I'm in the midst of preparing a talk on cameras which take photographs through mirrors. I don't mean mirrors in viewfinders and rangefinders, to qualify they must take the actual picture through one or more reflections. So this includes such things as stereo accessories which fit on the front of an SLR lens, mirror lenses themselves (catadioptric lenses) and many others. One of them is the ever-amazing Polaroid SX-70. This does indeed use a mirror to reflect the final image that's recorded, but even more fun is the optics of the viewfinder, so I took a picture of that using a semi-dismantled scrap SX-70, a laser pointer and a judicious touch of artificial smoke.
The beam enters lower right, passes through the main lens, reflects in a plane mirror down to a fresnel mirror (with split-image rangefinder feature) which is on the upper surface of a movable flap, presently covering the filmpack. From there it is reflected back to the upper mirror again, and across to a concave (magnifying) mirror in the viewfinder housing. From there it passes through the viewfinder lens and on to the eyepoint. When the shutter release is pressed, the flap carrying the fresnel mirror swings up to cover the sloping plane mirror. There is another plane mirror on the underside of the flap, so we still have a mirror under the large sloping surface of the camera. The light coming in from the lens is thus reflected downwards to the now-uncovered film surface. And still in focus, because although the mirror is now a few millimetres lower, because of the thickness of the flap, the receiving surface is also a few millimetres lower - presumably all calculated to be an equal distance!
Although I have seen diagrams of this before, I have never seen it in live action like this, so I thought it should be shared.
An occasional and irregular blog, mostly of photographic experimentation and photographic history.