I've done a quick search and couldn't really find anything related, so here comes a new thread.
A comment someone made in the PAD thread got me thinking. The comment in question stated that a shot they'd taken at f2.8 will of had an in focus area about 1.25" deep (assume unit is feet rather than inches).
Is there a constant guide that you can use to calculate what area of the shot is going to be in focus in this way? I.e. if I'm trying to get a shot of a couple of chaps talking to each other, but I'm not level with both, one being nearer and one being farther away - but for example, say they're both within a 3 feet band of each other. Is there some calculation I need to be aware of and mentally making to adjust the f-stop accordingly?
This relationship will change I'm sure dependant on sensor size (DX vs FX) but what about lens? Is it a constant calculation for DX across all lenses for example?
I hope that makes sense, because I'm at home recovering from knee surgery at the moment and still very much feeling the affects of the general anaesthetic, so forgive me if this is nonsense!