This question is related to this one; I am asking a very similar one because I didn't see in the answers how to arrive to a numeric answer in practice.
Suppose I'm going to shoot a portrait and have most other parameters figured out:
Sensor size: full frame; focal distance: 100 mm; subject distance: 1.5 meters; desired DoF: something like "would like the face to be sharp and the rest look blurred on 8x10 prints".
Now I want to compute the aperture that would help me take that shot. I need to clarify two things for that:
First, I need to make make the notions of "sharp" and "blurred on 8x10 prints" numeric; that is, I have to decide what's the size of the circle of confusion on a full frame that separates "sharp" from "blurred".
Second, I need to assign a numeric value to the depth of focus that would make the face sharp and the the wall behind the subject blurred.
Thus the questions:
What numeric values for the circle of confusion correspond to the above scenario?
What DoF values are appropriate for portrait shots?