When does side focus become necessary on a rifle scope.

[M.A.T]

When does side focus become necessary on a rifle scope.

[Edward J. Palumbo]

It becomes helpful or useful when it's counterproductive to remove your eye from the eyepiece. On a tactical scope, if you remove your eye from the target, you may lose the target. One does well to maintain visual contact with the subject or it may be difficult to reacquire the target. The same reasoning may be used with any maximum range/minimum target situation, but I don't regard it as a necessity.

[allegnmtn]

It depends on what you're hunting and the distance. I see no need for it on my whitetail rifle. If you're varmint hunting at 3-400 hundred yards though, parallax adjustment can make a big difference sometimes a couple of inches at 4-500 yards.

The same goes for a rim fire scope. If you're plinking or hunting squirrels at 25-50 yards, parallax adjustment is probably just another complication to those quick shots. But if you're shooting target ammo off the bench with a 9x scope, you'll see huge differences.

[Sarge01]

In the deer woods where my longest shot is 100 yards I use a 3X9X50 scope with a factory set parallax. I have a 4X12X50 on another rifle that I use when the shots are longer and then I usually have time to adjust the parallax when the animal shows itself and I start by having the scope parallax free at 100 yards.

[DakotaMan]

Focus becomes necessary when scope magnification exceeds 10x. At magnifications higher than 10x, the scope lens will cause the image to converge ahead of the scope reticle causing the cross hair to appear to be on target when it is not. When the scope is in this "out of focus" condition, the cross hair will move around on the target as you move your eye behind the ocular lens. This movement of the reticle while sighting is called parallax.

You know the focus knob is properly focused when the reticle doesn't move around the target as you move your eye. Most cheap scopes suffer from parallax even at lower magnification but better scopes will have no parallax once they are properly focused by moving the focus knob (whether it is on the side or on the objective lens).

I use a 2.5-10x whenever possible for hunting to give me big field of view and yet good parallax free sighting out to 400 yards or so. No fiddling with focus knobs is required on a scope of this magnification. My varmint and target scopes all have larger magnification and therefore require focusing via a side knob or objective lens focus. I use larger magnification for prairie deer, antelope and coyote hunting too but I leave my focus on 500 yards so I can see clearly out to any range I might shoot. Parallax is not too severe to miss shots at all other ranges.