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Author Topic: Lens Diffraction - When is it an issue with Medium Format Lenses?  (Read 15153 times)
Rob C
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« Reply #60 on: December 08, 2011, 03:06:58 AM »
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When I shoot Landscapes with my Mamiya RB67, I want everything in focus from front to rear.  So I work with the Hyperfocal distance, figure out what aperture I need, and then stop down one more stop for good measure.  Is this right?  Is this wrong?




If it works, it's right.

What else are you reasonably going to do?

Rob C
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KLaban
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« Reply #61 on: December 08, 2011, 03:51:55 AM »
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You must have some interest in this otherwise you wouldn't read...  

My experience is you don't "hit" diffraction, it creeps up on you. The science is interesting but perhaps a few more real-world observations would help?

Eric, the above is the opening line of my original post to this thread.

Macro vs. Infinity lens performance are not the same...

Doug's post highlights the importance of relevant testing.

I've nothing against testing as such, hell, I do my own, but I make damn sure it is relevant to my own work!

« Last Edit: December 08, 2011, 03:54:39 AM by KLaban » Logged

hjulenissen
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« Reply #62 on: December 08, 2011, 05:28:03 AM »
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Hmm, I don't know you from Adam, but I'll hazard a guess that you make your living as a scientist rather than an image maker?
Neither, actually. In what way does my occupation make my arguments more or less valid?

Both "hands-on" and "hands-off" approaches can be done in stupid ways, and easily ridiculed. Do you want to ridicule one but not the other?

-h
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ErikKaffehr
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« Reply #63 on: December 08, 2011, 05:44:03 AM »
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Hi,

Doug's posting referred to oneliner. The original posting clearly indicated that the Dollar bill was shot at three meters with a 100 mm lens.

Best regards
Erik

Eric, the above is the opening line of my original post to this thread.

Doug's post highlights the importance of relevant testing.

I've nothing against testing as such, hell, I do my own, but I make damn sure it is relevant to my own work!


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Doug Peterson
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« Reply #64 on: December 08, 2011, 07:26:14 AM »
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The recommendation was to shoot it at 3 meter distance with a 100 mm lens, so it's hardly macro. The reason I suggested the Dollar bill is that it has a lot of fine detail and is quite challenging in many ways.

Of course, my posting was a one liner, but it refers to this posting: http://www.luminous-landscape.com/forum/index.php?topic=59978.msg484272#msg484272

Your point is very valid. Thirty times focal length is still a bit on the short side for testing lenses.

My apologies for missing that detail.

I guess my general issue with "lab" tests is that they only tell you the answer to questions you know to ask. The "known unknowns" as Cheney would say. Often the "unknown unknowns" are often the ones that bite you in the butt.

At Capture Integration we get a lot of odd-ball testing requests from photographers who are going to use the camera exclusively for landscape. Generally I try to convince them going out and shooting a landscape is the best way to determine how a camera will perform when shooting a landscape :-). Ideally with the different equipment you are considering and alongside someone who knows where the pitfalls/misuse/mistakes/limits of each system are (e.g. don't stop down that canon lens to f/22).
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theguywitha645d
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« Reply #65 on: December 08, 2011, 07:27:35 AM »
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Interesting - to me that image is too soft for my purposes.  I wonder if you have tried working this image using the detail slider in Lightroom or ACR or one of the other deconvolution programs?


You mean soft at 100%? 100% is not a real world viewing condition. So you have made a 3 foot print? Besides, I this is a low contrast scene I shoot as a test, the test I mention in the thread, to see what the result would be at f/22 and a bunch of other apertures to show the effect of diffraction. Result: diffraction is overstated.
« Last Edit: December 08, 2011, 07:40:17 AM by theguywitha645d » Logged
theguywitha645d
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« Reply #66 on: December 08, 2011, 07:38:20 AM »
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It may appear funny to you, but it is based on the laws of physics. Nathan Myhrvold is no crank, but a genius with a PhD in physics form Princeton University. He is not only an accomplished scientist but an accomplished photographer and gourmet cook--a true renaissance man. That is why I chose to quote him. Of course, one can stop down further with a MFDB than a full frame dSLR, but a few calculations are instructive.

The pixel pitch of the IQ-180 is 5.17 μ, the resolution is 7816 x 10380 pixels, and the sensor dimension is 40.4 x 53.7 mm. The Airy disc diameter for light at 530 nm is 28.5 μ. The sensor becomes diffraction limited when the Airy disc diameter is 1.4 to 2 times the pixel pitch. Diffraction limited resolution of a lens at f/22 is 70 lp/mm at the Rayleigh limit (about 9% MTF) and 33 lp/mm at 50% MTF (figures from Roger Clark). Rayleigh resolution in an astronomical telescope can separate binary stars in this high contrast setting, but is not very useful for terrestrial photography, where an MTF of 50% is more reasonable.

One can use these figures to calculate the resolution of the IQ-180. The Rayleigh resolution at f/22 is 2840 x 3375 lp; since it takes 2 pixels to resolve 1 lp, that corresponds to 2702 x 3592 pixels or 43 MP. For a MTF of 50% the figures are 2702 x 3592 pixels or 9.7 MP.. For the Canon 1DsMIII that Dr. Myhrvold quoted in his post, the Rayleigh resolution at f/22 is 1687 x 2351 lp 17 MP. The MTF 50 resolution is 803 x 1204 lp or 3.9 MP, somewhat better than his prediction of 2MP.

One needs to correlate these figures to the perception of sharpness. According to David Pogue's tests, megapixels are not as important as often believed, so it is reasonable to get good results at the 9.7 MP you would get with the IQ-180 at f/22.

Regards,

Bill


Nathan may be a genius, but optics and imaging are not his fields. It would not be the first time someone making statement about something outside their field have made errors.

But anyway. Perhaps you can add to the discussion by showing examples of this megapixel equivalency hypothesis. Numbers are not results, especially if the underlying model has errors.
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bjanes
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« Reply #67 on: December 08, 2011, 08:44:43 AM »
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Nathan may be a genius, but optics and imaging are not his fields. It would not be the first time someone making statement about something outside their field have made errors.

But anyway. Perhaps you can add to the discussion by showing examples of this megapixel equivalency hypothesis. Numbers are not results, especially if the underlying model has errors.

Perhaps you should read his article and then point out where he made errors. Your own results seem to verify the hypothesis (vida infra).

Pentax 645D, D FA 55mm, ISO 200, f/22. The scene. 100% crop (and I am unsure if it is from the plane of focus). A plot of a small section of the image which is also inset in the plot. The image is resolving features 12 micros or a less on the image plane.

Your test shot does show light and dark bands likely representing resolved areas of the bark or other tree structures. Aliasing and demosaicing artifacts as well as excessive sharpening can produce false detail, but diffraction at f/22 should have eliminated most aliasing and I assume we are seeing true resolution.

Resolution is measured in lp/mm or cycles/mm--that represents peak to peak. Measuring peak to peak, I get 25 μ which would be 40 lp/mm. You seem to have measured peak to trough. From my previous calculations, the resolution of a diffraction limited lens at f/22 is 70 lp/mm at the Rayleigh limit and 33 lp/mm at 50% MTF. Your resolution is somewhere between these two values and is consistent with theory. The Nyquist frequency of your sensor is 84 lp/mm and you are achieving considerably less in your shot. A statement regarding resolution is incomplete unless the contrast achieved for that resolution is stated. Resolution at 50% contrast (MTF 50) correlates best with perceived sharpness.

BTW, what software did you use for your measurement? Photoshop extended? Anyway, nice work.

Regards,

Bill  

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theguywitha645d
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« Reply #68 on: December 08, 2011, 09:16:54 AM »
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The software is ImageJ, free for anyone who wants to use it. Google and name and you should find it easily

This is not a test target and so you need to dig a little further. The plot of the image is far more complex, which is why just using resolving power to make some sort of grid to define "equivalent MP" does not really work--you are not going to find convenient line pairs and line pairs in and of themselves do not describe an image (it is just a quantification of system that allows comparison, but does not have enough detail to express image formation or perception). Since the 645D has a pixel pitch of 6um, the image does actually provide clear separation of detail between 1-2 pixels or 6um-12um. You can see that in the image and if you analyze the plot, it is there as well--as in other areas of the image. You will also see it is defining edges very well. Just taking peaks on the plot will not work.

This is also a low contrast target which would lower the resolving power of the system. Which suggests that the Rayleigh criterion/limit, which you use in your post, is far too crude to model real systems. Actually, the Rayleigh limit is really just a useful learning tool, but it is out of date.
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KLaban
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« Reply #69 on: December 08, 2011, 09:47:06 AM »
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Doug's posting referred to oneliner. The original posting clearly indicated that the Dollar bill was shot at three meters with a 100 mm lens.

My point is shooting a dollar bill at three metres with a 100mm lens isn't going to help the OP, or me, or anyone using a 28mm lens - or whatever - at infinity, and that's before even considering the multitude of available MFD backs in various sizes.

I've tested all of my lenses with my MFD back at distances that are relevant to my subject matter and my output. I recommend that anyone who is concerned about diffraction does the same.
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John R Smith
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« Reply #70 on: December 08, 2011, 10:01:11 AM »
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I've tested all of my lenses with my MFD back at distances that are relevant to my subject matter and my output. I recommend that anyone who is concerned about diffraction does the same.

Well spoken. 'Tis the only way to do it. And once you have done it, internalise the results and stop worrying about it.

I did some testing recently off a tripod using a subject with mixed distances from about 30 feet to infinity, to check a couple of Zeiss 60mm lenses which I was servicing. Folks, don't you worry too much about f22. In this case it was f4 which was scary - I was horrified how the image fell apart when those lenses were wide-open. The corners with these old Distagons are not that great at the best of times, but . . .

John
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theguywitha645d
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« Reply #71 on: December 08, 2011, 10:42:46 AM »
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I've tested all of my lenses with my MFD back at distances that are relevant to my subject matter and my output.

That's just crazy talk!
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bjanes
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« Reply #72 on: December 08, 2011, 11:06:49 AM »
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The software is ImageJ, free for anyone who wants to use it. Google and name and you should find it easily
Thanks. I do use ImageJ extensively, but did not recognize the output in your graph.

This is not a test target and so you need to dig a little further. The plot of the image is far more complex, which is why just using resolving power to make some sort of grid to define "equivalent MP" does not really work--you are not going to find convenient line pairs and line pairs in and of themselves do not describe an image (it is just a quantification of system that allows comparison, but does not have enough detail to express image formation or perception). Since the 645D has a pixel pitch of 6um, the image does actually provide clear separation of detail between 1-2 pixels or 6um-12um. You can see that in the image and if you analyze the plot, it is there as well--as in other areas of the image. You will also see it is defining edges very well. Just taking peaks on the plot will not work.

Your test method seems quite convoluted and I doubt that it is validated. Why don't you use a validated method? Some choices are:

ISO 12233 Chart: allows visual determination of resolution by inspection and the slanted edge can be used with Imatest and other software. However, one must calibrate for the magnification factor if using the visual method. With the slanted edge target, the magnification is not critical.

Koren Resolution Chart: Can be used visually and with ImageJ, where one measures peak to peak. The magnification factor must be accounted for.

Van Der Wolf modified Siemens star: Target can be used at any distance and the slanted edge can be used with Imatest.

This is also a low contrast target which would lower the resolving power of the system. Which suggests that the Rayleigh criterion/limit, which you use in your post, is far too crude to model real systems. Actually, the Rayleigh limit is really just a useful learning tool, but it is out of date.

The Rayleigh  criterion is hardly out of date and, along with the Dawes criterion, is a fundamental concept in optics. The old USAF charts measure resolution near the Rayleigh limit and this is the result you get when using this method. As I stated, a MTF of 9% is too low to be used in terrestrial photography. The Dawes limit is where MTF is zero. A MTF of 50% corresponds best with perceived sharpness. What MTF did you get with your method. Resolution without reference to contrast is not that helpful.

Regards,

Bill
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DeeJay
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« Reply #73 on: December 08, 2011, 11:16:01 AM »
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Depends from scene to scene. Lighting to lighting. Lens to lens.

I see it a more on my P65 with V Series lenses more so than the H Series which I don't really see it.

It's always cleaned up with some sharpening though and hasn't ever really been a problem.

It starts to appear at f16.

It's never been an issue when I used the V with a P45. Probably more exaggerated with the IQ180.

« Last Edit: December 08, 2011, 11:19:25 AM by DeeJay » Logged
KLaban
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« Reply #74 on: December 08, 2011, 11:42:18 AM »
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I've tested all of my lenses with my MFD back at distances that are relevant to my subject matter and my output. I recommend that anyone who is concerned about diffraction does the same.

That's just crazy talk!

Yup, I know, I'm insane.

What I should be doing is taking a hands-off approach, and using blind subjective testing.

« Last Edit: December 08, 2011, 11:50:22 AM by KLaban » Logged

theguywitha645d
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« Reply #75 on: December 08, 2011, 01:15:45 PM »
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Your test method seems quite convoluted and I doubt that it is validated. Why don't you use a validated method? Regards,

Bill

They are my tests for myself--you had challenged us to show results. Also, test charts do not provide the information I need. These test give very useful data. And the final part of the test is always real output.

Bill, lets see some of your tests on diffraction.
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bjanes
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« Reply #76 on: December 08, 2011, 05:05:09 PM »
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They are my tests for myself--you had challenged us to show results. Also, test charts do not provide the information I need. These test give very useful data. And the final part of the test is always real output.

Bill, lets see some of your tests on diffraction.

Certainly. I used Bart's target with my Nikon D3 with the 60 mm f/2.8 AFS Micro-Nikkor. I rendered the images with ACR 6.5 with the default settings, which include a slight amount of capture sharpening.

Here is the result for f4.0 with Bart's method and with Imatest. The blur circle is 98 pixlels giving a resolution of 55 cy/mm or 0.47 cy/pixel, near the Nyquist limit of 0.5 cy/px. Bart's chart shows aliasing particularly well, but I think that the resolution results are for a relatively low MTF, perhaps near Rayleigh. Maybe Bart can comment if he is following this thread. Bart's chart also allows visual inspection of the test images; the low contrast at f/32 is evident at a quick glance, more so than the reduced resolution. The Imatest results are self explanatory. The resolution per picture height is not valid since I used cropped images, but the other figures are valid. Imatest allows calculation of the MTF at various resolutions. The MTF at 50% contrast is considered most important for image quality.The effects of diffraction would be more marked with the D3x or other higher resolution camera. At larger apertures, the lens out resolves the sensor with the D3.





And for f/32. The blur circle is 109.5 mm, corresponding to a resolution of 41 cy/mm or 0.42 cy/pixel. Note that the aliasing is completely eliminated.
Also note that Imatest shows the diffraction limit for f/32, which is closely approximated by the curve.





It would be most interesting if you would shoot Bart's chart at optimal aperture and the minimum aperture and post the results.

Regards,

Bill



« Last Edit: December 08, 2011, 05:21:59 PM by bjanes » Logged
BJL
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« Reply #77 on: December 08, 2011, 05:56:15 PM »
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...  f/22 ... The image is resolving features 12 micros or a less on the image plane.
This fits a pattern I have noticed in various field observations: the diffraction effects coming in on a length scale in microns of about half the f-stop. Fitting roughly with the wavelength of visible light being about half a micron. This seems to fit fairly well with the theoretical figures based on 50% MTF, or am I remembering that incorrectly?

Of course the effect is incremental, not a brick wall, and some more fastidious observers draw the line one stop earlier (like f/8 with 5.6 micron pixel spacing, and some willing to go about a stop further, especially with high pixel count MF images expected to be printed at high PPI.
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BartvanderWolf
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« Reply #78 on: December 09, 2011, 06:39:53 AM »
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Certainly. I used Bart's target with my Nikon D3 with the 60 mm f/2.8 AFS Micro-Nikkor. I rendered the images with ACR 6.5 with the default settings, which include a slight amount of capture sharpening.

Here is the result for f4.0 with Bart's method and with Imatest. The blur circle is 98 pixlels giving a resolution of 55 cy/mm or 0.47 cy/pixel, near the Nyquist limit of 0.5 cy/px. Bart's chart shows aliasing particularly well, but I think that the resolution results are for a relatively low MTF, perhaps near Rayleigh. Maybe Bart can comment if he is following this thread.

Hi Bill,

Yes, the limiting resolution (which approaches Nyquist) is at a relatively low MTF. It is closer to the human visual resolution limit, at approx. 10% response on the MTF, than to the 50% MTF metric (which gives a more overall perception of sharpness and contrast, and is closer to the maximum constrast sensitivity of the human eye). The Rayleigh criterion is more relevant for separating 2 point light sources, such as stars, and works out to something like a 20% contrast at that limit for a diffraction pattern.

To place that in perspective, a subject with 10:1 contrast at the 10% MTF point will exhibit a 1% resulting contrast, and that's at the perception threshold of detection (without sharpening) for human vision. The unaltered test chart, when printed on glossy paper will have something like a 100:1 contrast, and therefore plenty to produce aliasing even at very low MTF responses.

Quote
And for f/32. The blur circle is 109.5 mm, corresponding to a resolution of 41 cy/mm or 0.42 cy/pixel. Note that the aliasing is completely eliminated. Also note that Imatest shows the diffraction limit for f/32, which is closely approximated by the curve.

Sharpening will raise the MTF curve somewhat above the diffraction limited MTF, as long as there is sufficient signal left for sharpening. Low subject contrast combined with diffraction will result in loss of resolution, unrestorable even for deconvolution sharpening.

Cheers,
Bart
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bjanes
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« Reply #79 on: December 09, 2011, 09:10:12 AM »
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Yes, the limiting resolution (which approaches Nyquist) is at a relatively low MTF. It is closer to the human visual resolution limit, at approx. 10% response on the MTF, than to the 50% MTF metric (which gives a more overall perception of sharpness and contrast, and is closer to the maximum constrast sensitivity of the human eye). The Rayleigh criterion is more relevant for separating 2 point light sources, such as stars, and works out to something like a 20% contrast at that limit for a diffraction pattern.

To place that in perspective, a subject with 10:1 contrast at the 10% MTF point will exhibit a 1% resulting contrast, and that's at the perception threshold of detection (without sharpening) for human vision. The unaltered test chart, when printed on glossy paper will have something like a 100:1 contrast, and therefore plenty to produce aliasing even at very low MTF responses.

I repeated my experiment at f/22, but omitted capture sharpening. The resolution calculated from your target was 57 lp/mm and the MTF 10 calculated by Imatest was 59 lp/mm, confirming the approx. 10% figure you quoted. It is interesting to note that the MTF10 at f/22 is at Nyquist, which would imply that deconvolution sharpening could restore some of the lost detail imposed by shooting at this small aperture.





I am somewhat confused by your statement that MTF at Rayleigh is around 20%, since I had always thought that it was aroung 10% as stated by Roger Clark. However, on checking in Wikipedia, they do mention a figure of 20%. How does one resolve these conflicting values?

Regards,

Bill
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