OK, on my first cup of coffee, against my better judgment...
If you do not have the sharpest target image that your scope can be adjusted for, and zero parallax at the same point of objective or side focus adjustment, it is very likely that your eyepiece is ever so slightly out of adjustment. That is it in a nutshell. The first question that comes up from this is sort of a how can this be, or the stronger you don't know what you are talking about, the eyepiece adjustment is only to focus the reticule, and has nothing to do with parallax, and it says so right in the instructions. To that I reply, that those instructions were developed for deer hunters with 3 x9 scopes. Get real, most shooters can't even correctly tell you what parallax is, and are surprised when you tell them not to rely on the distance markings on their objective adjustment.
The root of the problem is that shooters stare at the reticule for an extended period of time while attempting to correctly focus the eyepiece, and their eyes make the final adjustment instead of their seeing a discrepancy that needs further adjustment.
The proper method involves repeated brief glances that do not give you eye time to adjust. You can stand right by someone, tell them this, and he will nod his head, and look through the scope 10-20 seconds at a time, apparently thinking that you are not being literal when you tell him to only look for a second. In any case, the end result is an eyepiece that is focused slightly out of the plane of the reticule.
Last item, how do we fix that? Make a small eyepiece adjustment, either in or out, and refocus the objective (or side focus), and then check for parallax. Has it been reduced, or made worse? If it has been reduced you are going in the right direction and just need to continue making very small adjustments, refocusing, and checking until you have peak target sharpness, and absolutely no parallax at the same point of adjustment. If it is worse after the first try, you either made too big a move in the right direction, or you went the wrong way, and need to make your next effort in the other direction. Once you have the eyepiece properly focused, you are good to go at any distance. Just adjust for your sharpest target image.
There can be slight dependencies due to expansion and contraction of metal, caused by temperature change, so if you are doing serious benchrest work, it is not a bad idea to check for parallax first thing in the morning, before the heat waves make seeing slight amounts impossible.
Another little, unrelated scope matter...in hot weather, if you installed your scope in the comfort of air conditioning, you may want to loosen and tighten one ring cap, after things have heated up, to relieve the stress of the scope tube getting longer due to the heat. This is best done when you have a bench and rest available so that you can do the slight resighting that is likely to be required afterward.
There you go. Now to my second cup.
