Just out of curiosity, I looked up depth-of-focus calculation today. Leaving aside the confusion, of many, with depth-of-field, I am trying to understand how to apply the formula, which is not as exotic as I imagined. What I found is:

t = 2Nc(v/f)

In this equation, t equals the total depth of focus, N equals the lens f-number, c is the circle of confusion, v equals the image distance, and f represents the lens focal length.

Questions:

1. What circle of confusion would I be using for my 135mm (4x5 neg)? would it be the same as my favorite online depth-of-field calculator's, i.e., 0.108 mm?

2. Is v the distance from the middle of the lens to to film as i sits in the neg carrier?

Thanks.