A few years back I purchased a Nikon Prostaff 4-12x40, for the same reasons OP did. And as OP, I was initially flustered about the "unsharp" view at 50m distance, when set at 12 power. As I "zoomed out," the image got sharper. By the way, the reticle remained in focus. What gives?
Then the light bulb went on: This was much like optics in photography - we are talking about depth of field (the distance between the nearest and farthest object in front that are in acceptable sharp focus) which diminishes as the magnification increases. Conversely, the depth of field increases as the magnification decreases. Ok, so there's nothing wrong with my scope (Nikon Prostaff is, by the way, it's nice glass for the money.) As it turns out, anything beyond 100m (to infinity) is in acceptable sharp focus at the full power range.
In other words, this was never intended to be a short-distance target scope. It works as designed.
Now, time to don Nomex: I think there is a lot of confusion and misinformation about parallax (the difference in the apparent position of an object when viewed along two different lines of sight - with rifle scopes, this refers to any movement of eye position away from the optical center of the scope) and focus (converging the light rays in a manner that brings clarity and definition to the object(s) we are looking at.) These are two different things: Sharp focus requires movement of specific lens elements towards or away from the viewer; Eliminating parallax error requires the reticle (crosshairs) to be placed precisely at either the first OR the second focal plane inside the scope (the two separate points where light rays "cross" each other inside the scope tube.) So the two are related, but not the same.
With lower magnification scopes we generally never worry about parallax, as the potential error is always minimal. The reticle is positioned at the factory to the "best" setting for the intended use of the optic. When hunting, the bullet placement variance of 1/4" at 200 yards makes no difference.
With higher magnification scopes and precision shooting, the ability of the user to adjust reticle placement is important. This is achieved either by moving the front lens group (adjustable objective, or AO scope) OR by moving the reticle back and forth (side focus, or SF scope.) In other words, AO scope works much like a camera lens focus by rotating the objective lens (the lens elements further away from the eye) on threads so they move back and forth, while the SF style moves an inner lens group containing the reticle back and forth. Different processes, same results.
Finally, what about the ocular (the lens elements closest to the eye) diopter adjustment? That is simply there to bring an image that we know is sharp in focus to fit your eye. With scopes, this is easiest done by pointing the scope to any "blank" area, closing your eyes for a moment to rest them, then looking through the scope. If the reticle is in focus, the diopter adjustment is good. Lock it in place and forget it.
Please bear in mind I'm not an expert, so do your own research to verify (or disprove) my statements. I won't cry if I'm off-base.
Having said that, if someone had explained the above to me years ago, I would have saved a lot of frustration - and money as well... .
Let the games begin!
