Key Takeaways:
- A 2" eyepiece provides a wider true field of view than a 1¼" eyepiece of equivalent focal length when used with the same telescope.
- The difference in true field of view is attributed to the larger field stop diameter possible within the 2" barrel size compared to the 1¼" barrel.
- Magnification remains unchanged for eyepieces of the same focal length, regardless of barrel size (1¼" vs. 2").
- A wider true field of view is advantageous for visual observation, particularly with non-motorized telescopes, as it reduces the frequency of manual repositioning to track celestial objects.
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.
