Find the value of magnification. The value for magnification can be found by dividing the value of the focal length of your telescope by the focal length of the eyepiece. Assuming that your telescope has a focal length of 750 mm and the focal length of the eyepiece is 10 mm, the magnification for your telescope will be 750 / 10 = 75. It is usually denoted as 75X. The focal length for this model is 900 mm.
Divide the value of apparent field of view by the value of magnification to get the value of field of view. For example, if the eyepiece has a value of apparent field of view of 50 degrees and a magnification of 75X, then the value of the actual field of view is equal to 50 degrees / 75X = 0.66 degrees.
