No, there's no need for diopter adjustment. There's no need for close focusing. Your eyes focus on the target, not on the reticle (dot). The red dot is collimated, so it's projected in the same plane as the target. Your eyes focus on the target, not on the red dot.
Red dot sights are very simple to use. You look through it at the target, place the dot on the target, pull the trigger. If you see the target clearly and in focus, the red dot is in focus too.
Not to complicate the issue, but parallax was mentioned. Most of these RDS are billed as "parallax-free". That's not exactly true, but parallax error is pretty minimal, especially at 50 yards and beyond. Closer than that and the parallax error is never any greater than the optical window (about 30mm) and far less than that if you have any kind of reasonably consistent cheek weld.
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