Exit Pupil
The diameter of the shaft of light exiting the optic through the ocular lens. The bigger the exit pupil, the better the low light performance. The relationship is geometric, not linear; that is, if you double the exit pupil, you get 4x the low light performance. Exit pupil is calculated by dividing the diameter of the objective lens by the magnification, so optics with large objective and low magnification do better in low light.
A very small exit pupil can make it difficult to align binoculars with your eyes (it's not usually a problem with spotting scopes because you're usually using a tripod). This comes into play somewhere around 3mm. We're not big fans of 10x25 binoculars for this reason, and also because, with such a small exit pupil, they're essentially useless in less than full daylight.

Eye Relief
The distance from the ocular lens to the point where the user's eye can see the full image. Generally, more is better, but more usually comes at the expense of field of view. Eyeglass wearers should choose an optic having at least 15mm of eye relief.

Field of View
The width of the field you can see at a specified distance, usually measured in feet at 1000 yards for binoculars & spotting scopes. Generally, wider is better, but wider usually comes at the expense of eye relief.

Objective Lens
The lens closest to the object being viewed.

Ocular Lens
The lens closest to the user's eye.

Prisms
Almost all binoculars & spotting scopes use one of two types of prism system: Porro Prisms, used in most optics with an offset barrel design; and Roof Prisms, used in most optics with a straight barrel design. Roof prism optics are generally lighter and more compact than porro prism. All else being equal, roof prisms will pass slightly less light than porro prisms, because the light has to bounce off one more prism surface. But the difference is slight, and won't be noticed by most users.
Phase Coating is applied to the prisms in better quality roof prism optics (not applicable to porro prisms) to cancel the phase difference of light passing through the prisms; this increases sharpness, contrast, and resolution.

Waterproof / Fogproof
Optics that are filled with nitrogen gas and completely sealed are waterproof, and will not fog internally due to temperature change. Get waterproof optics if you'll use them in adverse weather, or near open water.

Questions? Contact us!