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#31
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"Cockpit Colin" wrote in message ... Hi all, I have a nagging suspicion that the focus on my new 20D isn't all that it should be. Do any of you have any suggestions on the best way to test this? I was thinking of photographing a sheet of A4 "Math - graph" type paper. If the camera's focus is out, will it be out consistently when focusing on close and distant objects? Should I have the lens wide open or stopped down for the test. Is it possible for an image to be in focus through the viewfinder, and yet be out of focus at the CMOS sensor? Any other thoughts (I only have the one kit lens at this time, so I can't do any lens comparisons). Thanks for your input. Cheers, CC Stop worrying. If you cannot tell from normal shots then it is fine. But 'geek and ye shall find' However if the measurebator urge is too strong, then go on over to www.dpreview.com and search on bf and ff and you'll find your comrades. Indeed there is a lot of very useful information on dpreview also try www.photozone.de . There are plenty of charts on dpreview which you can get off on - shoot. You should know that it if there is a problem it could be the lens, the camera or both. Some wide angles lenses do not focus too well at a distance. Things look sharper in the viewfinder than on a PC screen on which they tend to look soft and even unsaturated and using a wide-angle cheapo lens will make them look even softer and duller. Also, you MUST post process the images to reveal their potential - that is an art in itself. You need some original images from the *same* camera and lens to compare with yours. Try on www.pbase.com use their search function - look for full-size unprocessed images. Stop worrying. It is most likely you simply don't know much yet... |
#32
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"Cockpit Colin" wrote in message ... "John Passaneau" wrote in message ... At F29 or so almost any lens will be un-sharp. This is due to diffraction from the small aperture. It is not a lens defect, it is a law of physics. At F29 or so the depth of field really means "every thing is fuzzy". The best aperture to check for focus errors would be wide open or if the lens not of the highest quality, at 1 or 2 stops down from the maximum aperture. Also with a zoom lens use the longest focal length available. If your basing the possible focus problem on a shot at the smallest possible aperture of your lens, I would expect it to be fuzzy. Thanks for that. Unfortunately the "nature scene" wasn't intended as a focus test - it was one nice scene that I wanted as a "keeper". I wanted the best possible DOF so I went for smallest aperture - lowest ISO - shot in RAW - put the camera on a tripod - and set the camera for a delayed shot. I knew that a large aperture means poor DOF - I didn't know that small aperture has issues as well (something else to add to my bucket of wisdom!). So far, using an unsharp mask @ 300% @ 0.3 as suggested by Colin D seems to be producing a good result (I didn't know you could select anything under 1 pixel). I'll have to do some more tests to see it F8 to F11 are any better for me. Cheers, Try www.bobatkins.com Dr Atkins has lots of info on lenses, dslrs, etc.... or try www.photo.net which he now runs. |
#33
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David Littlewood wrote: snip and of course f/45 in a 150mm lens is entirely different from f/45 in a 50mm lens, for two reasons; one, that diffraction is a function of the actual diameter of the aperture, and a longer lens has a larger aperture for the same f-number than a short lens; Not true. I used to think this until I actually went and studied the physics behind it, but diffraction is purely a function of f-number, not absolute diameter. Very interesting. I dug out my old Ilford Manual of Photography (that shows how long I've been involved in this caper!) and looked up Airy Disc calculations, and you're right. I am guilty of not reading the formula correctly umpteen years ago. Since d, the aperture diameter, is a component of the equation, I assumed that diffraction was simply proportional to d - but I overlooked that v (image distance) is also in the equation. Since v/d is focal length over diameter, and v/d = f-number, then diffraction is indeed purely a function of F-number. My old Dad used to say "the more you know, the more you know you don't know." After 50 years, I am still learning ... thanks. Colin D. |
#34
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David Littlewood wrote:
Thanks for this. Unfortunately, split prisms and micro-fresnel screens are not much improvement for what I want to do. If this page explains how to remove the factory screen, send to to Maxwell precision optics and have him make you a new screen. I'm don't know if you've ever used one of his screens, they are amazing. Very easy to see the focus plane yet still is a very bright screen. I doubt you'd need any kind of focus aide with it. Give him a call and tell him what your needs are, I'm sure he can come up with a very good solution. -- Stacey |
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