Calculate telescope magnification and exit pupil. Choose the best eyepiece for planetary or deep-sky viewing based on your telescope's focal length and aperture.
Enter the focal length of your telescope (usually printed on the tube) and the focal length of your eyepiece (in millimeters).
Magnification is calculated as Telescope Focal Length / Eyepiece Focal Length. For example, a 1000mm scope with a 10mm eyepiece gives 100x magnification.
Note that 'higher' isn't always better. The maximum useful magnification is generally 2x per millimeter of aperture (or 50x per inch).
It is generally accepted as 50 times the aperture in inches (or 2 times the aperture in mm). Beyond this, the image becomes dim and blurry without adding any new detail.
A Barlow lens multiplies the effective focal length of your telescope. A 2x Barlow doubles the magnification of any eyepiece you use with it.
The exit pupil is the diameter of the beam of light exiting the eyepiece. It should ideally match your eye's pupil diameter (5-7mm in the dark) for optimal brightness.
Yes — about the aperture in millimeters divided by 7. Below that, the exit pupil grows larger than a dark-adapted 7mm eye pupil and some of the collected light misses your eye. For a 200mm telescope the floor is roughly 29x, with the maximum near 400x.