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Image Scale - Image scale is the physical size that an object appears at the focal plane. It is a function of the object's angular size (how big it appears) and the focal length of the optical system. Therefore, for a given object size, image scale is determined by focal length and a longer focal length provides more magnification. Image scale is usually measured in terms of some angular size per physical size. For example, image scale might be measured in arcseconds per micron. An image scale of 2 arcseconds per micron means an object with an apparent size of 2 arcseconds will appear 1 micron wide at the image plane. This convention is used currently because CCD camera pixel sizes are measured in microns. In the days of film photography, arcseconds per millimeter was a more common unit if measure.

Image Shift - When focusing a telescope, the image may shift slightly due to certain components of the telescope moving from side to side. This is most often seen in a Schmidt-Cassegrain telescope (SCT) which uses a movable primary mirror to focus. Poorer quality rack-and-pinion focusers may show shift as well, but good ones will not. Accessory focusers are sometimes used on SCTs to eliminate image shift.

Image Stabilization - Certain binoculars (notably those from Canon) have an integrated image stabilization system that electronically compensates for the shake induced by hand-holding binoculars. This makes high-power viewing possible without the use of a tripod. Many cameras also have an image stabilization feature.

Interocular Distance - This is spacing between an observer's eyes. Binoculars and binoviewers have an interocular adjustment to accommodate different observers.