To fully appreciate the stereo window, we must first turn our attention to the means by which our eyes can ascertain depth in a stereo image, that is, homologous points. These are a pair of points, one in each image, which correspond to a single physical point in space in the original scene. Let us look first at a parallel stereo image. Once we remove the light leak in this image we have two image windows one on the left and one on the right.

Now we need to consider our eyes. We have two eyes approximately 6.5cm apart. The way a stern pair of images records depth in through what are called Homologous Points.

To fully appreciate the stereo window, we must first understand, homologous points. These are a pair of points, one in each image, which correspond to a single physical point in space in the original scene.

Consider a camera with lenses or view via a splitter is separated by 65 mm. The homologs which are recorded for very distant objects will be recorded on the film 65 mm apart. The homologs for nearer objects will be recorded on the film plane at a distance greater than 65 mm. When this happens parts of the image appear in front of the window. This of course can be used for artistic effect, but it is why stereo splitters cave separate lenses for less than 2 meter distances and greater than 2 meter distances. Of course if you post an image on social media, for example you have a close tree branch crossing the two edges of the frame you may get someone else declaring a ‘window violation’ and sounding rather smug for spotting it and telling you you must respect the stereo window, not that the Victorians who invented stereo photography cared too much about this.

Think of the frame or the edges of the stereo-image as an open window through which you are looking. Everything in the image should be beyond the window frame, unless the picture contains elements that can come through the window at the spectator. For instance, imagine a postman handing a letter through the open window. The letter and the postman’s arm can come through the window, but the rest of his body cannot because in real life this is impossible. Similarly, the end of a tree branch can come through an open window, but not the trunk of the tree since it is rooted in the ground.

Things are different when these images are projected on a large screen and “window violations” can become painful to look at because projection is unforgiving.

But to help you understand how the stereo window works let us look at a diagram showing what is happening here.

It should be noted having read what I have written above that you can easily change the stereo effect. I use Stereo Photo Maker, a free piece of software to align images.

If the image is coming out of the stereo window and want everything further behind it in the parallel-view format you move the RIGHT image to the RIGHT to push everything behind the window (RECEDE), likewise if you moved the left image to the left it will also recede. If you recede it too much though you’ll cause too much deviation between the two images’ backgrounds, which will not look great in stereo; your brain will struggle to fuse the images and you’ll make a small kitten cry. Try playing with the alignment of your stereoviews until you get used to seeing what is comfortable to view and behind the stereo window.