Someone told me:
To compare the amount of DOF of two lenses I just would need to calculate the size of the entrance pupil. The lens with the bigger entrance pupil has less DOF thus creates nicer backgrounds.
(read my thoughts about the focus distance - which is totally neglected - below). I try to understand that. I think it's false. Lets look at two lenses:
Lens A: 100mm F/2.0
Lens B: 200mm F/4.0
Both lenses have an entrance pupil of 50mm. So the statement above claims them to have the same amount of DOF. The question is: At what focus distances? When we compare both lenses at the same focus distance then lens B has a smaller DOF than lens A because focal length has an higher impact on DOF than the F-stop. But it also creates a complete different image of our subject.
To get the same subject magnification we need to adjust the focus distance of lens B by a factor of 2 (since lens B has twice as much focal length as lens A). At that point lens A has a smaller DOF than lens B. Actually the DOF of lens A at a given focus distance is half of the DOF of lens B from double of that focus distance.
So what (correct) statement can be made to compare the DOF of two lenses (with adjusted focus distances) based on the size of the entrance pupil?