Originally posted by macTak Finally, wombat2go, can you give us the formula for calculating the entrance pupil?

Hi Mac,

From my reading, I would answer that question as follows:

When a parallel beam of light from infinity arrives at a lens, the entrance pupil is the diameter that gets through the stop and focusses on the image plane.

That diameter is the diameter projected back through the rear nodal plane N2 and onto the front nodal plane N1. ( that is, it is measured at N1)

A real lens may have more complicated paths, and from the geometric optics diagrams it can be seen how the real diaphragm can be smaller than the entrance pupil,

However I think the following always stands, for light from infinity:

By definition, the entrance pupil is calculated by the f/ number the lens is set to

For example a 50 mm lens at f/2, the entrance pupil is 25 mm

A good reference for all this is Arthur Cox " Photographic Optics" and I see it is still available