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02-09-2012, 10:01 PM   #16
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one important difference i never see mentioned is lens hoods. a lens hood designed for ff will be shorter than one designed for aps-c, so if you use a ff lens hood with aps-c cameras you get extra light coming in at the edges, causing extra lens flare and lowering contrast. if you use an aps-c lens hood on a ff camera you get vignetting.

02-09-2012, 10:46 PM   #17
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QuoteOriginally posted by atnbirdie Quote
So it seems that so long as I use my APS-C camera, I will get the same results irrespective of the basic design of a lens (FF or APS-C) so long as the focal length is the same.

Have I finally got this right?
Yes, all other things being equal, this is correct.
02-10-2012, 09:23 AM   #18
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All good info above. We can use the same lens on medium-format, full-frame 35mm (135/FF), half-frame (135/HF = ~APS-C), and even m4/3 cameras, and the only difference is that smaller frames crop more of the projected image. If their images are shown at the same magnification, which is only a function of focal length and lens-subject distance, then DOF is exactly the same. This is illustrated in anthony mazzeri's post above.

DOF *is* affected if those images are enlarged by different factors, say to fit a same-size screen or page. DOF is also affected if we try to shoot the same subject whilst filling the frames for each of those formats, because the lens-subject distance will vary with each. And perspective is determined solely by lens-subject distance, so the perspectives of those frame-filling shots will differ.

There is one important adjustment to be made when shooting an FF lens on a HF / APS-C camera. The DOF scales inscribed on the lens are calibrated for FF. For predictable DOF on an APS-C cam, adjust those scales by one f-stop, or just a little more for paranoid hyperfocus. (Hyperfocus is zone-focus where the zone reaches infinity.) So if I set the aperture to f/11, I read the scales at f/8; and if I want infinity DOF, I set the infinity mark to around f/7, just to ensure that infinity *IS* in focus. Paranoid hyperfocus, eh?

My favorite examples: I put my 21mm M42 lens on my camera, set the aperture to f/11, set the infinity mark just inside the f/8 mark; now I'm prefocused to 2m, with DOF from 1m to infinity. That's hyperfocus. For a different zone focus, I put a 50mm lens on the camera, set the aperture to f/11, prefocus to 5m, and read the DOF scales at f/8 -- DOF extends from ~4m to 9m, that's the zone. I can be sure that subjects within that zone will be acceptably sharp. Those setups are good for different types of 'street' shooting.
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