There are others more knowledgable about this than me, so hopefully someone will clarify this or correct me if I'm wrong, but the three factors that affect DOF are apperature, focal length and subject (or focus?) distance. So, technically, if the subject is a set distance, the focal length doesn't change (i.e. you keep it at wide angle) and you change the aperature, the DOF will change. However, at short focal lengths, the hyperfocal distance (the subject distance where your far focus reaches infinity) is pretty near to the camera (at least for my camera) - 2 to 5 feet. That means that if your subject is farther away from the camera than that, the effect of changing aperature will be minimal, and not noticable. The near focus distance will change a bit, but your background will focus to infinity and therefore won't be affected.
The two factors that seem to have the greater impact are subject distance and focal length. That's why the two conditions you mentioned - shooting in macro mode, where your subject is close, or shooting at full zoom, were the focal length is longest - will have a greater initial impact on DOF.
Here are a couple of links that might help you get a better feel for the impact of each of these factors on DOF:
Focus Pocus DOF Calculators
DOFMaster - Depth of Field Calculator (also has a link to some articles and faqs)
Hope this helps.... |
