Re: Seeing the field stop

"Tony Flanders" <[EMAIL PROTECTED]> wrote in message > Well, I am
baffled.  Yes, I say that I cannot see the entire FOV at
> one time, but I have no trouble whatsoever counting cinder blocks
> all the way across the FOV; all that is required is to keep track
> of where I was when I move my eye.  In fact, if I were inside a
> cinder-block house, I could easily count cinder blocks all the way
> around me, 360 degrees, and *nobody* will claim to see 360 degrees
> at one shot!
>
> Sounds to me like a cognitive problem, having nothing at all to do
> with the physiology of the eye.
>
>     - Tony Flanders

I want to be clear on what I meant when I said that I was "counting cinder
blocks".  I hold the eyepiece up to one eye, and with the other eye open and
looking at the wall, I see a bright circle with a black ring around the
outside (the field stop) superimposed on the wall.  This is because the eye
looking through the eyepiece sees the field stop, and the other eye sees the
wall, and the two images are merged by the brain, i.e. binocular vision.  I
then walk/move my head backwards and forwards, trying to keep my head level,
until the image of the field stop frames something that I can measure or, in
this case, count (the cinder blocks).  I can then compute the
tan(afov_of_eyepiece) = (the height of the framed area computed by counting
the cinder blocks) / (the distance to the cinder block wall).  One can do
this quite accurately with a tripod to assist in positioning and good
measurements.

My friend has such a difficult time seeing the field stop in a 50degree
plossl that this method of determining afov is, for him, not practical.  It
sounds like he can see parts of the field stop, but only by really trying.
For him, a 50degree afov is as difficult to take in at one time as a
80degree afov is for (most?) other folks.  In fact, I think that I could use
this method to measure the afov of my BW-Optic 30mm 80degree eyepiece.

Tony, do you think that you have the same difficulty?

Still confused,

Dennis