Was doing a ladder test today, took about an hour and a half. Later in the test it became very difficult to focus on the target, like it was WAY out of focus, reticle was clear but couldn't make out circle on target at all. I looked around a minute blink my eyes and looked again and was clear again. This happened a few more times towards the end of the test and the last time I just kept concentrating on the target and it gradually cleared up. Changing the focus eye piece while it was blurry didn't help either. This has happened to me before while working up a load at a long distance where I am concentrating very hard on the target circle. I do not wear glasses or anything, and I don't feel like I need them in normal circumstances. The scope is a vortex viper pst 6-24 FFp. I was at 24 power. So my question is... Is this normal when looking through a scope and focusing real hard on the target? In other shooting conditions I havnt noticed this problem such as at a match. Would better glass solve this problem, or do I maybe need glasses?