(This was posted in response to a question on a forum.)
Ideally, your
head will be placed so that eye relief is perfect and you are looking directly
through the center of the scope, also the ocular and parallax are focused
exactly on your target. In this case, you would see target and reticle together
in exactly the same focal plane.
Realistically, you should focus on the reticle rather than the target and make sure that the image through the scope looks exactly centered in the scope to eliminate any parallax. If your eye relief is a little off, you will see shadows at the edges of the image in the scope; make sure these shadows are equal all the way around the image. Imagine the ocular of the scope is an aperture rear sight and the reticle is the front sight.
Realistically, you should focus on the reticle rather than the target and make sure that the image through the scope looks exactly centered in the scope to eliminate any parallax. If your eye relief is a little off, you will see shadows at the edges of the image in the scope; make sure these shadows are equal all the way around the image. Imagine the ocular of the scope is an aperture rear sight and the reticle is the front sight.
No shadows means
your eye relief is correct. If your eye is too far off center, you’ll still be
able to detect it. Correct eye relief gives you the maximum image and reduces
parallax. I try to mount my scopes for the correct eye relief.
As Anthony has stated, it’s easy to get your scope correctly adjusted.
You set your eye relief when you mount your scope. First, you want the scope far enough away that it won’t hit you under recoil. Next, you want as close to the correct eye relief as mechanically possible. Your cheek weld will be different distances from the scope with different shooting positions. Generally, you’ll be farthest away from the scope shooting offhand, and closest shooting supported prone. You can either mount for the position you’re going to use the most, to keep the scope as far away from the eye as possible (remember recoil), or some compromise position. You may run into scopes with very short eye relief or critical eye relief where only a very narrow band of distance from the scope will give you a good image. The better scopes have fairly long and forgiving eye relief. I prefer to mount mine for the prone position.
When your eye is centered with the axis of the scope the image will appear centered. How centered your eye is to the axis of the scope is a function of the relation between the comb on the stock and the scope. Many high-end sniper rifles have an adjustable comb. Otherwise, you can try different mount heights for the scope, use an add on cheek piece, or permanently alter the stock. Duct tape and foam rubber have been used successfully to adjust the stock comb.
Adjusting the ocular puts the reticle in focus. Look through the scope at a white wall or empty patch of sky (nowhere near the sun) and quickly note if the reticle appears in focus. Your eyes will bring the reticle in focus if you give them time but maintaining that focus will fatigue them so you must look quickly. Adjust the ocular and check again. It will take several adjustments to get the reticle in focus but it’s a one time deal. Once the ocular is focused, lock it in position or, if it’s a non-locking ocular, make note of the setting or mark it on the scope so you can find it again if the scope gets out of adjustment. You shouldn’t have to change this adjustment again. If you wear corrective lenses, you will get different settings with and without your glasses.
High-end target and tactical scopes have adjustable parallax. The parallax adjustment puts the target in focus. Focus on your target and you’ll be parallax free at that range. At this point, both reticle and target should be in focus and in the same focal plane. You should reset your parallax every time the range to your target changes. If your scope doesn’t have a parallax adjustment it will only be parallax free at the preset range- on commercial hunting scopes that’s generally 150 yards or so. By keeping your eye perfectly centered with the axis of the scope, the image will appear perfectly centered and parallax won’t affect your shot for ranges other than the preset.
As Anthony has stated, it’s easy to get your scope correctly adjusted.
You set your eye relief when you mount your scope. First, you want the scope far enough away that it won’t hit you under recoil. Next, you want as close to the correct eye relief as mechanically possible. Your cheek weld will be different distances from the scope with different shooting positions. Generally, you’ll be farthest away from the scope shooting offhand, and closest shooting supported prone. You can either mount for the position you’re going to use the most, to keep the scope as far away from the eye as possible (remember recoil), or some compromise position. You may run into scopes with very short eye relief or critical eye relief where only a very narrow band of distance from the scope will give you a good image. The better scopes have fairly long and forgiving eye relief. I prefer to mount mine for the prone position.
When your eye is centered with the axis of the scope the image will appear centered. How centered your eye is to the axis of the scope is a function of the relation between the comb on the stock and the scope. Many high-end sniper rifles have an adjustable comb. Otherwise, you can try different mount heights for the scope, use an add on cheek piece, or permanently alter the stock. Duct tape and foam rubber have been used successfully to adjust the stock comb.
Adjusting the ocular puts the reticle in focus. Look through the scope at a white wall or empty patch of sky (nowhere near the sun) and quickly note if the reticle appears in focus. Your eyes will bring the reticle in focus if you give them time but maintaining that focus will fatigue them so you must look quickly. Adjust the ocular and check again. It will take several adjustments to get the reticle in focus but it’s a one time deal. Once the ocular is focused, lock it in position or, if it’s a non-locking ocular, make note of the setting or mark it on the scope so you can find it again if the scope gets out of adjustment. You shouldn’t have to change this adjustment again. If you wear corrective lenses, you will get different settings with and without your glasses.
High-end target and tactical scopes have adjustable parallax. The parallax adjustment puts the target in focus. Focus on your target and you’ll be parallax free at that range. At this point, both reticle and target should be in focus and in the same focal plane. You should reset your parallax every time the range to your target changes. If your scope doesn’t have a parallax adjustment it will only be parallax free at the preset range- on commercial hunting scopes that’s generally 150 yards or so. By keeping your eye perfectly centered with the axis of the scope, the image will appear perfectly centered and parallax won’t affect your shot for ranges other than the preset.
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