How to Free-Fuse Stereo Imaging
Each stereo view consists of two images, one for each eye. Free viewing is the technique that will allow you to direct each of these images separately and simultaneously into each eye without using a stereoscope. If you have presbyopia and need reading glasses, put them on.
The image to free fuse looks like this:
Cross your eyes, so that the pair of images will double to four, and it may be out of focus. At some point, the two pairs of images you are seeing will begin to overlap. Another way to accomplish this is to use a pencil or fingertip laid on the paper at the black bar. Bring the pencil forward while focusing on the tip keeping it clear. The background images will blur:
The part of the image in color indicates the beginning of an overlapping view. That area of overlap is where you will fuse the left and right images of the pair. Continue to bring your pencil or fingertip towards you until the fusion effect (while still blurry) looks like below:
At this stage below, your eyes are crossed just the right amount (convergence) but the focus is off (accommodation).
You must dissociate the focus (accommodation) from the convergence while keeping the center image fused together. Try tracing the details such as blood vessels or the rim of the optic nerve head to help you gain a clear focus. Continue to keep your eyes crossed. In a few moments your visual system will redirect your eyes to focus on these details without uncrossing. When you have achieved it, you should be able to perceive the 3 dimensional depth of this optic nerve head cup.
This technique can create eyestrain as accommodation and convergence are not designed to work at differing distances. Some people cannot simply achieve the stereopsis effect. If you have amblyopia, strabismus, are monocular or suppress one eye then you won’t be able to see in 3D.
Stereo viewers with variable prism greatly assist fusing these images. With practice this free fusion technique develops more quickly.