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Diffraction in the real world


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#1 Alex_Mustard

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Posted 07 July 2008 - 12:41 AM

There have been some very useful discussions on Wetpixel recently comparing the relative benefits of full frame and cropped sensors.

One issue that comes up frequently in discussions is that of diffraction, an issue that gets worse with small photosites (high megapixels, small sensors). Low pixel density FF sensors (such as the 5D, D3, D700) are the best to combat the loss of sharpness due to diffraction at small apertures. And the theoretical limits of systems are often quoted.

For my own camera, an aged D2X (with high pixel density), a diffraction limit of F11 is cited (to quote Thom Hogan "On a D2x it seems to be around f/11. Up to f/11 and the D2x's acuity appears as you'd expect. Beyond f/11 and you'll likely start saying to yourself that the results don't look like you're getting the full impact of more DOF.").

However, this is not what I see in my underwater images. And my only conclusion is that by the time you are shooting through port glass, murky water etc, diffraction is not the biggest of your worries. Even in the lenses sweet spot (F8-F11) you loose acuity for these other factors. So stopping down the issues of diffraction are not so significant?

On Friday I was diving in Plymouth, UK. It was horrible weather, strong southerly 20Kt wind. Driving rain. Yum, yum. So we were stuck hiding being the breakwater, diving there! By the end of my third dive, bored with a lack of cooperative subjects, I decided to shoot a few frames with the intention of looking for diffraction. Annoyingly, now, I did not shoot a range of apertures, instead I just wanted to look at what my camera could produce at F25, when diffraction should be crippling my D2X beyond F11.

So here is one of the shots. Not great as it was on kelp, in the shallows as I was being moved around by waves and current. Nudi on kelp bryozoan:
Posted Image

Obviously everything looks sharp at web rez. So I prepared the file as I normally would for output. My standard image delivery size is 45x30cm (c. 18"x12") @ 300DPI, which is just under 19 megapixels. With a D2X (12MP) this requires up-rezzing (which is why I want a new camera), which I tend to do in one step in Lightroom, then re-sharpen, lightly in Photoshop (leaving final sharpening to client when image repro size is decided).

Here are two large 100% crops from the above 19MP up-rezzed image. For me the sharpness here is perfectly acceptable and without a pixel peeping comparison I would not be able to tell that from a F8 shot from the same 3.5 year old camera. There may be a slight lack of sharpness compared with F8-11, but for me this is not going to affect print quality and is not a significant trade off for the extra depth of field. Generally I shoot macro stuff between F16 and F22, but I think I will start going up more beyond F22 from now on.

Posted Image

Posted Image

It is not a perfect controlled conditions comparison, and I did it for my own purposes, hence the up-rezzing to my standard delivery size. But then when is underwater photography controlled conditions? Rarely, here in the UK! What do people think?

Alex

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#2 dhaas

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Posted 07 July 2008 - 05:04 AM

Thank goodness our plucky Brit exceptional underwater photographer brings these measurbating discussions some clarity :lol:

I have give up arguing with people who want to debate specifications and when I usually ask politely to show me where visually you can see said differences most commentators fold like a cheap tent :lol:

I got a headache reading the full frame versus APS-C discussion.....I'm sure the Nikon D700 will produce wonderful phtoos when used with decent glass. But for 99% of us underwater shooters Alex's examples show theory and real world image results aren't bad at all!

For the limited macro shooting I do, not tele-macro like many here, I always stopped around f11 due to the supposed diffraction problems on APS-C sensors. Now I'll revisit f16 and maybe even f22 and see if the results will be like Alex found on his D2X camera.

Thanks for taking the time to test this theory!

Kudos to the master :lol:

dhaas

Edited by dhaas, 07 July 2008 - 05:04 AM.

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#3 Paul Kay

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Posted 07 July 2008 - 05:23 AM

Ahh. The REAL world!

I shoot at f/16/22/+ on both 60 and 100 macros on FF Canon. I used to shoot at wider apertures (well f/11) and still do if I want background delineation, but in practice there are so many variables (not sure of the effect of uprezzing on MTF but I'll bet its severe!) that I've found differentiating between f/16 and f/22 isn't as easy as theory makes it sound.

However, I CAN see a difference in images shot at f/11/16/22 on my Leica M (1.3 crop factor) when I'm shooting on a tripod using the self timer, etc., etc.. There is A difference but even here its not massive, just a gentle and extremely slight softening of detail. But this is using stunningly good optics in an extremely controlled way.

Looks like you've got better vis in the south-west than we have in north Wales, Alex.
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#4 scorpio_fish

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Posted 07 July 2008 - 05:40 AM

I guess one of us needs to get off our lazy, uh... butt and do some testing. But I must clear that I am not volunteering.

My perception is that as I stop down I lose a little quality, but gain depth of field. Once I pass f/22, the gain in DOF doesn't offset the diffraction loss. But I could be very wrong.
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#5 james

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Posted 07 July 2008 - 06:17 AM

Thanks for posting a great example Alex. The effect will be different depending on the camera's AA filter, the water clarity, port glass, etc.

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#6 cor

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Posted 07 July 2008 - 06:33 AM

One can only wish for the end of measurbating. Im very happy with my D2x, and will not hurt its feelings by calling it 'aged'!
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#7 Paul Kay

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Posted 07 July 2008 - 06:46 AM

The effect will be different depending on the camera's AA filter


James I should have said that the Leica M8 has no AA filter and the results of diffraction do appear to be visible (if you really look carefully at comparative images). Underwater, I'd be hard pressed to say that an image taken at f/22 was 'not acceptable', which is perhaps what the discussion should really be about rather than being a 'comparison', and equally I'd be hard presed to decide what factors really degrade an image if there wasn't another actual comparative image at a different aperture to see.

We have exceptionally good gear today. To be blunt we need to try using it to its limit, something which is not always as easy as it may appear.

Cor, I still have D1X (which is a really 'aged' camera) images that I'm happy with - funny thing is that I can't remember anyone worrying about diffraction when the D1X was new. We now have much 'better' cameras and worry about diffraction. Its a funny world sometimes......
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#8 Alex_Mustard

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Posted 07 July 2008 - 06:57 AM

Thanks for the feedback.

I think theoretical discussions are very valuable. I think that they can guide a lot of decisions about gear selection. Certainly theoretical discussions here on Wetpixel have taught (and continue to teach) me lots.

I think that the only downside is when theoretic rules are not understood. The fact that the D2X has a diffraction limit of F11, does not mean that all photography ends at this point. This is where real world examples from our specific area of underwater photography are invaluable.

Of course these examples are nothing unusual. 1000s of underwater photographers have shot macro on DX cameras above F22. And are very happy with the results.

To me these images are acceptably sharp (for my camera, uprezzed to 19MP and viewed at 100%). But that is today. Maybe in the future with greater and greater cameras I will be embarrassed by them.

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#9 craig

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Posted 07 July 2008 - 08:26 AM

I have also suggested that ports and water contribute to softening starting back in the Kodak 14n days and believe that a high resolution camera could do without an AA filter underwater. The Kodak did not have one. I have pursued removing the AA filter on my bodies but the industry for doing that is focused only on Canon.

In order to do a meaningful test you would need a range of apertures, good shooting conditions, and a subject with plenty of high resolution content (preferably with red details). We don't have that here. I once shot at these apertures and did these tests many years ago. I wonder if this image would look just as good if shot with a D100.

Saying that a D2X is limited to f/11 is arbitrary. Such an aperture is no more than the point where diffraction reaches a certain level relative to pixel pitch. It does not say that image quality significantly degrades with smaller apertures only that it begins to. Trading slight loss in resolution for more DOF is sometimes a good one though. It seems too few people understand this.

Nevertheless, it does NOT matter what a person's perspective on that issue is as long as they provide equal perspective to other potential systems. The consistent failing of virtually everyone is their unwillingness to provide the same standards when comparing different systems. It is easy to understand that meaningful, theoretical evaluations can be made of different systems and understanding be obtained without regard to analysis such as this.

Now, if the claim is that DX is suitable above f/22 then I would say the following: (1) DX will always be capable of more DOF than FX with the same lenses, (2) there will never be a need for more than 12MP or even that much resolution, and (3) print sizes are clearly limited. I say that because such diffraction limits overall resolution to what is already achievable with the D2X/D300 today. I would also say that anyone who believes that would question why lens designers of 35mm stopped at f/32 with film since their tolerance of diffraction is higher than what engineers assume.

The whole point of larger sensors and higher pixel counts ultimately is more resolving power and greater quality at bigger enlargement sizes. Understanding that, it is easy to see how many photographers truly see no need for more pixel count, FX, or discussions of diffraction limits. They are not wrong, but if they assert that those improvements make no difference they are mistaken.

Finally, I feel terms like "pixel peep" and "measurebate" are words used by those afraid of the result to denigrate those that aren't. These words are attributable to the Reichman's of this industry and are nothing more than ad hominem arguments. If by looking at an equation I can understand the result I am neither guilty of studying a pixel too closely (whatever that is) nor engaging in some sort of sordid sexual activity with my photo gear. Fortunately, Wetpixel doesn't stoop to that level.
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#10 Paul Kay

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Posted 07 July 2008 - 09:37 AM

Craig - To take a practical approach, I'd say that you are about right about the 12MPixels being a good 'limit' for "35mm" styled cameras (that is, cameras the design of which follows those designed for 35mm film). I'd also suggest that whilst there are theoretical, mathematically derived parameters which we can use to determine specifics about our equipment, what fails to be taken into account are the more subjective, viewer's requirements.

IMHO many photographers train themselves to visually appreciate nuances of image detail and acceptability (subconciously). None photographers on the other hand are more interested in image content than image technicality. So an interesting macro image which has been taken as carefully as possible at f/22 of an fascinating subject is likely to be very acceptable to the vast majority of viewers.

I'm realising more and more that extracting every last nuance of detail and tonality out of a modern dSLR is quite a task, especially allied to creating an interesting and perhaps even aesthetically pleasing image. When I scan the web, I see all sorts of discussions about which camera is better, which lenses to use, etc., etc.. But I sometimes think that photographers are in danger of forgetting that at the end of the day it is the image which is important. Alex's post shows that sometimes its worth checking on perceived and accepted wisdom because there is more to an image than just mathematics. When I look at 'great' images taken in the past I am often bemused to realise how many are technically imperfect - not always of course, but many are - and technical imperfection is overlooked when image content is sufficiently strong. I'm not suggesting that we shouldn't shoot to the limits of technical perfection as an aimpoint, but its still the image content which counts most.
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#11 dhaas

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Posted 07 July 2008 - 10:01 AM

Not to argue with Craig, BUT........

Paul has said it best, and I echo his points on only discussing technical details while totally ignoring the impact of an image. This sometimes leans the discussion to the extremes instead of how good is good enough?

For some, it'll never be enough. On paper, in theory or an individual perception of their images. For many (if not most UW shooters) we're there already in terms of super image making gear.

I would guess there are likely less than 10 people TOTAL on Wetpixel who want to discuss specs in super detail versus image making is all I'm saying. Which is fine.....

I just abhor the thinking that technical details is all that will make a great photo......As Paul said many, many great images could be deemed "technically deficient". To me the visual impact is what any photography is all about.....Give me grain, implied movement sometimes, etc.

I will even go so far as to state very few underwater photographs shown today from digital cameras haven't been shot on fine grain slide film already. It's just easier to get them and share them visually with digital. I'm guilty of mentally duplicating these sub conscious images myself on trips versus pushing visual ideas.

So shoot whatever you desire and to what level perceived or shown in print, on the web or wherever.

Should be plenty of room for varying opinions :lol:

Back to lurking........

dhaas

Edited by dhaas, 07 July 2008 - 03:29 PM.

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#12 vazuw

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Posted 07 July 2008 - 10:45 AM

Nice show alex. That helps make me comfortable in keeping my d2x underwater for a while longer. There is an old saying: the proof is in the pudding

#13 craig

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Posted 07 July 2008 - 11:19 AM

I agree with you, Paul. It is not my intent to argue whether f/25 on DX is acceptable or to suggest that diffraction issues are more important than the image itself. As a side note, the diffraction limit on the D100 was f/27. When I started I fixed my aperture to that for every shot. The D100 was soft anyway so there was little reward in opening up.

I try my best to discuss diffraction in relative terms so as not to endorse any view regarding so-called aperture limits. I'm personally fine with shooting about these "limits" of f/9 - f/13 that others in other forums like to talk about. Those low numbers don't come from me.

Paul has said it best, and I echo his points on only discussing technical details while totally ignoring the impact of an image. ... I just abhor the thinking that technical details is all that will make a great photo.

I would hate to think that anyone believes that.

Unless you find shooting above f/22 with DX is important (and not all macro lenses even allow that), I believe that differences in macro DOF between formats aren't something that you should even concern yourself with. If you do, you may be interested in these old wetpixel threads:

http://wetpixel.com/...wtopic=2156&hl= and http://wetpixel.com/...wtopic=2918&hl=

In these threads I worked though the DOF equations to actually compare different formats using identical field of view and matching "spatial resolution". The results showed that at low magnifications they were similar and at high magnifications larger formats offered modest advantages. This benefit somewhat offsets the "1 stop" advantage DX has with available lenses at the highest f-numbers.

For those that accept f/22 as the accepted limit, I am curious as to why. Do you believe in f/22 because that's how it used to be with film or because f/22 is the DX equivalent of f/32? Perhaps another reason?
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#14 DiveMasterMark

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Posted 07 July 2008 - 11:36 AM

I have to agree with Paul (and dhaas) in that too much attention can be placed on the technical capabilities and never-ending gear race.

I have a long way to go with regards to mastering the equipment I have now. There is much more potential to be wrung out of my gear. (Gee, I hope I just didn't jinx myself -- I've had enough flooded equipment!)

It's the image and it's ability to grab the viewer's attention.

It reminds me of some shrimp boats at a dock I shot many years ago on out-of-date Kodachrome, on a lousy, cloudy day. I stuck an orange filter on the lens just for fun. It was my best-selling picture that year.
It's all a matter of perspective...

#15 loftus

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Posted 07 July 2008 - 12:27 PM

I have not seen anywhere in these discussions, anyone implying having superior equipment makes one a better photographer. I think it is quite normal for any artisan, concerned about the quality of his craft (professional or amateur) to want to understand his tools and to be interested in using the best tools he can afford for the task. So I am all for these discussions no matter how detailed, it's up to each individual to take what he wants from them, or to choose whether he even wants to participate.

Edited by loftus, 07 July 2008 - 12:27 PM.

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#16 james

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Posted 07 July 2008 - 12:41 PM

To get the best out of an image, you have to get the best out of yourself as a photographer and you have to get the best out of your equipment.

Ignoring either of those two factors would mean you can do better - something we all strive for.

If you think that using the gear properly and spending some additional $$$ to make a better image is not worth the modest improvement you can see then just don't worry about it.

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#17 Trevor Rees

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Posted 07 July 2008 - 02:01 PM

This may be the same slug, also from Plymouth in the Uk, a few weeks ago.

DSC_8615c2r.jpg
DSC_8615c3r.jpg

I'm not sure posting these adds to the discussion. However, I've used f20 @ 1/200, ISO100 on a D80 with a 105 +4T diopter. It's all dissapointingly soft to me and I really wonder sometimes why I bother shooting these wretched things. I'm really not sure how to get a sharper image.

I usually aim for a least f22 for max depth of field this type of thing but hold back from f32 because of a supposed loss in optical quality.
With this shot I might have focussed too far forward but I'm not sure.
Perhaps at least I can discount diffraction related softness as the main problem?

#18 dhaas

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Posted 07 July 2008 - 03:36 PM

James,

Not to argue a point, but I think Alex's original post was challenging supposed "known" or "calculated" diffraction limits and his experience in going against the grain.....

All I was trying to point out is wringing one's hands over all these technical details will not necessarily produce a stellar image. Hell, as I posted maybe 2 years ago a Pulitzer Prize winner shot his photo with an Olympus C5050! And I doubt that anyone would debate that is not a major recognition of one's photography.

As many Wetpixel shooters push into the extremes of tele-macro, I doubt you'll see super magnified totally tack sharp images. Unless they're shot on a flat plane.....Even at f22 and higher you're down to miniscule increments of DOF gain.

I think your shots are fine with an isolated amount of focus Trevor :lol:

Just thinking out loud.....

Hmm...........

dhaas
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#19 craig

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Posted 07 July 2008 - 04:24 PM

David, I've never heard of a single photographer who's work suffered because he learned too much about how his equipment worked. There are plenty who's work could benefit from learning more.

I don't think anyone is wringing their hands over this. If Alex posts a series of images that show that his result benefits from shooting f/25 then I will be more convinced. As I said earlier, I'm not certain the example provided is any better than it would have been with a D100. A major difference between the two cameras is resolution so it's relevant to understand what is necessary to take full advantage.

Of course, when faced with a shot that you can't get without compromising IQ you take what you can get. That's a choice we have to make all the time and that gets to the heart of what Alex is saying. A high resolution camera forces us to compromise more often but at least it is capable of greater IQ when the opportunity arises. There is nothing wrong with shooting high f-numbers contrary to what some theorist says is above your limit. The alternative is no shot at all.

Finally, I don't believe Alex is going against the grain here at all. How far you will stop down is a matter of degree and I doubt that many actually limit themselves to f/11. If f/11 is the conventional wisdom them I really will have learned something and will be greatly disappointed.
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#20 DiveMasterMark

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Posted 07 July 2008 - 07:00 PM

I feel my comments may have been misconstrued. I in no way meant to denigrate what the dear doctor said here, but was attempting to agree with the point that observing the rules was not as important as the impact of the image.

If we all followed the perceived rules as law, the world would still be flat, and many of the outstanding underwater vistas would remain unseen.

Sheepishly retreating to the sidelines, where I might listen and learn more from the masters...
It's all a matter of perspective...