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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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