Started Oct 10, 2012 | Questions thread
Tom Axford Veteran Member • Posts: 3,760
DOF and blurred background

There are two different effects that you need to consider to get the sort of pictures you want.

First, the depth of field (DOF) which is proportional to the F number.  You'll halve the DOF in going from F2.8 to F1.4, for instance.

Second, the amount of blurring of a distant background (when you have focussed on something close) depends on the F number inversely.  So the blurring increases when going from F2.8 to F1.4 (it roughly doubles).

However, the background blurring also increases as the focal length increases (it is proportional to the focal length).  So telephoto lenses at the same aperture give more background blurring than standard lenses, which in turn give more than wide-angle lenses.  This assumes that you always have your subject in focus and the same size in the frame (so the longer the focal length, the further away from your subject you need to be).

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