If your sitting in you car in the drivers seat and look at your speedo and its on 55mph and at the same time someone in the passenger seat looks at the speedo itll look like your going 52 or so when your still doing 55, that optical illusion is called a parallax.


Scopes all have parallax's in them. The non adjustable kind are usually set at 100yrds unless its a shotgun scope which is usually set about 50 or 75 yards. Your scope will let you chose where to set it where there is no opticle illusion and your x-hairs will be on exactly what you are looking at. Most people I know set the adjustable objectiveon about 100yards but you can taylor it to you hunting situation. If you take longer/shorter shots most of the time you can set it where you like.Generally scopes of 12x power and higher have the adjustable obective A.O. because you can see more detail further away.

This is a very short explanation and if you search the net a little you can prob get alot more indepth with the answer or someone here with a little more time will add a little more you can always count on that.