getom59,
I am not an optometrist, and I don't even play one on TV, but I am a land surveyor, and when I used to be out in the field, as an instrument man, I could spend as much as 12-14 hrs a day looking through the instrument (which operates optically, just like a telescope and a rifle scope). It can, and will ruin your eyes (although computers, in my opinion are MUCH worse). I can use alot of fancy words, and post some diagrams, but I like to keep things simple. Your cross hairs must be in focus. Quite a few people think that their cross hairs are in focus, when they are not, your eye is an incredible thing, and has the ability to compensate for minor deviations, without you realizing that it is doing it. Your cross hairs must be crisp and clear AND your back ground must be crisp and clear also. I'll say this again, because this does get over looked and or ignored - your cross hairs must be crisp and clear with your back ground being crisp and clear, together. Your eyes are seeing the two objects at different planes, or distances, and they both need to be focused, so your eyes see them as being at the same plane, or at the same distance. A good way to test if they are both in clear focus together, is, while you look at your cross hairs, slowly move your head from side to side. If the cross hairs appear to move across or "dance" on whatever you were sighted in on, even slightly, the cross hairs and/or the back ground are out of focus. You need to check this every day. Your eye can, and does, change focal points from day to day. It can change from the beginning of a day, to the end of the day, by just being dehydrated. When I train newbies on an instrument, almost every one of them don't understand parallax. Even "experienced" hunters don't always understand it, let alone know about it. But not only does it wear on your eyes, but it can affect your accuracy as well.
Now, even with the scope properly adjusted it will still wear on your eyes. You need to exercise your eyes. Focusing on the pencel at arm's length is good, but you also need to focus on objects at different distances. Focus on the pencil, then a tree out at 200 yards, and then again on the pencil, and then at a rock at 50 yards, then at the tree, and so on. But you must focus on things at different distances where the distances vary greatly. If you do these things, your eyes shouldn't tire as easily. I hope this helps.
