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Author Topic: Diffraction confusion.  (Read 6702 times)
stamper
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« on: February 06, 2013, 06:56:53 AM »
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I think I have a good understanding of diffraction and it's drawbacks. As a rule of thumb an aperture smaller than f/11will mean softness in an image. Yesterday I was looking at a book by David du Chemin and he has a couple of images that look sharp - at least to me - that he took at f/22. The reason he used f/22 was to give himself a shutter speed of 15 seconds in a long exposure. I understand what he was doing but he obviously ignored the "problems" of diffraction. Brian Peterson in his book about Exposure doesn't think diffraction is a problem. So how did David get his image to look sharp? I can't obviously post a copy.
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Slobodan Blagojevic
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« Reply #1 on: February 06, 2013, 07:22:11 AM »
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Because there is nothing to compare it to. If he took another shot at, say, f/11, and you were able to see it at 100% on your screen, you would see it is sharper. Besides, we are talking about a book reproduction (offset?) at a drastically reduced size (as compared to pixel peeping). Under those conditions, almost everything looks good enough. Also, defraction effects can be sharpened away with a proper technique (deconvolution).
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Slobodan

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hjulenissen
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« Reply #2 on: February 06, 2013, 07:38:51 AM »
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Not all scenes contain the kind of detail that is needed to spot diffraction degradation. Or if they do, our minds may be willing to forget about it if the image/scene still feels satisfying?

The distinction between true detail and apparent detail is blurry (pun intended). If we perceive the scene to be "sharp", does not mean that a really large amount of information about the true scene was preserved.

-h
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stamper
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« Reply #3 on: February 06, 2013, 07:41:20 AM »
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Slobodan I understand about deconvolution and I assumed that was possible. A lot of photographers make a big issue about not going smaller than f/11. What you state makes sense.  Smiley
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ErikKaffehr
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« Reply #4 on: February 06, 2013, 07:54:13 AM »
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Hi,

I agree with Slobodan.

Diffraction starts to have a negative effect already at f/8, the better the lens the earlier it is seen. But it is a gradual degradation of image contrast. Diffraction is bening to sharpening.

Check left column on the link below, it shows effect of diffraction:

http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=1

The link below shows the effect of sharpening:

http://echophoto.dnsalias.net/ekr/index.php/photoarticles/49-dof-in-digital-pictures?start=2

Best regards
Erik


Slobodan I understand about deconvolution and I assumed that was possible. A lot of photographers make a big issue about not going smaller than f/11. What you state makes sense.  Smiley
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Rob C
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« Reply #5 on: February 06, 2013, 08:01:29 AM »
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In a real world, ignore the theory. Use the diaphragm stop that will give you the depth of field that you require, and that's all you need worry about.

Folks have to realise that they are dealing with a photograph and not a laboratory test. Unless they actually do see their photography as a sequence of laboratory tests, in which case, good luck to them, and have fun.

Rob C
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HSakols
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« Reply #6 on: February 06, 2013, 08:13:56 AM »
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If you look at William Neils work you will see that he commonly uses f16 or f22.  I asked him about why he uses such small f stop and said that it is easier to "repair" resolution with sharpening software.  He uses Noise Ware.
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bjanes
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« Reply #7 on: February 06, 2013, 08:50:45 AM »
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I think I have a good understanding of diffraction and it's drawbacks. As a rule of thumb an aperture smaller than f/11will mean softness in an image. Yesterday I was looking at a book by David du Chemin and he has a couple of images that look sharp - at least to me - that he took at f/22. The reason he used f/22 was to give himself a shutter speed of 15 seconds in a long exposure. I understand what he was doing but he obviously ignored the "problems" of diffraction. Brian Peterson in his book about Exposure doesn't think diffraction is a problem. So how did David get his image to look sharp? I can't obviously post a copy.

Although diffraction has the same effect on the image plane with any format size, its effects on the final print are greater with small sensor formats, since they have to be "enlarged" more to produce the final print. Afterall, users of 8 x 10 inch film used to expose at f/45. What sensor is Mr du Chemin using?

Bill
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RSL
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« Reply #8 on: February 06, 2013, 09:32:28 AM »
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Folks have to realise that they are dealing with a photograph and not a laboratory test.

Which reminds me of what I see on Nikonians.org's D800 forum. Evidently the majority of D800 owners bought the camera to test rather than to make photographs. The forum is a laugh a minute.
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Slobodan Blagojevic
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« Reply #9 on: February 06, 2013, 12:26:47 PM »
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Just in case we forgot Michael' article on the subject (from the film era):

Stop It Down!

And its conclusion:

Quote
   
Is this shot sharp enough for almost any purpose? Yes!
   
Would it have been even sharper if I'd shot at f8 or f/11. Yes, probably.
   
Does it matter? No, not in the slightest.

Stop worrying! Take Pictures!

Enough said.
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Slobodan

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Petrus
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« Reply #10 on: February 06, 2013, 01:01:40 PM »
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There used to be a group of photographers who always (?) used f:64... Very sharp photographs. They were mostly using 8x10, by the way.

There is no rule that diffraction starts at f:11 or f:8 or whatever, as it depends on the sensor/film size and the wavelength we are most interested in (green usually). There is always diffraction, but it does not bother us until the sensor is sharper than the diffraction spoiled image. With D800 the limit is around f:8, with best APS-C cameras at f:5.6, with tiny sensors in some P&S and video cameras even below f:4. Actually some small sensor cameras can not resolve as good as the sensor pixel count promises, as the low speed lenses have too much diffraction even at full open.

Like already said, diffraction has always been with us and it does not bother us unless we know better = there is something you can compare it to. We have not been able to see much of it before digital photography and today's sensors, which give us the ability to pixel peep at huge enlargement size and make fast comparison series at different apertures without cost. Nobody bothered to shoot Technical Pan-X to make tests, and if somebody did, there was no Internet to spread the bad news...

Here is the tutorial again, look at the table near the end...

http://www.luminous-landscape.com/tutorials/resolution.shtml
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ErikKaffehr
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« Reply #11 on: February 06, 2013, 02:23:44 PM »
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Hi,

Somebody actually did...
http://www.photodo.com/topic_138.html

In real life, sharpening also matters a lot. Diffraction is benign to sharpening and sharpness is far from always the most important factor in imaging.

Best regards
Erik

Nobody bothered to shoot Technical Pan-X to make tests, and if somebody did, there was no Internet to spread the bad news...

Here is the tutorial again, look at the table near the end...

http://www.luminous-landscape.com/tutorials/resolution.shtml
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Petrus
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« Reply #12 on: February 06, 2013, 02:39:36 PM »
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Hi,

Somebody actually did...
http://www.photodo.com/topic_138.html


No surprise really, but now EVERYBODY is doing them...
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ErikKaffehr
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« Reply #13 on: February 06, 2013, 02:46:45 PM »
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Hi,

What is interesting with the quoted article is that the authors demonstrates the effects by microscope photographs on film. (135, 120 and 4x5" film)

Best regards
Erik

No surprise really, but now EVERYBODY is doing them...
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Jim Pascoe
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« Reply #14 on: February 07, 2013, 03:27:03 AM »
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In a real world, ignore the theory. Use the diaphragm stop that will give you the depth of field that you require, and that's all you need worry about.

Folks have to realise that they are dealing with a photograph and not a laboratory test. Unless they actually do see their photography as a sequence of laboratory tests, in which case, good luck to them, and have fun.

Rob C

Absolutely!  Once they have mastered the basics of the technical side, a photographers work is going to be judged by the subject, the composition, the light, the mood - anything except the difference in diffraction effects between f8 and f22.  Technical competence is important, but technical excellence is secondary to the other qualities of a photograph.

Jim
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stamper
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« Reply #15 on: February 07, 2013, 03:53:34 AM »
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Although diffraction has the same effect on the image plane with any format size, its effects on the final print are greater with small sensor formats, since they have to be "enlarged" more to produce the final print. Afterall, users of 8 x 10 inch film used to expose at f/45. What sensor is Mr du Chemin using?

Bill

In the photograph he used a Canon 5D mark II. I think he now has jumped ship to Nikon. For anyone who hasn't read his books he is  - imo - a very fine photographer and worth a read. I often do long exposure work with ND filters which mean I use f/16 a lot to get the shutter speeds I want. Reluctant to got to f/22 because of possible softness but not now!
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hjulenissen
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« Reply #16 on: February 07, 2013, 04:04:15 AM »
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I often do long exposure work with ND filters which mean I use f/16 a lot to get the shutter speeds I want. Reluctant to got to f/22 because of possible softness but not now!
If you have sufficient ND filtering available, why do you have to use f/16?

-h
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stamper
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« Reply #17 on: February 07, 2013, 05:31:52 AM »
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I like to control my initial shutter speed to 1/125 second in aperture mode which I know will give me about 20 seconds in manual mode. This tends to give you a small F stop.  In sunset shots for clouds and water It suits what I am shooting using a 10 stop ND filter. With the 10 stop you have to focus and expose in aperture mode  - screw the filter on - and switch to manual so setting exposure means you are limited. If I were to use a larger F stop then the shutter speed would be shorter but if I use a 6 stop or 3 stop or both combined then I am more flexible about setting the F stop because exposing and focusing can be achieved with the filters on the lens.
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bjanes
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« Reply #18 on: February 07, 2013, 07:39:44 PM »
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Just in case we forgot Michael' article on the subject (from the film era):

Stop It Down!

And its conclusion:

"Is this shot sharp enough for almost any purpose? Yes!
   
Would it have been even sharper if I'd shot at f8 or f/11. Yes, probably.
   
Does it matter? No, not in the slightest.

Stop worrying! Take Pictures!

Enough said."


Michael makes a point, but according to the excellent tutorial by Osuan and Garcia if Michael stopped down his Phase One IQ180 he would turn his expensive 80 MP camera into an 8 megapixel device with hardly more resolution than an iPhone.

BTW, an iPhone can take nice pictures.

Regards,

Bill
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ErikKaffehr
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« Reply #19 on: February 07, 2013, 10:58:15 PM »
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Hi,

I think it is OK to stop down when needed, but I really think that stopping down beyond f/16 causes considerable loss of sharpness, that can be compensated with sharpening to e certain extent.

Best regards
Erik


Michael makes a point, but according to the excellent tutorial by Osuan and Garcia if Michael stopped down his Phase One IQ180 he would turn his expensive 80 MP camera into an 8 megapixel device with hardly more resolution than an iPhone.

BTW, an iPhone can take nice pictures.

Regards,

Bill
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