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Scope Positioning

10K views 3 replies 4 participants last post by  ltcnav 
#1 · (Edited by Moderator)
I find that scope positioning forward or back is going to depend on multiple factors including the rifle and stock, the scope, and how you plan to use the rifle.

In my hands, if shooting from rest with the rifle supported on bags or a tripod, the eyepiece can be quite a bit further back than if the rifle is going to be shot freehand or from multiple positions (standing, kneeling, sitting, prone). The prone position especially tends to require the shooter's eye to be quite a bit further forward than shooting standing or kneeling or from a bench. And when prone, most people find it difficult to move the head further forward or back while keeping the butt properly mounted in the shoulder pocket. If you find you can easily move your head toward or away from the muzzle in a prone position, at least one of your parents was probably a turtle, so I wouldn't brag about it too much.

Properly used, the term "eye relief" is how far you can have your dominant eye positioned from (behind) the eyepiece and still have a full field of view through the scope without vignetting. Vignetting is the dark rings, arcs, or complete blackout that occurs when you do not have your eye properly positioned relative to the eyepiece. Eye relief is going to be different with different scopes, and with variable powered scopes, eye relief will decrease as magnification is increased.

If you get your eye too close to the eye piece you will also get an incomplete field of vision. The distance forward and back that you can position your eye with any given scope and magnification and still have a complete field of vision is called the "threshold" and this typically shortens up with an increase in magnification. It is also necessary to position the eye centrally with regard to the axis of the scope tube to get a complete field of vision. The "eye box" (which would probably better be called the "eye cylinder") is the cylindrical space within which your eye must be placed to get a complete field of vision through the scope. The diameter of this cylinder is the "exit pupil" and the exit pupil is the diameter of the objective lens divided by the magnification power. Therefore, scope with larger objective diameters have wider exit pupils, and for any given objective diameter, exit pupil diminishes with increasing magnification making precise eye positioning less forgiving of error.

Consider also that for unsupported shooting you will want to achieve a good cheek weld on the comb of the stock. The stock configuration can therefore partially determine proper scope position. Most scopes will result in a rise in the sight line relative to the bore line compared with iron sights, so if you are adding a scope to a rifle that has never had one, you may need to build up the stock comb in some fashion. The larger the objective lens, the higher the comb will need to be to achieve a good cheek weld.

If you plan to shoot from multiple positions, I would get into the position which places your head furthest forward with the least ability to adjust eye position forward or back. For most this would be prone. For a few it might be sitting. Standing and kneeling usually allow more latitude in head positioning. Once you get in this position, put the scope on the highest magnification setting. This will narrow the eye box and shorten eye relief to their minimums for that particular scope. Now move the scope forward and back until you find the position at which your eye is farthest from the eye piece while still maintaining a full field of view. With the scope in this position, as you decrease magnification, eye relief and exit pupil will both increase enlarging the exit pupil and allowing easier adjustment of dominant eye position.
 
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#2 · (Edited)
Good info.
A few other things.
For hunters.
Right hand hunters should be turn to get their bodies to the left and note how eye relief sets up in scope when shouldering rifle and then turn to the right. The scope will be farther from your eye when turned to the right. It isn’t better to have to push back in your stance a bit vs having to force your stance to get closer to the scopes eyepiece. So I recommend keying on scope distance as you are straight and turned to the right somewhat if right hand shooter. Left hand shooters do the opposite.
Also clothing worn is a biggie.
If deer hunting or varmit/small game hunting in colder weather, definitely consider wearing coat to set up your eye relief (scope position). If you don’t you’ll catch yourself reaching to get proper eye distance when in field.

The more comfortable the better. Remember when the pressure is on to make a good shot, all things easier will help you.

Variable power scopes are popular. Imo some of the higher power on some likely wouldn’t ever be used or hardly used. Take this into consideration when setting mag level when setting up your scope. Some times a medium range mag setting is better to use to set up your scope. Some scopes eye relief changes moreso when magnification is lowered and raised.

Be sure to have your eyepiece already set to proper focus before you go any further when setting up scope, as this will affect where scope will be on rifle distance wise.

Set your gun up for you and your eyes not your buds or wife’s eyes.
 
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