One point here, when talking about the N = f/D formula to calculate the f-number, D is entrance pupil diameter. That is, it is the apparent diameter of the imag

One point here, when talking about the N = f/D formula to calculate the f-number, D is entrance pupil diameter. That is, it is the apparent diameter of the image of the aperture when viewed from the front of the lens, not the actual aperture diameter.