Eye Position Relative to Rear Sight

[JerryX]

How close should my eye be to the rear sight on a 10M rifle? I've always gotten as close to the sight as possible -- is that correct?

[Tim S]

It depends. The aperture plays a part in focussing your eye; it narrows but elongates your focus so the foresight and target are sharper. If your eye is too close to the aperture, you lose some of the depth of focus, but if your eye is too far away, the sight picture may be too dark.

You have to work out by trial and error the eye relief that is sharp enough, and bright enough. Adjustable iris apertures are popular to fine tune the balance.

[patriot]

+1 ... but also taking into consideration the amount of white you want between the outside of the front sight and the inside of the rear iris. I like just a fine line. The amount of light coming in from around the rear sight is another consideration for how your eye focuses, fatigue, plus glare on a shooting glass lens.

Mark

[JerryX]

IIf your eye is too close to the aperture, you lose some of the depth of focus, but if your eye is too far away, the sight picture may be too dark.

In my experience, the closer my eye is to a small aperture, the more depth of focus I have. I just tried an empirical experiment. I used a pin to punch a small hole in a piece of opaque cardboard. I then took my glasses off (I have 20/600 vision in my dominant eye with my glasses off). The closer I hold this hole to my eye, the clearer my vision.

[Tim S]

It's the opposite way. The aperture increases depth of field because it is so small, if you increase the size (by moving it closer), you decrease depth of field. What you do get by having the aperture closer to your eye is a much bright sight picture; this isn't the same as having the correct sight picture for aiming. Remember you are trying to make two items approximately 9m apart appear in almost focus at the same time; without the aperture almost no one can barely, or not even, see the target if they focus on the foresight.

[JerryX]

It's the opposite way. The aperture increases depth of field because it is so small, if you increase the size (by moving it closer), you decrease depth of field.

This seems counter to my empirical results. I have a friend who has a PhD in optics -- I'll ask his opinion.

[Tim S]

Jerry,

looking through a hole at say your living room is not the same as aiming at foresight and target. Moving the aperture nearer to your eye will make the image brighter, which you may be confusing with sharper focus on the foresight and target. For aiming however, you need a certain depth of field to see a sharp foresight and a reasonably clear target. Some individuals can achieve this with a short eye relief, but a longer eye relief will always increase depth of field.

[conradin]

No real answer to that about eye relief. After you get a diopter you simply just have to play with it until you find the optimum comfort.