As I understand it, M series stereos are infinity corrected. The bottom objective focuses to infinity, as does the zoom module above it. The binocular head then has a pair of tube lenses at the bottom, which make an image that is picked up by the eyepieces. This makes sense both from what I've read about Leica stereos and with the way accessories introduced in between the zoom body and the binocular head do not increase magnification.
Here is the IC module, with the cover on and then removed:
With the above in mind, here are my thoughts on how this is laid out:
1 is the beamsplitter, which allows half of the light to go through while half is redirected towards the camera.
2 is the tube lens, perhaps identical to the one in the bottom of a binocular head.
3 is a prism to get the light to turn that corner, and maybe to make sure the distance is correct.
4 is a reducing lens, much like any other for a small-sensor microscope camera.
5 is the camera sensor.
6 is an extra prism which does...something? I assume it makes the image in the other eye match, though it doesn't seem to visibly do anything. Very curious.
That all was about what I was expecting, except I was expecting them to use a different tube lens to directly project onto the small-sensor camera, not a pair of lenses. When I remove the camera and hold an eyepiece where the reducing lens (4) was it gives me a nice image that seems to capture the FoV nicely.
My thought was that, since the image is focused to infinity, it shouldn't matter too much how far it travels before it hits a tube lens (I know it does have some effect in tests of high-resolution lenses, but this is a stereo after all and that distance isn't respected by other accessories). So I could either remove the camera and the tube lens, and then introduce my own tube lens on the outside of the camera module to directly project onto my camera sensor, or I could move the tube lens to be just a bit further along the light path so it would project the image somewhere accessible outside of the module.
Both the camera and the tube lens are just held in with some screws, so removing them was very easy. However, it did not perform as I was expecting.
In this configuration:
if I hold my aps-c camera up to the hole where the camera was, I get an image that is not parfocal but easily fills the sensor. But...
In this configuration, I can get a focused image, but it is significantly smaller than the image in the other configuration. It contains the entire field of view, just shrunk down into a circle maybe half the diameter. I can't get it anywhere near the size it was in the first configuration even if I pull the camera way back. Sorry, no pictures since I was handholding it with the top of the module open in a dark room.
This has confused the heck out of me, and I'm wondering if anyone can point out where my thinking is wrong. Worst case I can probably put in some sort of ocular-sized hole so a normal eyepiece adapter can be used, but it's not what I was hoping for.
