Telescope Magnification

It only takes focal length and eyepiece focal length, so to compute aperture-dependent numbers like exit pupil, Dawes limit, and light-gathering power it assumes a typical f/8 focal ratio and backs into aperture as focal length ÷ 8. If your scope is a faster f/5 reflector or a slower f/10 refractor, the assumed aperture — and every value derived from it — will be off, so use those specific figures as a rough estimate unless you can enter your real aperture into a calculator that supports it.

True field of view is calculated as the eyepiece's apparent field of view divided by magnification, and magnification rises as eyepiece focal length falls. So a shorter eyepiece boosts power but proportionally narrows the patch of sky you can actually see through it — there's no way around that trade-off with a fixed apparent field of view.

Exit pupil here is the assumed aperture divided by magnification, so it shrinks as you increase power with shorter eyepieces. For planetary and lunar detail, 2–5mm is typical since a smaller cone of light on a bright target minimizes glare; for faint deep-sky objects, you want it closer to your dark-adapted pupil size (up to about 7mm) to gather as much light as possible without wasting any past what your eye can use.

Max useful magnification is estimated as roughly 2× the assumed aperture in millimeters — beyond that point you're just enlarging the same amount of captured light and existing blur (from diffraction, seeing conditions, or optical imperfections), not resolving any new detail. The image gets dimmer and mushier rather than sharper, so it's rarely worth owning an eyepiece that exceeds this number for your telescope.