Depth of field and focal length
Depth of field doesn’t change with focal length – though it looks like it
■ It seems counter-intuitive, but depth of field doesn’t change with focal length. And yet we all experience what seems to be shallower depth of field when we’re using a telephoto lens, so what’s going on? It’s because telephotos are normally used to magnify a distant part of a scene, and wide-angle lenses to take in more of it.
If you were to shoot the same part of the scene by moving forward with the wide-angle or stepping back with the telephoto, the depth of field would be identical. So, while strictly speaking focal length isn’t involved, in practice, because of the way most of us use wide-angles and telephotos, it does matter.