The typical hunting scope has the parallax set at 100 yards. A rimfire scope is usually set at 50 yards. At other distances, especially shorter distances there will be some aiming error if your eye is not perfectly aligned with the center axis of the scope. The parallax adjustment knob on the saddle adjusts the distance of the parallax setting. An adjustable objective lens performs the same function. Very handy if you are shooting at short ranges as in moving from an open area to a wooded area. Most target scopes will have a parallax adjustment method. This is one of the reasons a good consistent cheek weld is so important. A good and consistent cheek weld is harder to achieve with the high rise or see through scope mounts and rifles that have too much drop.
"I would not be an old man if I had not been an armed young man." JTJ
Patron Member NRA
"If you have always believed that everyone should play by the same rules and be judged by the same standards, that would have gotten you labeled as a radical 60 years ago, a liberal 30 years ago and a racist today"