45 deg partial mirror in image path. How/why does it work?

[Beatsy]

I recently bought an old Zeiss polaroid film carrier for the Prontor mount and shutter mechanism that was attached to it. It works as a 0.25 to 0.8x focal reducer too - which was the main attraction. Anyway, with a bit of bodgery, it works well to give me an extra imaging path on my ICM-405 (through the trinocular head as well as the front photo port).

In removing the unneeded bits, I came across the component pictured. It's basically a 45 degree mirror that sits right in the image path and directs some of the light to an off axis sensor for auto metering and exposure. The light destined for the sensor/film goes straight through the mirror and out of the hole in the back.

I was very surprised to see that the glass is 0.7 mm thick!! Surely that must have an effect on image quality. Shouldn't it? Or is it special glass? The obvious thought is shooting through it on the macro rig and using the mirror to direct light in from the front (filling in the "lens image" you always get in insect eyes for instance).

I will be experimenting with this of course (but only high mag objectives given the size), but I'm about to go out for the rest of today. So thought I'd solicit some comments in the interim.

[Lou Jost]

Is the set-up somehow creating an "infinity space" between two component lenses, with the mirror in that space?

[ChrisR]

I've noticed that an Olympus CH30 and Nikon Diaphot both have a very thick ~1cm lump of glass in the back of their turret. I don't know how that works either!

[Beatsy]

Is the set-up somehow creating an "infinity space" between two component lenses, with the mirror in that space?

The mirror sits directly above the ocular with no other lenses between. There are two lenses "after" the mirror that do focal reduction (and maybe something else too). If the light coming from an ocular is already "infinity space" (I don't know if it is or not), then I guess it does.

[Lou Jost]

I don't know about that arrangement, but if it is in the infinity space, the thickness doesn't matter at all, so that's why I suggested that it might be.

[Pau]

Is the set-up somehow creating an "infinity space" between two component lenses, with the mirror in that space?

The mirror sits directly above the ocular with no other lenses between. There are two lenses "after" the mirror that do focal reduction (and maybe something else too). If the light coming from an ocular is already "infinity space" (I don't know if it is or not), then I guess it does.

Yes, when working at its right distance of the objective the image of the eyepiece is infinite focused, the "0.25X" upper lens (63mm FL) focuses it onto the film/sensor, so it is an infinite space indeed. This is what we call afocal photomicrography

[Pau]

I will be experimenting with this of course (but only high mag objectives given the size), ...

Place it behind the objective, ideally inside the infinite space if you have it