A little more on DOF: It's the range of distance from the camera lens (actually from the lens' optical center) where objects are acceptably in focus. DOF is a complex product of photography, presentation, and perception: beside what you do with the camera, how the image is displayed and how people see it makes a difference. But you can't control presentation and perception when you shoot. There ARE factors you CAN control:
* For THICKER DOF, use a shorter lens and/or tighter aperture and/or further subject-lens distance
* For THINNER DOF, use a longer lens and/or a wider aperture and/or closer subject-lens distance
Those rules tell us a few things. A short focal-length lens has thick DOF, and when its aperture is stopped down, everything from near to far looks sharp. Most P&S cameras have very short lenses, often fairly slow (tight apertures), thus their crisp-looking pictures. And shooting macros at very close distances gives very thin DOF. So when shooting teeth, it's very hard to get more than one tooth in focus unless you stop-down the aperture quite a bit. This often requires flash, to have enough light for a fast handheld exposure.
You can also influence DOF by subject-background placement, and lighting. DOF seems thicker if the subject and background are close, and thinner when they're further apart. And DOF seems thicker if subject and background are about the same brightness, and thinner if the subject is lit and the background is dark. These are tricks of subject isolation, to make the subject stand out from their surroundings.
As mentioned, stopping-down to a tight aperture isn't always a good idea. Light bouncing off the edges of the aperture's iris blades causes diffraction, an increased fuzziness. This might not be noticeable is the image is displayed small but becomes pronounced with enlargement. Lenses on our dSLRs are usually sharpest at f8-11, acceptable sharp up to f/18 or so, and rather fuzzy from f/22 onwards. Again, how well the subject is lit can make an apparent difference.
Lens choices have already been covered above. I'll just note that a zoom with a constant f/3.5 maximum aperture is only slightly slower, with only slightly less DOF control, than a f/2.8 zoom, but may be considerably less expensive. I'm a cheap bastard; I notice stuff like that. But f/3.5 AF zooms are rare. Oh bother.
Last edited by RioRico; 10-20-2011 at 08:15 AM.
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