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Author Topic: Diffraction limitation in 35mm lenses.  (Read 19140 times)
Ray
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« on: January 07, 2006, 07:06:24 AM »
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First let me say I think the new site is a vast improvemnt over the old and I for one will be renewing my subscription to the LL video journal which pays for this site.

As Jonathan Wienke frequently mentioned, there's no free lunch, and I don't expect to use valuable bandwidth on someone elses site for free.

The facility of now being able to download images directly from one's HD is a major upgrade which is most welcome.

With this in mind, I'm hoping to demonstrate a curious phenomenon which basically knocked my socks off.

Most of us are not scientists, and even if we were we'd have a difficult time making sense of all the contradictory data regarding lens performance, due to lack of reliable, up to date information in the public domain.

I recently bought a Canon 5D and the 24-105mm zoom lens, which I believe is a great combination. During the past 6 weeks or so, I've taken about 4,000 images in Nepal, Tailand and Cambodia using mostly the 24-105 lens, but occasionally the Sigma 15-30.

I'm now processing these images and in the process of doing so I noticed that images I'd taken at f16 were no less sharp than the same image taken at f11, in any respect, but the f16 image exhibited greater DoF.

This actually flies in the face of common  understanding that f16 is softer than f11 and f11 is very slightly softer than f8.

The following images demonstrate that this is not necessarily true.

The image that brought this to my attention was a shot of the Bayon temple near Angkor Wat. I took 2 shots, one at f11 and one at f16. The shutter speed was the same (100th) in both instances (which of course meant I bumped up the ISO to 200 for the f16 shot).

Examining these 2 shots of the same scene, having focussed on the same spot in each exposure (guesstimating the hyperfocal distance as I always do), I find the f16 shot is sharper in all respects. Geez! That can't be right! What have I done! F16 sharper than f11, or at worst equally sharp!!!

So here's the proof. I should also add that the following images have had zero sharpening and zero luminance smoothing, I've also adjusted the shadows and contrast sliders to ensure no clipping of highlights and shadows. These are not images adjusted for printing, and as a matter of fact my monitor is not even calibrated because my system is 64 bit (the calibration people are seriously lagging behind technological developments)   .

Here's the overview.

[attachment=135:attachment]

Here's the foreground showing a portion of the Lion's mane (sorry for another cat shot). It's a bit blurry.

[attachment=136:attachment]

Now here's the foreground of exactly the same shot but taken at f16 and ISO 200 to maintain shutter speed of 100th.

[attachment=137:attachment]

You should be able to see that it's noticeably sharper and that shooting at f16 has had its advantages. But what about the background? Well here's the background at f11 and f16.

[attachment=138:attachment]

[attachment=139:attachment]

If you examine these images you'll see that the f16 images are sharper. The f16 foreground image is quite noticeably sharper. The f16 background image is marginally sharper.

So what! I hear you say. That's what you'd expect. And yes it is. But what you'd also expect is that at some point in between the nearest point and the furthest point the f11 shot would be sharper. You have to believe me on this, it isn't.

There's no point anywhere on the f11 shot that is as sharp or sharper than the f16 shot.

Conclusion? This lens (at 85mm, did I forget to mention that) is sharpest at f16. Now that's good news  because there are 2 interesting ramifications. (1) I can use f16 with impunity without agonising over any trade-off in resolution, (2) The 5D sensor (and the 1Ds before it) is really not as good as some of the lenses attached.

For those of you who are skeptical, who think maybe I am suffering from Parkinson's or dipsomaniacal tremors, I put my camera on a tripod indoors; used remote control and mirror lock-up, and produced the following results.

[attachment=140:attachment]

[attachment=141:attachment]

[attachment=142:attachment]

Essentially, maximum resolution of the 24-105 lens at 85mm is the same across f8 to f16. What differs is the DoF. I haven't wasted bandwidth by showing this. It's too obvious.

I should qualify the above statement. Maximum resolution of the 25-105/5D system is the same across f8 to f16 at 85mm. I can only speculate on the reasons. My guess is that the 5D pixel size is too big to delineate (differentiate whatever) anything smaller than the f16 Airy disc. Please feel free to dispute this.

Okay! The images are too small. They are in fact around 100th of the area of the full image. If you need to see them, I can provide bigger images. Maybe I confused centimetres with inches in PS.

Yep! PS on my 64 bit system was set to cms instead of inches. Sorry about that, but I don't expect there'll be any clamour for larger images, but I do wish you Americans would get in line with the rest of the world   .
« Last Edit: January 07, 2006, 09:21:58 AM by Ray » Logged
jdemott
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« Reply #1 on: January 07, 2006, 12:44:06 PM »
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Your own examination and evaluation of dozens (or hundreds) of images is more important than looking at a few selected crops.  If you are satisfied, that is the main thing.  Whether someone can demonstrate measurable diffraction at f/11 on the test bench really doesn't matter if the results are not apparent in real world shooting.

Your conclusion seems quite reasonable and is about what I would have expected from my own experience.  I have the Nikon D2X which has about the same pixel count as the 5D, but on a much smaller chip.  The D2X begins to show diffraction beyond f/11--smaller apertures are still usable if you need the depth of field but you have to accept that there is a little softness by the time you get to f/16.  With the full frame sensor of the 5D and therefore larger photosites, I would expect that diffraction effects would be less of an issue, so your results are not surprising.
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John DeMott
David R. Gurtcheff
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« Reply #2 on: January 07, 2006, 03:24:31 PM »
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Ray: On New Year's Day I spent the day shooting seascapes with a 1Ds MkII and 24~105 (most at the wide end 24 to about 35mm). I purposly used mostly f11 because overall sharpness was a goal. Next time I will shoot at both f11 and 16 at the wide end. Your post has me re-examining how I work.
Thanks for the post.
Dave
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Bobtrips
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« Reply #3 on: January 07, 2006, 04:28:33 PM »
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Just a small request...

When posting images/crops for comparison how about doing a quick "merge" and present the samples side by side in the same frame?
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Sfleming
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« Reply #4 on: January 07, 2006, 05:10:32 PM »
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Would love to hear Michael's input on this.
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Ray
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« Reply #5 on: January 08, 2006, 12:18:20 AM »
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Ray: On New Year's Day I spent the day shooting seascapes with a 1Ds MkII and 24~105 (most at the wide end 24 to about 35mm).
[a href=\"index.php?act=findpost&pid=55442\"][{POST_SNAPBACK}][/a]

Dave, I'll make a prediction. The 1Ds2 will show a greater difference going from f8 to f16 because of its higher absolute resolving power. The lens has to be better at f8 but the 5D's resolving power with its lower pixel density is not so capable of revealing it.

Having read the reviews of the 24-105 on this site it seems clear that this lens has a sweet spot around 50mm. I couldn't resist doing a comparison at all stops at 50mm. I believe the differences are more noticeable at this focal length, but there seems to be no serious degradation till f22.

This time I've pasted the images together to make it easier for Bobtrips and others   .

Full image:-   [attachment=147:attachment]

Composite image:-  [attachment=148:attachment]


Each centre crop is a bit smaller than 100th the area of the full image (although the crops have a different aspect ratio). One should always bear such matters in mind when assessing the relevance of small differences at the 'pixel-peeping' level.

Even using this lens at its best focal length doesn't seem to result in more than a very marginal loss of image quality at f16. I think I might continue to avoid using f22 though   .
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Ray
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« Reply #6 on: January 09, 2006, 05:11:32 AM »
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Each centre crop is a bit smaller than 100th the area of the full image (although the crops have a different aspect ratio). One should always bear such matters in mind when assessing the relevance of small differences at the 'pixel-peeping' level.


[a href=\"index.php?act=findpost&pid=55457\"][{POST_SNAPBACK}][/a]

Actually, I seem to be out by a factor of 10. I should have written each crop is smaller than one thousandth of the area of the full image. With the ruler displayed around the 12x18" image at 240 ppi in PS, each crop is about 0.31" wide.

Viewing the enlarged crop in the above post, it appears on my screen 1.75"x3.5" in size. To see the crop this size on a print, the print would need to be 5.7 (0.31/1.75) x12" wide, ie., 68"x102". I therefore conclude that any small differences between f8 and f16 (with the 5D) is even less significant than I first imagined, or in other words, quite irrelevant.

Now I'd like to repeat this test with the Canon 85/1.2, if I can get my hands on one.  
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Ray
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« Reply #7 on: January 11, 2006, 01:08:35 AM »
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Actually, I'm rather surprised at the lack of interest in this topic. Am I the only one who's been inhibited in using f16 because of resolution fall-off? It apears so. Do I take it, if you all want the maximum DoF, you automatically stop down to f16, f22 and even f32, with narry a worry about resolution fall-off?

I'd like to know.
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francois
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« Reply #8 on: January 11, 2006, 03:25:06 AM »
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Actually, I'm rather surprised at the lack of interest in this topic. Am I the only one who's been inhibited in using f16 because of resolution fall-off? It apears so. Do I take it, if you all want the maximum DoF, you automatically stop down to f16, f22 and even f32, with narry a worry about resolution fall-off?

I'd like to know.
[a href=\"index.php?act=findpost&pid=55702\"][{POST_SNAPBACK}][/a]
I regularly go to f/16 but will go higher sometimes (1D Mk2 & 5D). In fact I rarely go beyond f/16 and if I do then I always double the shot (one at f/16 and one at f/22). Michael has an article about this subject but I can't find it anymore.
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Francois
Ray
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« Reply #9 on: January 11, 2006, 06:45:40 AM »
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I see! So I'm the conservative one. I've spent so many years with the small format D60 and 20D that I've forgotten that f16 can have a use with full frame 35mm without  resolution compromise.
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francois
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« Reply #10 on: January 11, 2006, 11:26:36 AM »
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I see! So I'm the conservative one. I've spent so many years with the small format D60 and 20D that I've forgotten that f16 can have a use with full frame 35mm without  resolution compromise.
Ray,
I found the article.
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Francois
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« Reply #11 on: January 11, 2006, 11:34:54 AM »
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I'm happy if I can do f8-f11.  But I have no issues going out to f16.  Beyond that I start to worry.  But if you need the DOF you do it and hope the results are good enough.
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Ray
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« Reply #12 on: January 11, 2006, 06:08:50 PM »
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... if you need the DOF you do it and hope the results are good enough.
[a href=\"index.php?act=findpost&pid=55749\"][{POST_SNAPBACK}][/a]

Sure, but I'd like a bit more certainty. It's supposed to be science after all.  
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Ray
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« Reply #13 on: January 11, 2006, 06:31:20 PM »
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Ray,
I found the article.
[a href=\"index.php?act=findpost&pid=55747\"][{POST_SNAPBACK}][/a]


Francois,
I remember reading that article. In fact I even remember commenting to Michael at the time that f32 on an MF camera is quite different to f32 on 35mm. In fact there's about a 2.5 stop difference, ie f32 on 6x7 is equivalent to around f13 on 35mm.

As I recall, in the days I was using 35mm film, f16 always produced rather disappointing results in terms of absolute resolution. I suspect the reason lies in the nature of MTF roll-off in film beyond 20 lp/mm.

My disappointment in 35mm film at f16 was easily carried forward to the D60 and 20D because these are smaller formats with 1.6x the DoF at the same stop and FoV. It's well known that as you go down in format, the smaller apertures become less usable. No 35mm lens supports f64 and no 2/3rds format P&S supports f16.

The fact that f16 appears to be fully usable on my 5D with no resolution fall-off of practical significance is a sort of bonus for me. I'm just surprised that no-one has been adding this feature to the list of advantages in upgrading from a 20D to a 5D.
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DarkPenguin
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« Reply #14 on: January 11, 2006, 07:39:07 PM »
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Sure, but I'd like a bit more certainty. It's supposed to be science after all. 
[a href=\"index.php?act=findpost&pid=55792\"][{POST_SNAPBACK}][/a]

It's magic.
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Ray
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« Reply #15 on: January 11, 2006, 11:41:13 PM »
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It's magic.
[a href=\"index.php?act=findpost&pid=55799\"][{POST_SNAPBACK}][/a]


Well, let's hope it's consistent magic for you.  "I wasn't feeling too well yesterday. Took a few shots at f22 though, to get maximum DoF. But the resolution's not too good. Just can't explain it. I used a fast shutter speed and IS so I don't know what went wrong. A week ago my f22 shots were just fine, as sharp as a razor."    
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pcox
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« Reply #16 on: January 16, 2006, 05:09:11 AM »
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It's basically an issue of pixel pitch - the smaller the pixels themselves, the sooner diffraction effects kick in. There's an excellent article on understanding diffraction and pixel pitch here: http://www.cambridgeincolour.com/tutorials...photography.htm

From that article, a 5D will start to lose resolution to diffraction _after_ f/16. In my own tests to verify this I found that I saw improved sharpness at f/16 over f/11, but lost sharpness at f/22. With my 20D, I found that f/11 was the minimum aperture I could use before losing resolution (using the same lens, the 24-70L).

That being said, as Michael says in his 'stop down!' article, if you need the depth of field, shoot at the appropriate aperture and worry about diffraction later.

It's also worth remembering that everything has to go right to get the sharpest possible shot. In a controlled environment with plenty of time and no discomfort, it's easy to get razor sharp shots. In the field, with wind, rain, soft ground, misfocussing (remember - AF still isn't as reliable as we'd like it to be for landscape work, especially) and photographer discomfort it's easy to get it wrong!

Cheers,
Peter
« Last Edit: January 16, 2006, 05:11:01 AM by pcox » Logged

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Dr. Gary
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« Reply #17 on: January 16, 2006, 12:16:14 PM »
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First let me say I think the new site is a vast improvemnt over the old and I for one will be renewing my subscription to the LL video journal which pays for this site.

As Jonathan Wienke frequently mentioned, there's no free lunch, and I don't expect to use valuable bandwidth on someone elses site for free.

The facility of now being able to download images directly from one's HD is a major upgrade which is most welcome.

With this in mind, I'm hoping to demonstrate a curious phenomenon which basically knocked my socks off.

Most of us are not scientists, and even if we were we'd have a difficult time making sense of all the contradictory data regarding lens performance, due to lack of reliable, up to date information in the public domain.

I recently bought a Canon 5D and the 24-105mm zoom lens, which I believe is a great combination. During the past 6 weeks or so, I've taken about 4,000 images in Nepal, Tailand and Cambodia using mostly the 24-105 lens, but occasionally the Sigma 15-30.

I'm now processing these images and in the process of doing so I noticed that images I'd taken at f16 were no less sharp than the same image taken at f11, in any respect, but the f16 image exhibited greater DoF.

This actually flies in the face of common  understanding that f16 is softer than f11 and f11 is very slightly softer than f8.

The following images demonstrate that this is not necessarily true.

The image that brought this to my attention was a shot of the Bayon temple near Angkor Wat. I took 2 shots, one at f11 and one at f16. The shutter speed was the same (100th) in both instances (which of course meant I bumped up the ISO to 200 for the f16 shot).

Examining these 2 shots of the same scene, having focussed on the same spot in each exposure (guesstimating the hyperfocal distance as I always do), I find the f16 shot is sharper in all respects. Geez! That can't be right! What have I done! F16 sharper than f11, or at worst equally sharp!!!

So here's the proof. I should also add that the following images have had zero sharpening and zero luminance smoothing, I've also adjusted the shadows and contrast sliders to ensure no clipping of highlights and shadows. These are not images adjusted for printing, and as a matter of fact my monitor is not even calibrated because my system is 64 bit (the calibration people are seriously lagging behind technological developments)   .

Here's the overview.

[attachment=135:attachment]

Here's the foreground showing a portion of the Lion's mane (sorry for another cat shot). It's a bit blurry.

[attachment=136:attachment]

Now here's the foreground of exactly the same shot but taken at f16 and ISO 200 to maintain shutter speed of 100th.

[attachment=137:attachment]

You should be able to see that it's noticeably sharper and that shooting at f16 has had its advantages. But what about the background? Well here's the background at f11 and f16.

[attachment=138:attachment]

[attachment=139:attachment]

If you examine these images you'll see that the f16 images are sharper. The f16 foreground image is quite noticeably sharper. The f16 background image is marginally sharper.

So what! I hear you say. That's what you'd expect. And yes it is. But what you'd also expect is that at some point in between the nearest point and the furthest point the f11 shot would be sharper. You have to believe me on this, it isn't.

There's no point anywhere on the f11 shot that is as sharp or sharper than the f16 shot.

Conclusion? This lens (at 85mm, did I forget to mention that) is sharpest at f16. Now that's good news  because there are 2 interesting ramifications. (1) I can use f16 with impunity without agonising over any trade-off in resolution, (2) The 5D sensor (and the 1Ds before it) is really not as good as some of the lenses attached.

For those of you who are skeptical, who think maybe I am suffering from Parkinson's or dipsomaniacal tremors, I put my camera on a tripod indoors; used remote control and mirror lock-up, and produced the following results.

[attachment=140:attachment]

[attachment=141:attachment]

[attachment=142:attachment]

Essentially, maximum resolution of the 24-105 lens at 85mm is the same across f8 to f16. What differs is the DoF. I haven't wasted bandwidth by showing this. It's too obvious.

I should qualify the above statement. Maximum resolution of the 25-105/5D system is the same across f8 to f16 at 85mm. I can only speculate on the reasons. My guess is that the 5D pixel size is too big to delineate (differentiate whatever) anything smaller than the f16 Airy disc. Please feel free to dispute this.

Okay! The images are too small. They are in fact around 100th of the area of the full image. If you need to see them, I can provide bigger images. Maybe I confused centimetres with inches in PS.

Yep! PS on my 64 bit system was set to cms instead of inches. Sorry about that, but I don't expect there'll be any clamour for larger images, but I do wish you Americans would get in line with the rest of the world   .
[a href=\"index.php?act=findpost&pid=55422\"][{POST_SNAPBACK}][/a]


Diffraction is a non varible property. In otherwords, it is the lens opening not the f number responsible. A 100mm lens at f8 has the same amount of image degrading diffraction that a 50mm lens does at f 5.6 or a 200mm lens does at f11. It is a property decided by the lens opening.

DrGary
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Slough
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« Reply #18 on: January 16, 2006, 01:02:08 PM »
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Diffraction is a non varible property. In otherwords, it is the lens opening not the f number responsible. A 100mm lens at f8 has the same amount of image degrading diffraction that a 50mm lens does at f 5.6 or a 200mm lens does at f11. It is a property decided by the lens opening.

DrGary
[a href=\"index.php?act=findpost&pid=56073\"][{POST_SNAPBACK}][/a]

That's true, but for a given F stop, reducing the focal length increases the diffraction due to the smaller aperture. To achieve a given FOV, a 35mm camera requires a shorter focal length lens than a medium format camera, hence at a given F stop and FOV, diffraction is greater.

To put it another way, you can use smaller F stops on MF gear, before diffraction starts to degrade the image quality.

Leif
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kenstrain
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« Reply #19 on: January 16, 2006, 01:05:38 PM »
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Regarding the principle, it is near enough correctly stated. Perhaps "opening" is a little ambiguous,  but as for the arithmetic    
focal length/ f-no.  aperture
100mm/8  =12.5 mm
50mm/5.6 = 8.9  mm (should have been f4, of course)
200mm/11 = 18.2 mm (should have been f16).

(Let's not worry about the true value for f11, as the correct answer only needs the
2^n f numbers that are exact.)

Note that the "opening" depends a little on the degree of retrofocus/telephoto - it is a long time since I calculated it from scratch, but anyway it makes only a very little difference in most cases (IIRC). Also we are talking about low magnification situations: with macro, as usual, the aperture behaves as if it were smaller than it really is (in the usual way, according to the magnification), so diffraction is worse than you might expect from the usual formula.  


Ken


Quote
Diffraction is a non varible property. In otherwords, it is the lens opening not the f number responsible. A 100mm lens at f8 has the same amount of image degrading diffraction that a 50mm lens does at f 5.6 or a 200mm lens does at f11. It is a property decided by the lens opening.
DrGary
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