Handgun scope question

[mut617]

On a fixed power pistol scope what is the purpose of the large amount of adjustment on the ocular bell? The scope is 3x with parallax set at 50 yards I think. The bell adjusts about an inch and has a locking ring where the magnification adjustment would be. Does it do anything?

[gunderwood]

I would guess diopter, so the reticle can be focused even if your vision isn't 20/20 (either way). Without knowing the scope brand/model, I'm just guessing as it could be just a particular construction technique. Hold the scope out at a fixed distance and turn the ocular bell. If the focus changes, it is diopter.

[mut617]

It is an out of production burris 3x. I had assumed it was completely fixed power since all it says is 3x and has no markings for anything else but sitting it at rest and adjusting the bell it looks like it actually goes from about 1.5x to 3x. Just guessing because I do not know how to measure it. I e-mailed burris but they have not responded.

[SgtBill]

I would assume like Gunderwood said it is a Diopter adjustment ring for Paralex correction.
Bill

[gunderwood]

I would assume like Gunderwood said it is a Diopter adjustment ring for Paralex correction.
Bill

Diopter and parallax correction are different. Wikipedia:

Parallax is an apparent displacement or difference in the apparent position of an object viewed along two different lines of sight

In a rifle scope this causes aiming errors. High end/long range scope have a parallax correction knob. They can also have diopter correction too. Diopter doesn't cause aiming errors, but rather corrects for vision impairment. Examples to provide clarity.

1. Set the parallax to the correct distance for the target (most fixed rifle scopes are 100 yards, handguns are commonly 50 yards). Turn the diopter adjustment and the image will go in and out of focus. However, if you move your head around the aim point will not move on the target.

2. Turn the diopter all the ways to one side (doesn't matter). Now adjust the parallax until the image is focused. Now move your head around. The aim point will move on the target even while the rifle is not moving.

Parallax causes aiming errors, diopter doesn't unless it causes you to incorrectly set the parallax on an adjustable model. E.g. my S&B is marked/calibrated in meters on the parallax adjustment. If I am shooting a 500 meter target, I just turn it to that and it is correct. If I have the diopter adjusted correctly, the image will be in focus. If I don't it won't be. Either way, my shot will go where I aim it. To adjust them correctly do the following.

On a good clear day with no mirage, set up a target at a known distance. 100 yards is fine. If you have a fixed model, set the target up at the correct distance for your parallax. If it is an adjustable model, turn it to the correct distance (make sure you know if it is yards or meters). Sand bag the gun and aim it at the target. If the parallax is correct, as you move your head around the aim point should not move. Now without moving the gun, adjust the diopter until the image provide is as crisp as possible while you can still focus on the reticle. From this point onward, only adjust the parallax.

On models that have adjustable parallax and no range markings you must fire the gun to determine if the parallax is set correctly. You can do this with range marked scopes too for confidence. Start with diopter adjusted to 0 (not always midpoint). Focus the target image with the parallax. Move you head. Keep doing this until the aiming point does not move. Now adjust the diopter to focus the image. The problem is you possibly adjusted too much parallax. I.e. it is set for 140 yards vice the 100 yards it should have been. Shoot the rifle with different cheek welds, but the same hold. Slight shifts in the viewing position should not cause the bullet to impact in different places. Back the parallax off until you get an impact difference or the reticle moves. Go back slightly and it should now be set correctly. Adjust the diopter to get the best image. Make sure to note what bar or dot is correct for that distance.

Generally, if you have a MOA scope the bars or dots on the parallax adjustment will have one that is really close to 100 yards. As this is a handgun scope, I doubt it has adjustable parallax. It either has dioper adjustment or zoom. Being that it is a Burris (good low cost) scope I doubt it has diopter. Probably is zoom. It more than likely is the older version of this scope: http://www.burrisoptics.com/handgun1.html#15x4x

Edit: I originally guessed diopter because it seemed implied that the scope was a fixed power model. Given the uncertainty of the fixed power assumption, I think it actually is a 1.5-3x scope. Diopter and parallax are confusing because they seem to do similar things; focus the image. However, where they are doing it has implications for aiming. You will get poor results using the wrong one. Parallax is usually on the scope bell or a third knob with the turrets. Diopter is usually a ring on the ocular housing. Usually the last lens in the system.

[gunderwood]

Here is a NF scope (NXS) which has the parallax correction knob next to the turrets. It is the left knob; top is elevation, right is windage. Notice there are no yardage markings, just bars until infinity (which is what the scope is set at in the picture).



Here is a NF scope (benchrest) which has the parallax on the scope bell. Notice the grip and yardage markings out in front of the El/W turrets. For a tactical scope the previous turret adjustment is king. The bell method generally is considered more precise and for benchrest you adjust it once and forget it. Tactical scopes are shooting unknown distance targets at a variety of ranges and you can't get off of your gun to look over the scope and adjust it.

Same image, but not shrunk for the forum: http://nightforceoptics.com/nightforces ... -42x56.jpg


The diopter adjustment on those is at the other end. You want it to adjust focus only! The benchrest is easy to see in the original picture. You can see a zero and then a + and - on either side. The zero should be 20/20. Diopter goes both ways, but usually has more adjustment for near sighted people due the population stats.

Edit: Diopter is like glasses. You are purposefully misfocusing the image so that when the light passes through your eye lens it compensates for the focus distortion and the image appears correct.