What is parallax? If your looking through a rifle scope, cross hairs are on the bulls-eye and you shift your head in just the slightest amount and the cross hairs appear to move around, that’s called parallax and we don’t want that.
Parallax can occur if the target is just a bit out of focus, so manufactures design scopes to be parallax free, but parallax free at a given distance. Typically this distance is 50 yards for rimfire scopes and 100 yards for centerfire scopes. Also, making a parallax free scope only works for those with lower magnification settings, usually 9X or less.
To avoid parallax at close ranges and high magnifications, rifle scopes are designed with a side focus or an adjustable objective. On scopes designed for airguns, these focus adjustments can go as low as 9 or 10 yards while on centerfire scopes the lowest setting is typically 35 or 50 yards.
Rifle scopes with a side focus or an adjustable objective will require the shooter to locate the target and then adjust the focus to bring it into a clear image.
On a rifle scope with an adjustable objective, this focus adjustment is found on a ring around the large objective lens making it a little more difficult to adjust while maintaining good control of the rifle.
On a rifle scope with side focus, this adjustment has been moved to the user’s left hand side of the rifle scope opposite the windage turret making it much easier to access. Although much more practical to have the focus on the side, it does add to the manufacturing cost as it requires the addition of a couple lenses inside the rifle scope.
Whether you have an adjustable objective or a side focus, both will be marked with distance readings. When the image is brought into clear focus, just a quick glance at the side focus or adjustable objective ring will give the user a reading on the distance to target.
These readings can be fairly accurate inside 100 yards, but even more precise when when used at higher magnifications and using a large diameter focus wheel. It should be noted though, many add on side focus wheels will require the user to mark their own distances. This adds for even a greater degree of accuracy when ranging distances with the side focus.