So I'm test-driving the Stokes DLS 10x42.
with eye cups all the way down, I get the blackouts when panning. What causes this?
WIth eye cups fully extended, It's as if I'm looking through a small tube/tunnel to get to the image.
For me, the cups have to be halfway out. But then they do not stay solidly
in place as other designs, but that's another story . . .
It's all about eye relief--blackouts occur when the eye gets too close to the eyepiece, the tunnel effect when it's too far away. That's why good quality adjustable eye cups, permitting exact positioning of the eyes, are important.
You did not say if you have glasses on. If you find the ideal position, you can use rubber bands or o-rings to hold them there. Take caer when storing, as the eye cups can be knocked off if stored the wrong way among other junk.