There is a long-standing myth in photography that focal length has an impact on the depth of field in your scene. This is what I was taught, and it is what I have, in turn, been teaching. In fact, early editions of Complete Digital Photography included this very myth. However, with a simple experiment, you can demonstrate that focal length has no impact on actual depth of field.
However, before you start dreading the need to relearn a bunch of old habits, stop. While the theory that has been taught for the last 150 years or so might have been technically incorrect, the practical upshot has been completely valid. So, this article is not so much about changing your hands-on technique. Rather, it’s simply presented to offer you a more accurate explanation of what actually happens to depth of field when you choose one focal length over another. Your everyday practice—using longer lenses to get apparently shallower depth of field—will still apply, but after reading this article, you might have a different understanding of why the depth of field appears different with different focal lengths.