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Author Topic: Lens Diffraction - When is it an issue with Medium Format Lenses?  (Read 13181 times)
hdomke
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« on: December 04, 2011, 06:23:58 AM »
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What is the best way to test a lens for diffraction?

Is it true that medium format lenses can go to much higher f-stops than 35mm DSLR lenses before diffraction is an issue?

I am asking this because I would like to avoid having to use focus stacking in my landscape images.

In the Luminous Landscape video series “Camera to Print & Screen” Michael Reichmann says: “With a pocket camera you may start to hit diffraction at f/8, with a DSLRs it is usually about f/16, but on a large or medium format camera you might be able to go to f/32 before you hit diffraction.”
Jeff Schewe: “Maybe, but that is something you have to test with that lens and sensor combination.”

Note: This is an approximate quote from about 4 minutes in to chapter 3 of “Camera to Print & Screen”
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theguywitha645d
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« Reply #1 on: December 04, 2011, 06:34:46 AM »
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Are you a pixel peeper? If not, your aperture limit is proportional to the sensor size, the larger the sensor, the more you can stop down. So are you using a 44x33mm MF sensor or a a 53x40mm MF sensor? That will have something to do with it. I am quite happy to use f/16 with my 645D. I can get excellent results at f/22 as well.

If you are shooting with a tech/view camera, you can also use tilt.
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Guillermo Luijk
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« Reply #2 on: December 04, 2011, 06:36:42 AM »
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Is it true that medium format lenses can go to much higher f-stops than 35mm DSLR lenses before diffraction is an issue?

Yes it is, but it is also true that medium format sensors need to go to higher f-stops than smaller format sensors to achieve the same DOF. In the end, there is no practical advantage/disadvantage for any format size regarding diffraction and DOF issues.
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ErikKaffehr
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« Reply #3 on: December 04, 2011, 07:53:53 AM »
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Hi,

It depends a lot on your expectations, your definition of an "issue" and what back you have.

Diffraction is usually defined by: r = 1.22*Lambda*N, where r is the radius of the Airy circle,  Lambda is the wavelength of light and N is the f-number, like 16 for f/16.

So, for green light around 0.5 microns and f/16 we would get 9.76 microns. But this is the radius, the diameter would be twice that, namely 19.5 microns.

 Recall that the Airy ring is a circle, so it's area would be r^2 * pi, that is 299 microns.

If pixel size is 9 microns, typical of many MFDBs around 20 MP the Airy ring would cover about 3.5 pixels.  O
n the IQ180 with 5.17 micron pixel pitch it would cover 11 pixels. Both sensors would loose some sharpness, but the 9 micron pixel sensor may loose little and the 5.17 micron sensor a lot.

So if you are very careful about having maximum sharpness and in utilizing your lenses and have a high resolving sensor like P45, P65+, IQ180, Pentax 645D or a Leica S2 you would consider diffraction. If you don't really care about maximum achievable sharpness it may be less of an issue.

This page illustrates diffraction on a 4.7 micron sensor: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1 . That would be pretty close to what you would see on an IQ180.

That said, diffraction is relatively benign to sharpening, as demonstrated here: http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=2

This is a good link for using the hyperfocal distance correctly: http://optechsdigital.com/Alpa_and_Hyperfocal.html the article also demonstrates diffraction effects.

These two recent threads here on LuLa forums discuss this issue:

http://www.luminous-landscape.com/forum/index.php?topic=59862.0

http://www.luminous-landscape.com/forum/index.php?topic=59930.msg483360

Best regards
Erik



What is the best way to test a lens for diffraction?

Is it true that medium format lenses can go to much higher f-stops than 35mm DSLR lenses before diffraction is an issue?

I am asking this because I would like to avoid having to use focus stacking in my landscape images.

In the Luminous Landscape video series “Camera to Print & Screen” Michael Reichmann says: “With a pocket camera you may start to hit diffraction at f/8, with a DSLRs it is usually about f/16, but on a large or medium format camera you might be able to go to f/32 before you hit diffraction.”
Jeff Schewe: “Maybe, but that is something you have to test with that lens and sensor combination.”

Note: This is an approximate quote from about 4 minutes in to chapter 3 of “Camera to Print & Screen”
« Last Edit: December 04, 2011, 09:17:22 AM by ErikKaffehr » Logged

theguywitha645d
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« Reply #4 on: December 04, 2011, 09:21:49 AM »
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And naturally, the size of the Airy disk to the number of pixels is not really that important. What is important is the size of the Airy disk to the permissible circle of confusion for the format--the eye has a limit to resolve.
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ErikKaffehr
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« Reply #5 on: December 04, 2011, 10:25:18 AM »
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Hi,

Yes but that limit is quite flexible, it depends much on how large you want to print, but also on subject. Sometimes a small detail is important, like a penguin in an arctic landscape. It's generally agreed that the eye resolves 1 minute of an arc. Let's assume a viewing distance of 1 m. One minute of arc is 1/60 of a degree, at one meter this would be 0.29 mm. Two tell two points apart you would need a point in between. So you need three points spaced at 0.1 mm. So if you print at 70x100 cm and look at the image at 1 meter you would need 10000 pixels along the longer side to match the resolution of the unaided eye. On a 49 mm sensor that corresponds to 4.9 microns.

On the other hand I'd agree that the requirement can be much relaxed.

My opinion is that in many cases it's better to have the essential subject as sharp as possible and the less important parts acceptably sharp, than having everything a bit unsharp.

Another interesting option is that sharpening can be selectively, smart sharpen in Photoshop can do deconvolution for out of focus images that works quite well.

This article compares three different methods to achieve extended DoF, stopping down to f/22, Scheimpflug and focus stacking, with real world examples. http://echophoto.dnsalias.net/ekr/index.php/photoarticles/29-handling-the-dof-trap

Best regards
Erik

And naturally, the size of the Airy disk to the number of pixels is not really that important. What is important is the size of the Airy disk to the permissible circle of confusion for the format--the eye has a limit to resolve.
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theguywitha645d
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« Reply #6 on: December 04, 2011, 10:58:04 AM »
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Erik, we seem to be bumping up against each other recently. Everything is based on standard viewing distance. The permissible circle of confusion is not something set in stone. And the permissible circle of confusion is not set at the eye's ability to resolve to point sources. Sharpness and resolution are not the same thing.

What MFD system are you shooting with where you are thinking you need to work at such tight tolerances? I just hung an exhibition of 5x4 foot portraits taken with my 645D. There is nothing to suggest that MFD has tighter tolerances than before digital photography--no one would say shooting with Tech Pan has less depth of field and is more susceptible to diffraction than TMax 400 just because it has finer grain and more resolving power. Nor with film was there any question you could use smaller apertures with larger formats. Digital has not changed how a human would perceive an image. The idea that an image has to be optimized to 100% monitor view really has no basis in any real condition an image will be viewed at, especially with the pixel numbers on MFD sensors.

Yes, technically you are right diffraction affects an image--after all, an image is created from diffraction. But the problem is relative one, not an absolute one based on pixel pitch.
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ErikKaffehr
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« Reply #7 on: December 04, 2011, 02:23:52 PM »
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Hi,

The example given was intended to illustrate what the eye can actually resolve.

An MFDB has tighter tolerance than film for five reasons.

- The first is the that the sensor is a flat device, while film has a curvature that varies between exposures.
- The second is that digital is said to have higher resolution than film. Joseph Holmes talks about a factor five.
- The third factor is simply that new optical systems are built for digital, sharper lenses, tighter tolerances.
- The fourth factor is that the digital image is a first generation product. With film there was always an additional optical step potentially degrading the image.
- The fifth is that we print large. Making large prints in the dark room was not easy.

Another factor is that there are people buying 80 MPixel backs, and specially calculated digital lenses with extreme performance. To utilize all that expensive technology the workmanship also needs to be on top.

You may check this article: http://optechsdigital.com/Alpa_and_Hyperfocal.html that describes a way to achieving optimal focus. Interestingly he also arrives at f/16 as minimal optimum aperture, using his P65+ plus on Alpha. It is a good article worth reading.

My take on the issue is that I'd rather have optimum sharpness on the main subject and have a bit extra sharpening on the out of focus areas than reduce overall sharpness.

The other point is that high end backs have more to loose than low end backs. With a P25 the loss at f/22 may be minimal, with a P65+ it is a lot.

Joseph Holmes articles are clearly worth reading, they are about making most of MF digital:

http://www.josephholmes.com/news-sharpmediumformat.html

http://www.josephholmes.com/news-medformatprecision.html

Best regards
Erik

Erik, we seem to be bumping up against each other recently. Everything is based on standard viewing distance. The permissible circle of confusion is not something set in stone. And the permissible circle of confusion is not set at the eye's ability to resolve to point sources. Sharpness and resolution are not the same thing.

What MFD system are you shooting with where you are thinking you need to work at such tight tolerances? I just hung an exhibition of 5x4 foot portraits taken with my 645D. There is nothing to suggest that MFD has tighter tolerances than before digital photography--no one would say shooting with Tech Pan has less depth of field and is more susceptible to diffraction than TMax 400 just because it has finer grain and more resolving power. Nor with film was there any question you could use smaller apertures with larger formats. Digital has not changed how a human would perceive an image. The idea that an image has to be optimized to 100% monitor view really has no basis in any real condition an image will be viewed at, especially with the pixel numbers on MFD sensors.

Yes, technically you are right diffraction affects an image--after all, an image is created from diffraction. But the problem is relative one, not an absolute one based on pixel pitch.
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« Reply #8 on: December 04, 2011, 03:37:54 PM »
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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?

Using an H3D11-22 and 80mm lens I find the optimum apertures are f5.6 to f8. There is very little if any difference at f11 and f16 is perfectly acceptable. By f22 there is a noticeable softening. Much the same applies to a super-wide such as the 28mm.

Generally speaking given a choice between a landscape image displaying diffraction and another displaying an obvious inadequate DOF I'd choose the diffraction limited image every time.

If I were predominately a landscape photographer I'd be using a tech camera; but thankfully I'm not.
 
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theguywitha645d
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« Reply #9 on: December 04, 2011, 04:40:38 PM »
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Erik, I shoot MFD. I have shoot film. The human eye has not evolved in my lifetime. Perhaps I have not printed large enough, but I think when you hit 12 feet, you really are at a point that larger is not going to be any different. My experience is that diffraction is very much overstated.

Personally, I don't know why Mr. Holmes finds this so difficult.

What MFD system do you use?
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ErikKaffehr
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« Reply #10 on: December 04, 2011, 06:54:16 PM »
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Hi,

No, I'm not shooting MFD, I think I have clearly stated this at least on of the discussion we both are involved in. Nor do I print large, 70x100 cm is my maximum, for practical reasons. My normal print size is A2 (around 18"x26").

I have been using Pentax 67 with film and I was considering to buy a Pentax 645D, but decided against it, basically because I don't print large.

The camera I shoot is the Sony Alpha 900, it has a pixel size similar to your Pentax 645D. The Alpha 900 has an OLP filter which the Pentax 645D does not have.

I also bought a DVD with images from a shootout made by Michael Reichmann, Bill Atkinsson and Charlie Cramer. I made actual prints from those images. The main reason I have so much interest in this is that I'm quite interested in optics.

I'm not entirely alone stating that diffraction starts to be an issue past f/16. I don't mean that you should never stop down beyond f/16 just that you start to loose a lot.

Another question is what can be regained with sharpening. That applies to both defocus and diffraction. I'd suggest that diffraction is more benign to deconvolution based sharpening than defocus. But adding sharpening to the discussion just makes the issue more complex.


Best regards
Erik




Erik, I shoot MFD. I have shoot film. The human eye has not evolved in my lifetime. Perhaps I have not printed large enough, but I think when you hit 12 feet, you really are at a point that larger is not going to be any different. My experience is that diffraction is very much overstated.

Personally, I don't know why Mr. Holmes finds this so difficult.

What MFD system do you use?
« Last Edit: December 05, 2011, 12:34:28 AM by ErikKaffehr » Logged

theguywitha645d
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« Reply #11 on: December 05, 2011, 08:28:07 AM »
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Erik, sorry if I missed you were not using MFD

All I can say is I use MFD and I print large with it. I also know about the concepts in photography that we have been discussing, which also support what I see. You can chalk me up to a lost cause if you are going to try to convince me otherwise.

And you will also not convince me that small print size means a larger format film or digital camera will have no impact on the quality of the image. Sampling theory is not enough. An 8x10 will look different depending on the format, whether film or digital. I have seen it and so have others. Make an 8x10 with a 6x7 and a 35mm negative and the images will not be the same. Again, print size is not the defining characteristic. Even the human visual system is not defined by a single characteristic. Resolution and detection are two different things where detection can actually perceive even finer detail than resolving power will imply. And resolving power, both in optics and the visual system, are not a fixed quality, but will change with object contrast--a lens will only resolve to its maximum ability, only if you also present it with a high-contrast target.

I have no doubt you have a genuine interest in this subject--the internet is probably the worst place to have conversations about topics people are passionate about. You certainly have experience. But this is not a simple optical problem. Photography is subjective and very relative because it all goes back to a viewer and their perception of an image. But I think we may be having a problem with trees and the forest...
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jeremypayne
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« Reply #12 on: December 05, 2011, 08:52:57 AM »
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... My experience is that diffraction is very much overstated...

So is gravity, in my experience.

If all the physicists would just stop the hysteria ....  Roll Eyes Roll Eyes Roll Eyes
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Rob C
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« Reply #13 on: December 05, 2011, 12:57:28 PM »
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And you will also not convince me that small print size means a larger format film or digital camera will have no impact on the quality of the image. Sampling theory is not enough. An 8x10 will look different depending on the format, whether film or digital. I have seen it and so have others. Make an 8x10 with a 6x7 and a 35mm negative and the images will not be the same.




Yes, and that's the main reason I used both 6x6 and 135. The other reason for using 6x6, in my case, was that much of my fashion work required that the client get contact sheets... using 135, the solution often meant my selecting a few better shots and making blow-ups of them instead, but how can you do that quickly without cutting your own standards and creating a poor impression, regardless of how much better the selected, finished prints might be? Time costs money, and spending your own isn't so clever in many cases.

But regardless of size, tonality was always won by larger format!

Rob C
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« Reply #14 on: December 05, 2011, 02:14:12 PM »
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I was curious about this and shot a set up with a ruler and a $2 bill with both my CF 39 Multishot and an Aptus 12 through the whole range of apertures.   I still have all the files so one day when I have time, I'll post a thread with the crops, but the summary is that I felt I could shoot to f/18 with the CF 39 in single shot, f/25 in multishot (yes its much different), and f/16 with the Aptus 12.     I did this test mostly for my graphite hands art series which has very fine detail in the fingerprints but because of the focus distance needed as much DOF as possible.    Previously I shot with a phase p20 which I thought I could stop down to f/20.     Note that these are my own personal values, where I felt the trade off in more DOF vs loss of sharpness was balanced for larger prints.  Everyone will have their own set of criteria.  I'm sure were I printing smaller or with subject that had less fine detail,  I might think I could stop down a bit more.
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« Reply #15 on: December 05, 2011, 03:01:46 PM »
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Hi,

Just a short comment, my initial posting which was third on this list began with:

"It depends a lot on your expectations, your definition of an "issue" and what back you have."

I also have a small anecdote. Once upon the time my best friend and I were shooting my friends Hasselblad SWC and my Minolta XE with MC Rokkor 24/2.8 in parallell. My friend was shooting Tri-X at 200 ASA (that was before ISO) developed in D-76 and I shot Plux-X, also at 200 ASA developed in HC-110. These were the films we used in those days.

We printed both images in his darkroom, with his enlarger using his APO-Rodagon. When we developed the smallish 18x24 cm prints (that was than) he was pretty sure that my picture was the one he shot with the Hasselblad. 

I did shot with Pentax 67 on film for a long time, but found that I worked in a different way with the 67 than with 135. One of the issues I found that it was essentially impossible to achieve the depth of field I was used to with 135. So I essentially gave up on depth of field.

When I started scanning I found out that depth of field was more limited than I initially thought.

Anyway I was perfectly satisfied with the Pentax 67, but than I went digital and never looked back.

Best regards
Erik


Erik, sorry if I missed you were not using MFD

All I can say is I use MFD and I print large with it. I also know about the concepts in photography that we have been discussing, which also support what I see. You can chalk me up to a lost cause if you are going to try to convince me otherwise.

And you will also not convince me that small print size means a larger format film or digital camera will have no impact on the quality of the image. Sampling theory is not enough. An 8x10 will look different depending on the format, whether film or digital. I have seen it and so have others. Make an 8x10 with a 6x7 and a 35mm negative and the images will not be the same. Again, print size is not the defining characteristic. Even the human visual system is not defined by a single characteristic. Resolution and detection are two different things where detection can actually perceive even finer detail than resolving power will imply. And resolving power, both in optics and the visual system, are not a fixed quality, but will change with object contrast--a lens will only resolve to its maximum ability, only if you also present it with a high-contrast target.

I have no doubt you have a genuine interest in this subject--the internet is probably the worst place to have conversations about topics people are passionate about. You certainly have experience. But this is not a simple optical problem. Photography is subjective and very relative because it all goes back to a viewer and their perception of an image. But I think we may be having a problem with trees and the forest...
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« Reply #16 on: December 05, 2011, 04:06:50 PM »
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I certainly wouldn't shy away from using f22 on a 22MP back when necessary.
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« Reply #17 on: December 05, 2011, 04:13:06 PM »
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I certainly wouldn't shy away from using f22 on a 22MP back when necessary.

Yes definitely, these are/were the best backs for small apertures.  I'd say if someone was doing primarily table top / product work or macro, then the 22mp multishot backs are perhaps the best they will find.  With multishot  one can stop down several clicks more and still get a crisp detailed shot.   
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« Reply #18 on: December 06, 2011, 12:24:33 AM »
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There is quite an interesting discussion on the Shneider web site that discusses the technical reasons that forced them to redesign their older film lenses to create the newer digital versions.

Basically, digital was quite clearly showing flaws that were hidden in the noise of the film base, among other things. I believe it discusses the MTFs required for both 35mm andIlarger digital sensors. They used a hypothetical 24MP 35mm full frame sensor shortly before the Canon 1Ds came out (among other sizes)  Interesting read.

FWIW, the digital lenses were optimized for a much wider aperture - between 5.6 and 8.0 - than the older lenses. You should be able to find the curves and discussion in a second paper there.  (too hard to search now on my phone.)

OK, one older paper:

http://www.schneiderkreuznach.com/knowhow/digfoto_e.htm#1.%20What%20is%20"Picture%20Sharpness"?

More general discussion that precedes the above:

http://www.schneiderkreuznach.com/knowhow/opt_quali_e.htm


Best,
Michael
« Last Edit: December 06, 2011, 12:43:08 AM by mmurph » Logged
Graham Mitchell
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« Reply #19 on: December 06, 2011, 01:58:58 AM »
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Theory is all very well but why not just do a test at different apertures with YOUR lens and back, and decide for yourself where the acceptable limit is?
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