Here is a BBC infomercial for a recent program(me) on the use of “3D”, AKA stereo, imagery by Photo Interpreters (PIs) in Operation Crossbow, the effort to find and destroy Hitler’s V-weapons.
What makes this interesting to me, is that I trained on a stereoscope exactly like the one shown (I still have it), and I knew PIs who knew Constance Babington Smith (“Babs”), the original “Air Spy” on Crossbow.
To do photoreconnaissance that will give you stereo coverage, you have to fly the mission so that the photo tracks overlap by just over 50%. Anything less and you are missing the stereo effect somewhere on the frame. In fact, they prefer a 55% or 60% overlap, in order to ensure good coverage, and so that you are not trying to interpret something right at the edge of one picture. Think of the plane taking a vertical picture while flying slightly to the left of the road, then coming back and taking a vertical picture while flying slightly to the right. This is exactly what your eyes do, and it’s why your two eyes are in the front of your head, instead of on the side, like a horse. More commonly, rather than shooting two side-by-side images, what they do is set the intervalometer to take a series of vertical pictures in quick succession, so that the overlap is in the direction of flight.
The timing of the shutter is a fixed value, depending on the planning factors for the mission. Actual coverage will depend on things like the true air speed, winds (head/tail/cross), and whether or not they are shooting at you. Here’s coverage of a large domed bunker that was part of a V2 storage and assembly area near Wizernes, in the Nord-Pas-de-Calais, France. The aircraft was flying horizontally across the page from left to right. If I am interpreting it correctly, the bunker can be seen on the R/H image, just in from the L/H edge, and about a third of the way up the photo. The facility had been attacked thirteen times in the four months preceeding these photos, and if you zoom in you can see the bomb craters.
RAF 542 Squadron, Spitfire, Sortie 106G_1181, Frames 3100 and 3101, 30/06/1944. Centre point of image | 50.708143 N 2.247203 E
With stereo imagery you can see the shapes of vertical objects. You can, to a certain extent, ‘see’ underneath camouflage netting, because your eyes tell you the netting is up here, while the object of interest is down there. It can also be useful when the image quality isn’t perfect, and when two parts of the scene have the same shade, making it difficult to tell them apart. On the other hand, overlapping, stereo, imagery, while highly useful, does burn up film. You use at least twice as much to get the same coverage.
And that brings us back to Hitler. There’s a side note that they don’t mention in the article. Not everyone can see 3D with a stereoscope. You have to be able to do extreme un-crossing of your eyes. Essentially, each eye has to be able to look straight ahead, instead of at the slightly converged angle that nature intended them to. I, by the way, am of the other persuasion. If a stereo pair has the proper separation, I can see stereo without the ‘scope. So, the story goes that Hitler couldn’t use a stereoscope — his eyes didn’t unconverge properly. As usual with management, if they can’t see it, it doesn’t exist, and if it doesn’t exist, why waste film (= silver) on it? So, no stereo coverage for the Luftwaffe, which severely limited their ability to see through our deception efforts before D-Day.
If you want to see what more recon photos from this era looked like, here is a good place to start. Why they keep them in Scotland is beyond me.