I'm curious about how my "area of acceptable focus" changes when the focal length of the lens I'm using changes as I zoom (or switch lenses). In particular, I'd like to know how the front & back focal planes change, thus changing the depth of field and the minimum focus distance.
Answer
While it is a fact that changing focal length from shorter to longer reduces DOF and using a smaller (less light) aperture in will increase DOF (providing format is identical) however there is a simpler way to think of it.
DOF decreases the larger the subject is in the frame regardless of the lens and increases with smaller apertures.
Example: If you shoot the same photo, say a headshot, with a 200mm lens and, at the same distance, with a 35mm lens. Then take the image from the 35mm and crop it to match the image from the 200mm you will find the DOF/image identical.
Of course this is an example assuming that the resolution would not be factor. Which is WHY we change lenses and don't just crop.
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