Okay, back over on the hot topic about the strange radial lines SE of Marfa (they're cattle trails folks), Shanea commented that sometimes things don't just jump out at you and he noted the fuzzy/wavy artwork of a decade or more ago that some found especially tricky to resolve.
The topic deserves its own spot...so here we go.
First, a short history and a primer...very non-technical.
Stereophotography has been around almost as long as cameras. Somebody figured out that if you took two slightly different angles of the same view you could do with photos what your eyes do to the world around you...render it in three dimensions. Once this was known, 3D photos became a social rage for a time in the 1800s with stereopticons (viewers) being ubiquitous. You can still buy old stereo views at antique shops and there are reproduction stereopticons available today to view those vintage scenes. Stereo photography quickly grew from a novelty to a valuable tool. One of its greatest expressions lies in aerial photography where the ability to perceive depth has numerous applications. These kinds of photos are typically shot on 9" color transparencies, but there are thousands of black & white prints from earlier efforts. These prints are so large that a stereoscope is used to make it easy to view, plus it adds the ability to magnify the image.
However, you do not need a scope to view these or most any other stereo pair. You just need practice.
The vast majority of the public is familiar with the Viewmaster 3D wheels. A Viewmaster is nothing more than a primitive stereoscope and it is needed because the 3D wheels have such tiny images that you could not view them without the magnifier.
When you look through any stereoscope (and most folks who took a basic geology class in college probably had the next step up in quality...a cheap folding plastic stereoscope that was entirely adequate for the intended purpose) all it really is doing is isolating the images so the left eye only sees the left image and so on for the right eye. Any magnification is a bonus, but not needed for depth perception. This separation-of-view aspect is important later own down the page.
Now, if you have large prints but no stereoscope, you can just hold them up about arms length and force yourself to see only one image with each eye, just like the stereoscope does. Initially, this can be difficult to do and a certain amount of eyestrain will probably result. It won't hurt you so take a break and try again. Once you get the hang of it, you mostly will never bother with the scope again. Whether doing it unaided or with a scope, the images MUST be in vertical registration otherwise it will be difficult or impossible to resolve. Horizontal registration is almost a non issue as the worst that can happen is you hold the photos too far apart. When trying to bring the photos into 3D if helps, if hand held, to move the images slightly and slowly up and down with respect to each other. You want to fixate on some prominent feature and the slight motion will cause the image to 'pop' rather quickly. If using a scope just slightly rotate the device back and forth around the center point and the same effect will occur.
Depth perception in real time is a function of your interocular distance...your eyes are a couple of inches apart and that is enough to produce depth. However, if you move the camera only 2 inches you will faithfully reproduce that distance and create some awfully dull images. This is where vertical exaggeration comes into play. While that is a term referring to the apparent relief you see in an aerial photo, it applies as well to horizontal shots of landscapes. Because the 'interocular' distance of an aerial camera can be very long due to aircraft speed...and the images are typically shot several thousand feet up, you get some incredibly impressive relief features when looking at the images. The exaggeration can exceed a factor of 10. It makes you wish you could live in that very vertical world where tiny depressions start looking like major canyons.
If you do this much exaggeration on the ground, you will get grotesquely distorted features and any people in the photos will end up with impossibly long arms if they are oriented across the view on a 45 degree angle or so. I have found that shifting about shoulder's width....basically leaning over the left foot and then rocking over to the right for the second image, will produce a very pleasing effect.
This brings us to the images below. These are all stereophotos shot with a digital camera, and handheld. There are tripod rigs to precisely shift the camera and there are even some really old 35mm stereo film cameras that will do it all with one shot. For me, that's just too much stuff and since I do this for my own entertainment, a little flaw here and there is acceptable. There are challenges to shooting handheld and taking two shots to create the 3D image. Here are the main bugs....people in the photo move slightly or grossly between shots (you have to warn folks to stand still while you do this), wind moving vegetation, objects in the foreground being too close to the camera and thus never being able to bring them into 3D when viewing (and it is very distracting when looking at the rest of the scene), excessive 'interocular' spread, loss of vertical registration between shots (a gridded viewfinder is a great help), lighting conditions change rapidly (clouds blowing over).
All of these things can be dealt with and most are solved through practice. You just learn what works. As an aside, transparencies and digital images can be successful projected, but you need two projectors and polarizing filters for each projector to match the glasses viewers will wear. You also need a metallic screen as polarizing filters do not significantly affect specular reflections (which is what comes off a metalized screen) thus the polarized image that hits the screen is reflected as polarized light, allowing your glasses to work correctly rather than filtering out the reflection as they would on a water reflection. A lenticular screen is what you need, but a very useable screen can be made by applying aluminum spray paint to a piece of masonite.
Okay, now it's time to look at the photos, but there is one last piece of information. If you look carefully at these images it is possible in a few cases to see that
these are not left and right pairs but, instead right and left, meaning they are swapped side-to-side. In playing with stitching the images together and viewing them on a monitor, I found it was easier to resolve them by basically viewing them cross-eyed. The reason you don't do this when looking at prints or through a scope is that you can control what each eye is doing. With the images directly next to each other, it is much harder to do that. So, sit back about an arm's length from the screen and stare at the stitch line. Move your head in and out and even slightly side to side (like rotating the scope). You want to cause a middle image to appear and then you have to concentrate on matching a common point. Once you do, you will have 3D. If you've never done this before you will initially find that the effect slips and you have to work to get it back. With practice you can keep a lock on it and if you had several photos stacked vertically on the screen you could even move among them without losing the effect. And since that is they way they appear here, you can try that out as well.
With that out of the way, here are 8 images of secret places few have seen (none of them are in a park, so there are no trails and no people). Enjoy, they are rather nice if I may say so.
