This is a video explains why focal length does not affect depth of field. Most people learn there are three variables that effect DOF, Aperture, Distance to Subject, and Focal Length (including myself). However, there are really only two variables that effect DOF and focal length is not one of them. Focal length only gives you the perception of a shallower DOF, but in reality your DOF does not change with focal length, it remains the same.

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