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  #21  
Old 06-18-2018, 02:49 PM
mhinagoya mhinagoya is offline
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There is a relationship between objective diameter and power.
When a scope is in use, there is a 'light pencil' that exits the rear of the scope and enters the shooter's eye. When we were kids, the iris of our eye could dilate to about 7mm or so. As we grew older, that number shrank and as a result, our night vision suffered. The iris of our eyes could no longer let in as much light as it used to. I'm 64 years old and mine will open to about 5.5 mm, but that's it.
Now, back to the mechanics of a rifle scope. You can easily calculate the diameter of the light pencil exiting the rear of the scope. Simply divide the objective diameter by the power. 56mm objective at 10X gives you a light pencil 5.6mm in diameter. That is about the limit for my eye to receive and utilize all the available light the scope has gathered. A light pencil any bigger than that and some of the light won't enter my eye, but will fall on the iris instead. This light is wasted. Now, for the kicker. The iris of my eye isn't dialated to the maximum amount during the day. It is actually much smaller (as it should be). The brighter the day, the smaller the iris. Hence, with a 56mm objective, I am nearly guaranteed that the light pencil is larger than the opening in my iris and part of the light is lost.
A light pencil smaller than the maximum size of my iris works pretty well, unless it is so small that I have trouble locating it, or eye position becomes very touchy. If the light pencil approaches 2mm or less, there is a good chance that I will be seeing the floaters in my eye. In other words, either extreme is going to cause issues.

If we set the lower limit of the light pencil to 2mm and want a 25X scope, then a 50mm objective does the job. What should the bottom power be for us to continue to utilize all the available light? Since we are adults, let's use 6mm as our upper limit. 50mm divided by 6mm is 8X (close enough), so the ideal mechanics would tell us that 8X-25X by 50mm would be just about right in our theoretical scope, allowing for adult eyes.

That is how I understand objectives and power ranges and it makes sense to me.

Bill.
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  #22  
Old 06-19-2018, 02:12 PM
recoillug recoillug is offline
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Very good explanation and example. Thanks!
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  #23  
Old 06-19-2018, 03:03 PM
Bayou City Boy Bayou City Boy is offline
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Quote:
Originally Posted by mhinagoya View Post
There is a relationship between objective diameter and power.
When a scope is in use, there is a 'light pencil' that exits the rear of the scope and enters the shooter's eye. When we were kids, the iris of our eye could dilate to about 7mm or so. As we grew older, that number shrank and as a result, our night vision suffered. The iris of our eyes could no longer let in as much light as it used to. I'm 64 years old and mine will open to about 5.5 mm, but that's it.
Now, back to the mechanics of a rifle scope. You can easily calculate the diameter of the light pencil exiting the rear of the scope. Simply divide the objective diameter by the power. 56mm objective at 10X gives you a light pencil 5.6mm in diameter. That is about the limit for my eye to receive and utilize all the available light the scope has gathered. A light pencil any bigger than that and some of the light won't enter my eye, but will fall on the iris instead. This light is wasted. Now, for the kicker. The iris of my eye isn't dialated to the maximum amount during the day. It is actually much smaller (as it should be). The brighter the day, the smaller the iris. Hence, with a 56mm objective, I am nearly guaranteed that the light pencil is larger than the opening in my iris and part of the light is lost.
A light pencil smaller than the maximum size of my iris works pretty well, unless it is so small that I have trouble locating it, or eye position becomes very touchy. If the light pencil approaches 2mm or less, there is a good chance that I will be seeing the floaters in my eye. In other words, either extreme is going to cause issues.

If we set the lower limit of the light pencil to 2mm and want a 25X scope, then a 50mm objective does the job. What should the bottom power be for us to continue to utilize all the available light? Since we are adults, let's use 6mm as our upper limit. 50mm divided by 6mm is 8X (close enough), so the ideal mechanics would tell us that 8X-25X by 50mm would be just about right in our theoretical scope, allowing for adult eyes.

That is how I understand objectives and power ranges and it makes sense to me.

Bill.

Does this all work equally well for both Tasco and, let's just say, Zeiss or NightForce scopes, and maybe everything in between?


-BCB
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  #24  
Old 06-19-2018, 04:44 PM
TinMan TinMan is offline
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Yes, a very good refresher course. Thank you and welcome to the board. I remember some of that from when I was looking for binoculars some years ago, looking at specs, and searching for "exit pupil" specs for the different brands and models.
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  #25  
Old 06-20-2018, 03:19 AM
carbon carbon is offline
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mhinagoya, the reasoning behind the second half of your post is what I was trying to adovocate for and to see if there are other factors beyond exit pupil that contribute to scopes with large eyebox.

Most of the time people pooh-pooh large objective scopes by using the first part of your post, i.e. old eyes “waste” extra light pencil diameter (exit pupil). I’m glad you didn’t go down that road.

Unfortunately, master optical engineers who post on forums are few and far between. I feel there has to be other factors beyond large exit pupils that makes some scopes (of the same magnification) easier to use. I know large eye relief and the ability to move one’s head around and still see through the scope are key to scope “friendliness”, at least to me.

I wonder what those additional factors are? Optical formula, scope length, scope tube diameter, etc?

Last edited by carbon; 06-21-2018 at 07:59 AM.
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  #26  
Old 06-20-2018, 02:46 PM
recoillug recoillug is offline
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While this has nothing to do with objective size, one thing that makes a scope "friendly" or not for me has a lot to do with the reticle's subtension. When shooting for precision, especially at long distances, the reticle has to be fine enough to allow for a precise aiming point (such as an F-Class x ring at 600 yards or Pdog at similar distances), but really fine reticles tend to wash out and become blurry after a while with these old eyes. However, heavier reticles, while easier on the eyes, just doesn't allow for those precise aiming points. So finding scopes with the reticle subtensions just right can be difficult and frustrating, at least for me.
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