To clarify, some widefield eyepieces have more spherical aberration of the exit pupil than others.
This is an issue which grows less objectionable with a smaller focal length, so it predominantly complained about in longer focal length widefields.
It can definitely make positioning the eye difficult, even when using the eyepiece mostly in the center of the field.
And different brands have more than others.
But human vision has about a 5° field of greatest acuity. We use this to examine details.
We have a 30° field of "greatest attention" and decent quality vision.
And we have up to a 140° field in the right-left direction for out peripheral vision, which is of low acuity.
So picture your vision like a spotlight on a stage in a 100° eyepiece. You can move the spotlight all around all over the stage (field) and look with high acuity
anywhere in the field. But to look at the edge of a 100° field pretty much means you'll be looking through the glass at a sharp angle.
You can't simply go to the left to look right, as you would with a porthole because then you move the pupil of your eye away from the exit pupil of the eyepiece.
So the normal technique, even if it is unconscious, is to keep the pupil of your eye in place and roll the head to the side to look straight at the edge with the center of your vision.
To be real about it, you do this on a 50° eyepiece as well, but the head movement is small enough you aren't aware of it.
And if you are looking at only the center 30° of field, your head can stay still and you can simply move your eye a tiny bit without losing the exit pupil.
And this is also true of the 76° Baader Morpheus and 92° Explore Scientific. If you want to look at the edge with direct vision, you're going to roll the head over to do it.
Obviously, the narrower the field, the less movement, of course.
That's true with 100° eyepieces as well. If you only look at the center 1/3 of the field with direct vision and all the rest of the field is merely contextual and in your peripheral vision,
then you can use the eyepiece exactly as you would a 50° eyepiece. But if you do want to "look over there" and "look down there" and "look up there", then you need to move the field of best visual acuity
to look at the new place in the field, and that will require rolling your head to look there.
I had used 82° eyepieces for almost 25 years when I got my first 100° eyepiece, so I didn't notice there was anything particularly unusual about using a 100° eyepiece--it merely had more field.
So I never was aware of how difficult they were to use for some people until I started reading comments on CN and other forums.
Most of the people who have difficulties are moving from 30-60° eyepieces straight to 100°, and since the techniques of usage are different, I can totally understand the issues.
However, it shouldn't take long to accommodate to the 100° field and the use of that wide an eyepiece.
But is the point of a 100° eyepiece to look straight at the edge of the field? I had to think about it, but I rarely look directly at the edge. My gaze seems to occupy the center 75% of the field
and it is my peripheral vision that becomes aware of the edge. The exception to that is watching a planet drift across a 110° field at 500x, where I may watch the planet from just inside one edge to just inside the other.
Edited by Starman1, 14 December 2017 - 06:45 PM.