Spotting Scope Clarity and Focus

[zrod]

Hey all,

I’m a spotting scope newb and I’m hoping someone can explain what I’m seeing and why it happens.

When looking through my Swarovski sts-65 hd I noticed that in order to get the object I’m looking at totally in focus I need to look through the eyepiece in a manner that blacks out a large portion of the field of view. For example, I’m looking at a knot on a tree 150 yards away and with proper alignment in the eyepiece (no shadow) the knot is just slightly out of focus. It’s more noticeable with magnification. If I cant my head to where the top right portion of the field of view is blacked out I get a crisp clear picture of what I’m looking at. I can achieve the same clarity by backing off the eye piece far enough that all but the very center of my field of view is blacked out.

Is this just a result of my eye interacting with a narrow exit pupil due to the 65mm objective lens?

Thanks