Larry Brown wrote in message ...

Do the manufacturers of 80-degree eyepieces have a new definition of eye
relief? Thanks!

Eye relief is a technical term which assumes an idealized
model of the eye. It is a well-known fact that the human
eye does *not* behave like this idealized model, so that
in practice your eye *always* needs to be closer than
the stated eye relief in order to see the whole field.
I get the following explanation straight from Al Nagler.

The reason is that as you slew your eye towards the
corner of the field, the pupil moves laterally, away
from the center of the eyepiece. If you slew 40
degrees to look directly at the field stop of a Nagler,
your pupil moves several millimeters, and when the
eye relief is small, that amount of motion may cause
the image to black out entirely.

Personally, I have considerable trouble seeing the
field stop in *any* Televue eyepiece, and have never
even come close with a Nagler. Granted, I have never
looked through a 32mm Nagler, which is where the field
stop is most likely to be visible. Therefore, I find
the useful FOV of most Naglers to be *smaller* than
in my Pentax XL eyepieces, with their 65-degree AFOV
and 20mm eye relief. Moreover, I find the inability
to see the field stop extremely vexatious in its own
right, since I routinely use it as a reference point
when estimating sizes, and also to place bright stars
out of the field when viewing faint objects.

- Tony Flanders