Looking at my own Mitutoyo 10X, I think you have made just a couple of small mistakes. First is that you assumed the entrance pupil is located on the lens side of the focal plane. The formulas do not distinguish between the two cases. Second is that when you intended to focus on the limiting aperture, I think you actually found a different aperture that is limiting when you look from far back, but would not be limiting from the standpoint of a focused subject. To see the actual limiting aperture from far back, you have to look from off-axis so that your line of sight to the aperture passes near the center of the focus plane. Or alternatively, let the lens serve as its own projector as I described earlier for the Mitutoyo 5X.michael_r wrote:From this i would like to conclude that the entrance pupil lies approx. 4 cm in front of the objective, which is in contradiction with my computation using conventional theory.
Using the projection approach and measuring with a ruler, I find the pupil projected about 7 cm in front of the objective, with a diameter of 24 mm. Using a rear pupil of 12 mm, that gives a pupil ratio of 0.5 (rear/front), which according to theory should place the entrance pupil at a distance of 20/0.5 = 40 mm away from the focus plane. This lens has a working distance of 33.5 mm, so 40 mm away from the focus plane, on the side away from the lens, would be a total of 73.5 mm = 7.35 cm from the front of the objective.
This all seems consistent, to within measurement error.
I suggest to take another look and see if you see the same thing. I will be very interested if you find something much different.
--Rik