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One of the first things we learn in photography is about aperture and depth of field and we all rail on about how a big number means small aperture and vice versa and then getting confused. Once we get it, we start isolating everything with narrow depth of field. It’s not a bad way of getting a handle on photography, working in aperture mode is perfectly acceptable.
Shutter-priority has advantages with birds and fast-moving objects and where depth of field may be a secondary concern.
In this shot of Orange Hawkweed my depth of field is very narrow. Had I been paying attention to details, I probably would have missed this shot because my tendency is to try for a larger depth of field for more realism in photographs like this.
At 420mm (300mm with a 1.4 tele-converter) f 7.1 is a relatively large aperture with a modest depth of field. The compression (telescope effect) of the lens combined with the fstop makes for a narrower depth of field at minimal focusing distances. Understanding aperture and depth of field is one thing, understanding them with respect to the lenses you use is another. It’s worth doing some testing to learn what happens at minimal focal lengths and infinity.
The same happens with large fstops where you would assume everything is in focus, when actually something less than your largest fstop and a distance shorter than infinity will give you a hyperfocal distance (meaning everything from your minimal focus distance to infinity is in focus). You need a special calculator to determine hyperfocal distance and there are many tutorials on-line to help with that (as well as apps for smartphones).
In short there is more than one chapter in the book on depth of field….