I'm no expert like some of the folk on here, but with regard to the T50, I have had one for about a month, and it does take some getting used to. With mine I have found there is a degree of backlash in the focussing mechanism that means it is imperative you always bring the image to sharp focus from one direction, and don't "hunt" the wheel back and forth a fraction.
I get most consistent results by turning the wheel up and away from me, that is to say, indicating shorter and shorter distances, and if I overshoot then go back up to a longer range and arrive at the point of best focus from the same direction again.
Only by knowing that sharpness the scope is capable of, can you be reasonably certain that you have gone far enough.
50x is in some respects pushing the scope to its limits and 40x has a bit more contrast which helps in finding the optimum focus.
I've also got a T35 fixed mag which is optically superior but only has a plain cross hair ret; the mildot ret in the T50 helps with allowing for wind.