect that a Raw converter has on the resolution of base image for Capture sharpening. Expressing these results in Gaussian Sigma blur units makes it easier to relate to practical consequences than a LW/PH metric would offer.
Right. The main difference in my mind being that in theory one cannot undo defocus (= also axial spherical and chromatic aberrations in this context) in post while, always in theory, one can undo diffraction.
Well, it depends. Defocus can be reversed with deconvolution, but it depends on the specifics (e.g. noise and blur quality) how successful one will be. The Chromatic errors that will diminish with narrower apertures may be harder to correct at wider apertures. But for focus stacking one can choose the optimum aperture, or allow a certain (known) level of diffraction blur.
Hence if one does not need the shallower DOF one would most likely be better off shooting the D3X+ApoT*135 at around f/7 and your 1DsIII+EF100 at f/8 instead of wide open: same linear spatial resolution, fewer aberrations, most likely 'sharper' result once properly rendered.
Yes, one can easily see the trade-offs with a chart like that. Residual aberrations with a wider aperture may produce the same amount of blur as stopping down beyond the optimum, but the cause of the blur may be harder to correct at the wide end than at the diffracted end. So if a certain level of blur is allowed, I'd err on the side of diffraction.
Cheers,
Bart
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Hi Jack,
I produce such charts for my lenses with the procedure outlined here (http://bvdwolf.home.xs4all.nl/main/foto/psf/SlantedEdge.html).
For additional precision, I take 10 readings (evaluate, 1 line down and approx. 10 pixels to the right, repeat) along the slanted edge, in order to avoid phase errors that the approx. 10x oversampling may produce. This is especially important to eliminate the effects of aliasing. See the attached overview of such a data collection effort. I base the readings on the only White balanced and Raw converted (unsharpened) image (no geometric distortion correction, since that will blur parts of the image, but CA correction is allowed because it improves sharpness). That allows to incorporate the effect that a Raw converter has on the resolution of base image for Capture sharpening. Expressing these results in Gaussian Sigma blur units makes it easier to relate to practical consequences than a LW/PH metric would offer.
Well, it depends. Defocus can be reversed with deconvolution, but it depends on the specifics (e.g. noise and blur quality) how successful one will be. The Chromatic errors that will diminish with narrower apertures may be harder to correct at wider apertures. But for focus stacking one can choose the optimum aperture, or allow a certain (known) level of diffraction blur.
Yes, one can easily see the trade-offs with a chart like that. Residual aberrations with a wider aperture may produce the same amount of blur as stopping down beyond the optimum, but the cause of the blur may be harder to correct at the wide end than at the diffracted end. So if a certain level of blur is allowed, I'd err on the side of diffraction.
Cheers,
Bart
Very nice, Bart. Out of curiosity, what's your criterion for the edge of the blur circle?
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So, there is resolution and acutance.
On acutance (http://www.strollswithmydog.com/deconvolution-vs-usm-capture-sharpening/).
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luscious looking cyclamen! Very nice indeed.
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I am working on an illustrated album of what I would call my “kit,” the lenses I regularly like to use, not that I carry them all around in a bag. It is funny. Once I get up into the stratosphere of highly-corrected APO lenses, there are still decisions to make as to which lenses I admire most. Many of these highly-corrected APO lenses are very flat, what I sometimes call “forensic” or “copy” lenses. Nothing wrong with that; in fact, that is what I have been striving for. Then I kind of look a gift-horse in the face and say, now, which of you lenses have character?
In other words, a lens can be highly corrected (like the Coastal Optics APO Macro f/4) and still not have character, which is a little unfair because first I want all aberrations, etc. (character) taken out of the lens, and then I rebuke it for just that. After all, I pay for expensive forensic-style lenses not to have any character whatsoever, to be perfectly transparent, and then I demand character. There is a disconnect here.
One of the lenses that is most highly corrected, not only in the visible spectrum, but in the near ultra-violet and near-infrared as well is the enlarger lens APO El Nikkor 105mm f/5.6, not to be confused with the same El Nikkor which is not APO.
This indeed is an incredible lens, very highly corrected, but one that also has all kinds of character. Go figure. Here is a stacked image (Zerene Stacker) I took with this lens probably taken on the Nikon D800E. You can see the almost 3D quality. It still needs some retouching, I know. Do you see what I am getting at here about character?
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In other words, a lens can be highly corrected (like the Coastal Optics APO Macro f/4) and still not have character, which is a little unfair because first I want all aberrations, etc. (character) taken out of the lens, and then I rebuke it for just that. After all, I pay for expensive forensic-style lenses not to have any character whatsoever, to be perfectly transparent, and then I demand character. There is a disconnect here.
Reminds me of discussions (http://blog.kasson.com/?p=9037)around high end audio equipment.
This indeed is an incredible lens, very highly corrected, but one that also has all kinds of character. Go figure. Here is a stacked image (Zerene Stacker) I took with this lens probably taken on the Nikon D800E. You can see the almost 3D quality. It still needs some retouching, I know. Do you see what I am getting at here about character?
Beautiful. Although, not to diminish the contribution of your outstanding lenses, I have a feeling that what we are perceiving more of is the character of your virtuoso performance in Zerene Stacker and PP ;)
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Reminds me of discussions (http://blog.kasson.com/?p=9037)around high end audio equipment.
Beautiful. Although, not to diminish the contribution of your outstanding lenses, I have a feeling that what we are perceiving more of is the character of your virtuoso performance in Zerene Stacker and PP ;)
I wish, but lens experts agree with my view of the APO El Nikkor 105mm. For example, here is a quote from macro-lens expert Klaus D. Schmitt, author of the very fine Macro Lens Collection Database, that was just exchanged yesterday. We were discussing the APO El Nikkors, in particular the 105mm’s big brother, the 210mm APO El Nikor, which weighs a couple of pounds!
http://www.macrolenses.de/
He writes:
“Yes, the APO El 105mm is indeed a fantastic lens. I consulted a friend doing macro and close-up shots of sparkling jewelry, and he’s blown away about the results he now gets. Very different than the Printing Nikkors, the latter being fantastic lenses themselves, but quite different in rendering, the AENAPO105 is more “alive,” hard to express indeed.”
So, I do believe that even very flat lenses that, because they are highly corrected, might be similarly transparent as far as character go, definitely can have real character INDEPENDENT of the skill of the photographer. Here is another shot, this one taken with the Nikon D810 and the APO El Nikkor 105mm enlarger lens, mounted on the PB-4 Bellows. Stacked with Zerene Stacker.
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Very nice, Bart. Out of curiosity, what's your criterion for the edge of the blur circle? And (bonus rhetorical question) what's the PSF of defocus?
Jack, I'll try and not hijack Michael's thread, but the question is relevant.
The Point spread functions of lens aberrations, defocus, and diffraction have different shapes. Therefore, if we were to look at them in isolation (e.g. for construction of a mathematical model), we would probably need something shaped like a (perhaps elliptical) Gaussian PSF, a disc shaped PSF, and an Airy pattern shaped PSF. We could then weigh in their contribution proportionally, although that will be computationally expensive because it requires integration to allow of positive and negative contributions to the overall MTF. And then there is the influence of the sensel aperture, and the demosaicing, and gamma pre-compensation for display.
As it happens to turn out, the composite of all that, looks almost perfectly like a Gaussian shaped PSF. When adding different PSFs, one very quickly get a Gaussian average, and the empirical results prove that to be true. An edge transition, like that of a Slanted edge target, takes on the shape of a normal Cumulative Distribution Function (CDF). We can therefore fit a CDF model to the edge transition, and the blur sigma is the result.
This blur sigma does not have a fixed diameter because a Gaussian has an infinite extent, so we will have to draw an arbitrary boundary at which we declare it a no longer acceptable amount of blur, e.g. the Circle of Confusion. One possible metric is the 10 to 90% rise of the edge profile which can be expressed in a radial distance in pixels, and for focus stacking one could define one's personal COC and thus pick an aperture that satisfies that condition. Since we are using a Gaussian shaped PSF model, it's easy to translate the 'blur sigma' to a 10-90% edge profile rise, and restate the y-axis values of the chart. A 10-90% edge profile rise (the difference between the 10th and 90th percentile points) of a CDF is approx. equal to 2.5631 x sigma, in pixels.
So, if we find a 2 pixel diameter to be our limit of acceptable blur on my above tested lens, then we should draw the limit at a maximum blur of 2 / 2.5631= 0.78 sigma, or f/6.3, and choose our stacking distance interval accordingly. It also shows that at optimum performance, at aperture f/4.5 or f/5.0, we will have a COC of 0.72 x 2.5631 =1.85 pixels for that specific lens and camera and converter combination. That suggests we need deconvolution sharpening for restoration of full resolution if we want uncompromised resolution at the pixel level. In the case of Michael who seems to do reduced size output, we can scale those requirements up accordingly.
Cheers,
Bart
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Bart: You are not high jacking the thread. I find your blogs amazing, even confounding. Of course, I can’t claim to understand it all. I would have to go and study, and for a long time, to do that. I am glad such explanations exist.
When you say “Michael who seems to do reduced size output,” what does that mean in layman terms? It would help folks like me if, after such an exposition, at the bottom, you had a “conclusion,” where you summed up for non-technical folks like myself, what you write so eloquently is actually all about.
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Jack, I'll try and not hijack Michael's thread, but the question is relevant.
The Point spread functions of lens aberrations, defocus, and diffraction have different shapes. Therefore, if we were to look at them in isolation (e.g. for construction of a mathematical model), we would probably need something shaped like a (perhaps elliptical) Gaussian PSF, a disc shaped PSF, and an Airy pattern shaped PSF. We could then weigh in their contribution proportionally, although that will be computationally expensive because it requires integration to allow of positive and negative contributions to the overall MTF. And then there is the influence of the sensel aperture, and the demosaicing, and gamma pre-compensation for display.
As it happens to turn out, the composite of all that, looks almost perfectly like a Gaussian shaped PSF. When adding different PSFs, one very quickly get a Gaussian average, and the empirical results prove that to be true. An edge transition, like that of a Slanted edge target, takes on the shape of a normal Cumulative Distribution Function (CDF). We can therefore fit a CDF model to the edge transition, and the blur sigma is the result.
This blur sigma does not have a fixed diameter because a Gaussian has an infinite extent, so we will have to draw an arbitrary boundary at which we declare it a no longer acceptable amount of blur, e.g. the Circle of Confusion. One possible metric is the 10 to 90% rise of the edge profile which can be expressed in a radial distance in pixels, and for focus stacking one could define one's personal COC and thus pick an aperture that satisfies that condition. Since we are using a Gaussian shaped PSF model, it's easy to translate the 'blur sigma' to a 10-90% edge profile rise, and restate the y-axis values of the chart. A 10-90% edge profile rise (the difference between the 10th and 90th percentile points) of a CDF is approx. equal to 2.5631 x sigma, in pixels.
So, if we find a 2 pixel diameter to be our limit of acceptable blur on my above tested lens, then we should draw the limit at a maximum blur of 2 / 2.5631= 0.78 sigma, or f/6.3, and choose our stacking distance interval accordingly. It also shows that at optimum performance, at aperture f/4.5 or f/5.0, we will have a COC of 0.72 x 2.5631 =1.85 pixels for that specific lens and camera and converter combination. That suggests we need deconvolution sharpening for restoration of full resolution if we want uncompromised resolution at the pixel level. In the case of Michael who seems to do reduced size output, we can scale those requirements up accordingly.
Cheers,
Bart
Excellent explanation, thank you Bart.
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Bart: You are not high jacking the thread. I find your blogs amazing, even confounding. Of course, I can’t claim to understand it all. I would have to go and study, and for a long time, to do that. I am glad such explanations exist.
When you say “Michael who seems to do reduced size output,” what does that mean in layman terms? It would help folks like me if, after such an exposition, at the bottom, you had a “conclusion,” where you summed up for non-technical folks like myself, what you write so eloquently is actually all about.
Michael, from what I've seen you mostly publish very nice images in PDF documents, and on the internet. I do not know if you also print large format output. If you only need reduced size output, then you can relax the resolution requirements a lot.
When one technically determines the performance of e.g. your Otus lenses at various apertures, it then becomes much easier to draw lines as to what's still discernible at certain output sizes, and what's not. When, as in my example, a 2 pixel blur is used but you only output to 50% of the original capture size, there will be no blur in the resulting image. In a 25% of maximum output size, a 4 pixel blur (blur sigma 1.56, or in my lens case f/32) should not be much of a problem.
We do need to also account for magnification factor or focus distance, so we get a narrower effective aperture the closer we shoot. The formula for that is; Effective aperture = Aperture * (1 + (imageMagnification / pupilFactor)), so we need to divide the aperture we read the chart with by (1 + (imageMagnification / pupilFactor)).
That's the technical approach to it, but one can of course also use a more empirical approach and guess that e.g. f/16 will produce acceptable results when output size doesn't need to be huge.
Cheers,
Bart
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Bart, I don’t print photos and never have, although I am actually thinking about doing so for a show. All of my knowledge is empirical, just from experience. I would not want to cut any corners for reduced-size output because the main reason I do this is the “see” the results in as perfect a form as I can on-screen. Of course, what I see from a 36MP shot on the D810 in Photoshop or Lightroom (I use both) cannot be represented fairly here at sRGB format for the web. It is a sad approximation IMO. This last piece you wrote is more understandable for me. Thanks!
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You should see the radiology monitors. These have replaced film for many years, and the highest grade mammography/breast ultrasound monitors now do both B and W and color images at 8 MP. I have seen a lot of film mammograms over the years, and the new digital monitors do seem to be just as good, but with the added benefit of being able to alter the "development" on the fly.
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Thank you so much for sharing your immense knowledge. One thing I noticed in your comparison with the Two Dollar bills (VH8AA): The left side of the bill is quite a bit greenish @ f2, and pretty much neutral @ f16. Did anything change (light, processing etc) between the 2 shots, or does it have something to do with diffraction?
You do beautiful work!
Markus