A 2″ eyepiece delivers a wider true field of view than a 1¼” eyepiece of the same focal length.
The field stop is the visible ring that sits in front of an eyepiece’s field lens. It, along with the focal length of the telescope you’re using, will determine the amount of sky you can see. Credit: Alison Klesman
What is the benefit of a 2″ eyepiece compared to a 1¼” eyepiece?
Tom Nagy
North Ridgeville, Ohio
A 2″ eyepiece delivers a wider true field of view (the actual amount of sky visible) than a 1¼” eyepiece of the same focal length. Two factors determine the true field: the telescope’s focal length and the diameter of the eyepiece’s field stop. A field stop is a centered, circular opening in front of the eyepiece’s front (or field) lens. The bigger the field stop, the more sky you’ll see.
The only limit to field-stop size is the inside diameter of the eyepiece’s barrel. So, for example, a 20mm 2″ eyepiece will show more sky than a 20mm 1¼” eyepiece through the same telescope. What’s more, the magnification through both eyepieces will be the same.
Most observers prefer a wider field that doesn’t sacrifice higher magnification. This really helps skywatchers whose telescopes lack a motorized drive. A wider true field of view means you have to manually move your scope less often to follow objects as Earth rotates.
Michael E. Bakich
Associate Editor
This question and answer originally appeared in the July 2011 issue.
