What is stereographic photography?

Answer 1

Stereographic Photography is a system for giving the appearance of 3D images using two 2D (conventional) images. Two images are taken from slightly different points (separated by roughly the distance between two eyes; around 4 inches).

Ideally, the two images are taken with two identical cameras with identical settings, but it is possible to get acceptable images using the one camera and simply moving it by a few inches between shots.

With the two images available, some way is used to present the left image to the left eye and the right image to the right eye. There are several techniques:

The "simplest" is to use a stereoscope - this device looks a little like a pair of binoculars; essentially it's two tubes, one for each eye, and the left image is at the end of the left tube, the right is at the end of the right tube. When held up to the eyes the images combine to form the stereoscopic image.

Another system combines the two images into a colour separated single image (nowadays using software) which is viewed using special glasses with coloured filters so that the left image is only seen with the left eye, the right image only seen with the right.

A similar system uses polarised glasses - the two images are projected simultaneously using a polarised filter (say horizontal in front of the left, vertical in front of the right) when viewed with matching glasses, again the left image goes to the left eye, the right image to the right.

There are also "active" systems - the two images are projected alternating; first left, then right. Special glasses blank first the right eye then the left in synchronism with the projection. Provided the alternation is fast enough, the brain will combine the alternating image into a single 3D image.

It should be noted that so called 3D films and images produced using these techniques aren't actually 3D; they're stereoscopic. In a true 3D image (such as a hologram) you can choose to focus on any part of the image and the eye can bring it into sharp focus. That isn't the case with a stereoscopic image - one of the reasons that some viewers report headaches when watching "3D" films is that the image is actually flat - projected onto a flat screen, but the eye "tries" to focus at different distances because it thinks it's seeing a 3D image, this creates eye strain.