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05/09/2009 09:47:07 AM · #1 |
Just went back after a long time to have a look at the Ken Rockwell website (please don't open a discussion about him). He has an article about throwing away your tripod (that's silly of course) because when you use small apertures, you get soft focus because of diffraction. Like when you almost close your eyes.
Of course he has a point there for not using a small aperture when not necessary. But when you need the depth of field, I guess that it's much easier to get rid of the soft focus from diffraction with unsharp mask than if you narrow the DOF. Did anybody try this out? |
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05/09/2009 10:30:46 AM · #2 |
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05/09/2009 11:30:10 AM · #3 |
I haven't tried it, but diffraction will cause a softness right across the image as opposed to just in the OOF areas so in theory an unsharp mask should help a lot better with diffraction softness than with DOF softness.
BUT
I can't think of a situation where you would want to close the aperture down that much, other than if you wanted to do a long exposure, or you're trying to get some DOF in a macro shot. Generally with other lenses you will get an almost infinite DOF before you get to the point where diffraction really becomes a problem. |
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05/09/2009 11:35:09 AM · #4 |
| It's rarely necessary to stop down so far that diffraction is that big an issue. You should be able to pretty confidently use f/16, although at that setting you probably will find diffraction effects if you closely compare to the same shot at f/8. Sharpening to compensate for diffraction softening will never bring back detail that wasn't recorded, and is a poor practice. For wide angle work, the combination of optimal aperture setting, say f/11, and proper use of hyperfocal focusing technique will give you sharpness from a few feet to infinity. |
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05/09/2009 11:42:56 AM · #5 |
Originally posted by kirbic:
It's rarely necessary to stop down so far that diffraction is that big an issue. You should be able to pretty confidently use f/16, although at that setting you probably will find diffraction effects if you closely compare to the same shot at f/8. Sharpening to compensate for diffraction softening will never bring back detail that wasn't recorded, and is a poor practice. For wide angle work, the combination of optimal aperture setting, say f/11, and proper use of hyperfocal focusing technique will give you sharpness from a few feet to infinity. |
Ummmm... I don't have the actual numbers to hand and am too busy to calculate 'em right now, but this diffraction issue is related to the physical size of the aperture, not the f/stop used... So f/11 on, say, a 16mm lens (quite WA) is gonna be a very small aperture and have quite a lot more diffraction than f/11 on, say, a 100mm lens.
Or to put it another way:
16mm lens, f/11 = 1.5mm diameter aperture, very small indeed.
400mm lens, f/32 = 12.5mm diameter, quite a bit larger.
So as you increase focal length, you can use progressively smaller f/stops to gain DOF (in fact you NEED to) without fear of diffraction causing critical loss of sharpness. On the other hand, I almost never stop my 10mm down past f/8, cuz I can see the difference when I do.
R. |
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05/09/2009 11:59:11 AM · #6 |
Originally posted by Bear_Music:
Ummmm... I don't have the actual numbers to hand and am too busy to calculate 'em right now, but this diffraction issue is related to the physical size of the aperture, not the f/stop used... . |
Actually, the diameter of the Airy disk (which is the preferred method of defining whether an optical system is diffraction limited) is proportional to the f-number, and *not* the physical aperture size. See this discussion for the basics, and see the fifth post in this thread for the math. The first link is a really great discussion, I just wish they would have included the math there for completeness. |
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05/09/2009 12:24:14 PM · #7 |
Originally posted by kirbic:
Originally posted by Bear_Music:
Ummmm... I don't have the actual numbers to hand and am too busy to calculate 'em right now, but this diffraction issue is related to the physical size of the aperture, not the f/stop used... . |
Actually, the diameter of the Airy disk (which is the preferred method of defining whether an optical system is diffraction limited) is proportional to the f-number, and *not* the physical aperture size. See this discussion for the basics, and see the fifth post in this thread for the math. The first link is a really great discussion, I just wish they would have included the math there for completeness. |
From your link:
"Since the physical size of the lens aperture is larger for telephoto lenses (f/22 is a larger aperture at 200 mm than at 50 mm), why doesn't the size of the airy disk vary with focal length? This is because the distance to the focal plane also increases with focal length, and so the airy disk diverges more over this greater distance. As a result, the two effects of physical aperture size and focal length cancel out. Therefore the size of the airy disk only depends on the f-stop, which describes both focal length and aperture size. The term used to universally describe the lens opening is the "numerical aperture" (inverse of twice the f-stop). There is some variation between lenses though, but this is mostly due more to the different design and distance between the focal plane and "entrance pupil.""
While I follow all this, to a certain extent it's contrary to my own experience, hands-on. That is to say, I lose sharpness dramatically at f/16, even f/11, with the ultrawide, and don't seem to lose much, if any sharpness at f/22 with the 200mmm, just to give two examples...
R.
Message edited by author 2009-05-09 12:27:59. |
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05/09/2009 01:32:55 PM · #8 |
| Many thanks for your answers! |
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