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Scope vs. Eye Question

Discussion in 'General Gun Discussions' started by PinnedAndRecessed, Sep 9, 2004.

  1. On an earlier thread someone stated that the rule to determine the largest scope lens size the eye can utilize is 7 = (28/4) for a four power scope.

    Therefore for a four power scope the largest lens the eye can utilize is 28mm.

    Does that mean if the scope is a 20 power it will be 20X7=140mm?

    Or what is the maximum lens size the eye can utilize?

    I know there's no rifle scope with a 140mm lens, but exactly how does the formula work?
  2. Vern Humphrey

    Vern Humphrey Well-Known Member

    On an earlier thread someone stated that the rule to determine the largest scope lens size the eye can utilize is 7 = (28/4) for a four power scope.

    The spot of light projected by the scope is called the "exit pupil." A small exit pupil meains the scope (or other optical instrument) will less than effective in dim light -- when the pupils of your eyes are dialated.

    The human pupil can dilate to about 7mm -- so any exit pupil larger than 7mm is wasted. An exit pupil smaller than 7mm, on the other hand, will not be good for dim light conditions.

    To calculate exit pupil, divide the diameter of the objective lens by the magnification.

    If you had a 20 power scope, and a 40mm objective lens, the exit pupil would be 2mm in diameter. If the objective lens were 80mm, the exit pupil would be 4mm. And so on.

    Other factors -- such as glass quality, coatings, and so on -- can affect light transmission. But all other things being equal, an exit pupil of 7mm gives all you the light you can handle.
  3. tanx vern.

    So I guess since I'm looking into a target scope with maximum efficiency I should find a 20X140 scope.

    That's gonna require some set of rings.
  4. Vern Humphrey

    Vern Humphrey Well-Known Member

    It that's what you want.

    But most people don't do any target shooting at twilight. :D

    For most purposes, a smaller magnification is the solution to the problem. For example, if you have a 3X9X32 scope, you can use the 9X setting during the day, and turn it down to 4X or so for twilight shooting.

    Let me point out that optics and brighness are secondary in importance in a scope -- if you use optics to FIND your target, you use binoculars or a spotting scope. In those instruments, clarity, brightness, and so on are paramount. You use them sometimes for hours at a time -- and tiny flaws can create headaches.

    The riflescope comes into play only when you find game, so ultra high quality optics aren't that critical.

    What IS critical in a riflescope is reliability. It has to stand up to rough handling and not lose its zero.
  5. rbrowning

    rbrowning Well-Known Member

    Most target shooting is done during the brighter portions of the day and the scopes ability to gather light isn't as critical. Under hunting conditions, at first or last light, is when this becomes critical (or at night if you are a tactical type of shooter). Most hunters prefer a variable and this is one of the good reasons for it, the increased field of view another.

    If you have a 4-16X56 scope you can use the 16X during the bright day and back it down to 8 durring twilight and still maintain a 8mm exit pupil, ensuring that your eye has as much of the light that comes out of the scope that it can use. Then you can back it down to 4X to increase the field of view when you are moving and more likely to get a quick shot and have to find your target very quickly.

    I have found on my varmint rifle with an 8-32X56 that I use it mostly in the 16-24X range during the day because the mirage is too much at 32X. In the cool of the morning and evening, well into the light, I use the full 32X with enough light and no mirage.
  6. Chuck Dye

    Chuck Dye Well-Known Member

    Note that a larger exit pupil will allow coarser eye to optics alignment. This is not much of a much when plinking or target shooting, but if you are trying to acquire a sight picture in a hurry on that once-in-a-lifetime critter under less than optimum conditions, having a larger exit pupil can more than make up for the added expense and weight of a larger objective.

    For those who would argue the parallax issue, if the critter is so close that you are taking a snap shot, parallax will be negligible. In fact, for large critters parallax is negligible at all ethical hunting ranges. See http://www.usoptics.com/sub_pages/parallax.php

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