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Actually its quite possible and very well documented. I could go into a lot of aberrations that would degrade your resolution yet be in focus. Most of the aberrations will only start having an effect at the sub-micron level, but there is plenty still. Spherical and Chromatic would be the the most obvious. Even at the mean focus, you are going to have a bell curve of that focus depending on the size and position in the field of view, and of each color. Better glass, better coatings, different internal gas and pressure...
There are reasons that in general there are cheap and expensive scopes.
In my field of occupation I believe what we call this parallax is telecentricity?
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