Things You'll Need
Instructions
Check the specifications for your Plossl eyepiece. You should find the focal length of your eyepiece, given in millimeters. If the focal length is not listed, you can calculate it by dividing your telescope's focal length by the magnification provided by the eyepiece.
Find the focal length of your barlow lens, if you are using one. Add this to the focal length of your Plossl. The resulting number is your total focal length.
Find the distance between the end of the Plossl eyepiece and the eye lens. Many eyepiece eye lenses are recessed into the body of the eyepiece to protect the lens. If the eye lens on your Plossl is recessed, measure the distance. Call this number the recess.
Multiply the total focal length from Step 2 by 0.73. The resulting number is the real eye relief. Subtract the recess number from Step 3 from the real eye relief to find effective eye relief, which is the optimum distance between your eye and the end of the eyepiece.