No, you're supposed to take the focal length divided by the diameter of the entrance pupil of the lens. Only when there is no glass in front of the aperture will that be identical to the diameter of the iris opening.Originally Posted by photomc
Think of it like the glass in front of the aperture is a magnifying glass, so the entrance pupil is larger than the physical opening.
Placing a single lens ell behind the aperture gives better field flatness and less distortion. Try the same cell in both locations, you will be able to see the difference on the negative!