Collimation - Binoculars
Collimation refers to the optimal and mechanical alignment of the binoculars. If binoculars are out of collimation, the result will be poor image quality or, for instance, double images. And what is more, poorly collimated binoculars can also cause headaches and eye strain. Binoculars constructed from materials of inferior quality can hardly ever be collimated properly.
It is fairly simple to get an idea of the collimation quality of your binoculars. Just aim the binoculars at the sun (never look directly into the sun through your binoculars). You can then project an image of the sun onto a white screen. The image can be focused using the focusing mechanism of the binoculars. Hold the binoculars very still (use a tripod, if necessary) and aim it at the sun so that the two images of the sun are in the shadow of the binoculars. Carefully move the binoculars until the image of the sun is precisely in the shadow of the ocular. If the collimation of the binoculars is satisfactory, the second image of the sun will fit exactly in the shadow of the other ocular.
A small deviation will not cause any problems. However, if the deviation is too great, it will cause serious problems when used on a daily basis.
The diagrams below are taken from http://sites.google.com/site/rchamon/home/sun-images-method-for-collimation-of-binoculars, where a wealth of information is available to those who are really interested in this topic.