← Browse this category
Equipment buying guides

Eyepieces — magnification, field and comfort

An eyepiece magnifies the image formed by the telescope. Its focal length determines magnification only together with that particular telescope’s focal length.

Choose a few useful powers rather than the largest number on a box. Apparent field and eye relief affect comfort.

Objective lens · Eyepiece · Observer’s eye123
  1. 1Objective lens
  2. 2Eyepiece
  3. 3Observer’s eye
Functional schematic, not to scale. Rays and component shapes are simplified; this is not a particular model’s optical prescription.

What is it good for?

Low powers for finding targets and wide views; higher powers for planets, the Moon and doubles in good seeing.

Limitations and trade-offs

Excessive power blurs the view and dims extended objects. Very wide eyepieces may need better edge correction with fast optics.

Check before buying

  • 1.25 or 2-inch barrel and diagonal fit
  • Focal length, field and eye relief
  • Eyepiece weight and edge correction

Magnification M = telescope focal length / eyepiece focal length. Exit pupil = aperture / M, using matching units.

Matching your setup

Example: a 1000 mm tube and 25 mm eyepiece give 40×. With 100 mm aperture, exit pupil is 2.5 mm. Choose that pupil deliberately for extended objects rather than continually increasing power.

True field can be roughly estimated as apparent field divided by magnification. A field-stop calculation is more accurate. A 2-inch eyepiece does not necessarily magnify more: the larger barrel mainly accommodates a wider field.

If wearing glasses, check eye relief. Two or three well-chosen eyepieces can be more useful than a case of overlapping focal lengths. Seeing often limits the benefit of high power.

Documentation and further reading
Browse this category →