Gator, all of the above responses are correct.
I think I understand what you are saying/asking. A full frame camera lens produces a 43mm fully-illuminated circle while you only need a fully-illuminated 18mm circle... so why not use an APS-C or even a Micro 4/3 lens instead? With an equivalent focal length and focal ratio, you would achieve equivalently sensor illumination with a smaller optic. BUT, if you COULD cram all the light gathered from a full-frame camera lens into a smaller circle, it should be brighter. And that is actually true. But it would require a different lens design, making it a faster focal ratio while reducing the size of it's image circle. The proof of this explanation: an f:2 full frame 50mm camera lens has a larger aperture (lens) than a 50mm f:2 Micro 4/3 lens... both would fully illuminate the NV sensor, but the Micro 4/3 lens produces a smaller focused image circle. If the Micro 4/3 lens design had the same diameter aperture as the full-frame 35mm camera lens... it would have a faster focal ratio.
What you suggest is a way to reduce the focal ratio of the full-frame camera lens so that light outside the 18mm NV sensor could be re-directed, rather than washing around the sensor, or being blocked by the C-mount obstruction. To my knowledge, no optical remedy exists to restructure the light cone from the camera lens objective, without effecting both the FoV (magnification) and the focal ratio. For example, using a tele-extender (similar to a barlow) on the camera lens will restructure the light cone from the native camera lens. It also changes the focal ratio by effectively extending the focal length of the camera lens. The opposite is true for a camera lens tele-compressor... similar to a telescope reducer. But neither of these devices alters the size of the camera lens image circle. Camera lenses are designed to provide a specific size image circle... originally to illuminate the film plane.
If I understand your post #1 correctly, you would use the camera lens as a prime objective with light focused into a typical telescope eyepiece with a large enough field lens to accommodate the full 43mm image circle, and have it exit the eyepiece as a smaller diameter image circle... effectively reducing the focal ratio of the camera lens for a brighter image (with correspondingly larger FoV). Theoretically, it is possible... realistically, you have a challenge to mate different optics that were not designed for each other, to work together compatibly. But this is essentially what we do when using an NV afocal system... using different eyepieces to achieve different magnifications with corresponding FoV at different focal ratios, which are then used afocally with our NV devices (one optical system looking into another optical system). But telescopes are designed to focus their light where the focuser resides and have a much bigger range of focus. Camera lenses are designed to focus their light at one specific focal plane which is usually only 35mm to 54mm from the lens mount, depending on camera brand, and since telescope eyepieces also have different focal planes, you would need a raft of close coupled adapters to make this work with different eyepieces. Would it be an improvement? I doubt it. Too many additional optical surfaces to consider with the potential for increased reflections and field aberrations, like astigmatism or coma. And, you would not actually be changing the size of the camera lens image circle... it would remain constant. The image would be brighter because a longer FL eyepiece would be used... just as with a telescope/eyepiece combinations.
Does this make sense?
Finally, we use old 35mm camera lenses mainly because they are readily available and often, very inexpensive. They allow us to tailor the focal ratio by using their aperture, AND by using them attached directly to our NVD as the prime objective, they can be filtered behind its mount using 1.25" filters in the C-mount adapter. Used as a prime lens on a NVD without a manual gain control, their aperture ring can be used instead, to control the brightness of the background sky. Yes, a good portion of the focused image they produce washes around the NV sensor, but the sensor IS fully illuminated. And to buy a comparable Micro 4/3 lens, to reduce the size of the optic, would cost a lot more $$$. Old manual, camera lenses remain a cost effective way to achieve short focal length, wide-field images at a fast focal ratio for a bright image.
Edited by GeezerGazer, 05 June 2020 - 11:43 PM.