Quite a while ago now, I had read the luminous landscape article on depth of field that presents the rule of thumb:
if the subject image size remains the same, then at any given aperture all lenses will give the same depth of field
After reading that, I had always thought of the main differences between long and short focal lengths in taking a portrait photo as:
- short focal lengths give more distortion (noses pointed toward the camera get big)
- long focal lengths mean you could stand far away from someone and act like a spy.
But there's another difference -- one which may actually be the thing behind the common belief that long lenses have a smaller depth of field. That difference is that a longer telephoto lens has a much smaller field of view than a shorter wide angle lens.
If the reason that you're using a large aperture on a shot is because you are trying to remove background elements from the photo (by blurring them out), then it seems like it will usually also use a long focal length in such a shot, because doing so will remove even more background elements from the photo (by "blowing up" the background, allowing only a smaller amount of it to show up in the frame).
