Magnification Useful calculators and formulae.

Notes:

1: Atmospheric seeing conditions (the sky) often limits the maximum usable magnification to 250-350x.

2: An exit pupil size (diameter of light beam as it exits eyepiece) over 7.5mm might be too large for telescope designs with central obstructions (i.e. Newtonian telescopes).

3: Exit pupils less than 0.4mm are impractical because eye floaters and eye-lashes can interfere with the view. They might also indicate the magnification is too high.