Newby2Cam wrote in post #11841556
Also keep in mind what FL you are shooting at. You don't necessarily need to have the focus point on the head for longer FL.
Longer FL (let's say 200 in this case), even if you focus on the head, there is a good chance the body will also be in focus (greater DOF). And vice versa.....if you focus on the body, the head could be in focus as well.
Shorter FL (let's say 70 in this case), you will more than likely need to focus on the head if you want the head to be sharper (shallower DOF at shorter FL)
I'm not sure I follow this. Are you talking about using different FL to fill the frame the same (thus, directly related to subject distance)? If so, it seems to me that with the same f-stop, the DOF will be the same.
I used one of the on-line DOF calculators and got the following results:
70mm, f2.8, subject distance of 10 feet: total DOF is 0.65
200mm, f2.8, subject distance of 28.6 feet: total DOF is 0.65
Why did I choose 28.6 feet - I assumed if you frame the shot at 10 feet with 70mm, then this is the distance that would get the same frame at 200mm.
I think there is more to the DOF than only the FL you're shooting with. I was aware that faster apatures (lower numbers) give thinner DOF and vice versa, but like I said, I'm not aware of a general rule about FL.