I'm on a space walk and comet Kohoutek is passing by and I've brought my specially designed space binoculars with me to look at it.
I left my Nikon Monarch 8x42 binoculars inside because with an eye relief of only 18.4 mm they would be nearly useless held against the faceplate of my helmet. I would be able to see an extremely tiny field of view; their 4.2 mm exit pupil at say 15 cm from my face would present an apparent angular field of view of not 51.3°, but only 1.6 degrees. I'd see tiny dots with stars in them, but it would be really hard to locate anything.
Question: How far would you have to hold "space binoculars" from your eyes in a space suit? I guessed at 15 centimeters but I have no idea, and probably different helmets for different suits present different distances.
These are going to be pretty crazy looking binoculars!
Screenshots from How to Adjust Your Binoculars (Presented by Nikon Canada) and Understanding Binoculars: Eye Relief showing how eyecups can be adjusted to place the exit pupil of the eyepiece at the entrance pupil of the eye. For those wearing eyeglasses (a glass barrier fixed in front of the eye) one retracts the eyecups.
A helmet with a transparent face plate would place the binoculars much farther from the eyes than eyeglasses do, so the exit pupil positions of the eyepieces of a pair of space binoculars would have to extend much farther.
What it looks like when you are too far away from the eyepiece. From Wikipedia's exit pupil. If you were looking for something in a field of stars this "tunnel vision" would make it a lot more difficult.
Cropped and annotated from here