Depth of field decreases as the subject to imaging sensor distance decreases. So, when you are working at close range with a long lens, near minimum focusing distance, depth of field is at a minimum and is usually measured in tiny fractions of an inch. So when you are close to a small bird or creating tight head portraits it is generally best to stop down considerably.

The corollary is that depth of field is far less of a concern with when working with relatively distant subjects. artie

Sorry, that has nothing to do with it 🙂