Fast Telephotos, front element size, and EF-S questions...

[wickerprints]

The entrance and exit pupils are not something that actually exist as physical objects. Only the aperture stop itself is a real object. The pupils are simply the images of the aperture stop as viewed from the corresponding end of the lens, and as such, they can appear at any position behind the end of the glass you're looking at. The entrance pupil can, for instance, appear behind the front element at a distance that is greater than the physical length of the lens.

That said, the entrance and exit pupils are in fact optically constrained by the diameter of the glass in front of them. That is to say, an entrance pupil of 100mm cannot be produced if the front element is only 90mm. It is impossible, in as much as it is impossible for the entrance pupil to appear in front of the front element.* If you reduce that front diameter through the introduction of a secondary stop, you will reduce the amount of light entering the lens, and therefore, the f-number of the system will change. Whether or not this results in noticeable vignetting depends on how the lens is constructed and where you put the field stop. In some cases (frequently, telephoto lenses), what you would see is simply a change in exposure. In others (e.g., wide-angle lenses), you would see hard vignetting whilst the central portion of the image remains unaffected.

Something else that you should consider when conducting your tests is that the f-number of the system pertains to the behavior of the lens when it is focused at infinity. If you take your telephoto lens and look through the front element from a few feet away, the entrance pupil will appear a certain size. If you set the lens down and walk away from it, the entrance pupil will seem to grow in size until it appears to take up the entire front element's diameter. This is because when you look at the lens, the light rays that form the entrance pupil correspond to the effective f-number of the system, not the relative f-number. When you are close, the light rays emerging from the lens and reaching your eye are not parallel and the actual light-gathering ability is less than what the nominal aperture would suggest. When you are far, the light rays are nearly parallel, and this corresponds to the case where N = F/D. The reason I mention this is because if you were to conduct your tests by imaging a subject at a short distance, you would not notice a significant change in exposure with the step-down ring because the effective f-number has already been increased. If you shot the lens at infinity focus, the camera absolutely would see less light if you blocked the periphery of the front element with step-down rings.

As for your question regarding the relationship to sensor format size, a smaller sensor could make part of a telephoto lens smaller but not the front element. With a shorter back focus distance and a smaller image circle, the lens could be made a bit more compact, but not drastically so, if the f-number is to remain fast.

*When I speak of the pupil diameter, I am speaking of the size of the pupil as it appears to us (or the camera sensor), not as it actually is. For example, a pupil that is 100mm in diameter at a distance of 50cm away looks the same size as a pupil that is 200mm in diameter at a distance of 100cm away. The light cone they produce has the same angle. A camera sensor cannot tell the difference between the two. Thus, you can have a pupil that is infinitely large yet infinitely far away, but it still appears to us to have a finite size (e.g., telecentric lens) and thus a finite light-gathering capacity. As the pupil diameter pertains to this capacity, we are therefore interested only in the angular size of the pupil--how large it appears to us, not how large it actually is, in relation to its distance from us.