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Outsized Objectives article in Petersens

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Category: Scopes
Forum Name: Rifle Scopes
Forum Description: Centerfire long gun scopes
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Topic: Outsized Objectives article in Petersens
Posted By: smitty
Subject: Outsized Objectives article in Petersens
Date Posted: November/10/2007 at 11:48

Interesting article. They explain exit pupil and relative brightness and describe a test you can do to see how much light is transmitted thru your scope.


The test is to cover the objective end of the scope with a piece of paper, then shine a flashlight thru the occular end and the paper will have the light projected on it. Measure the diameter of that projection in mm.  That is the objective to use for calculating the exit pupil.


I did the test on my K4, WPI 8x24 binoc and Safari 8x30 binoc.  Here are the results:


K4 Scope:

Diameter of the projecton is 37mm

actual magnification of the K4 is 3.7x

There for exit pupil is 37mm divided by 3.7 which = 10mm  That's pretty good considering most people eyes can use only 7mm.  I was happy about that!

The relative brightness is exit pupil squared which is 10 x 10 = 100. That doesn't mean anything to me, yet!


Now the WPI binoc:

Diameter of the projection is 24.6mm

magnification is 8, therefore exit pupil is 24.6mm divided by 8 = 3.1mm

Retative brightness is 3.1 x 3.1 which = 9.4.....that's a small number compared to the 100 above!


The Steiner Safari:

Diameter of the projection is 29.5mm

magnification is 8

Exit pupil is 29.5mm divided by 8 = 3.7mm

Relative brightness is 3.7 x 3.7 which = 13.6, also pretty small compared to the 100 of the K4.


If I did this right, my K4 scope is many times brighter than both of my hunting binoculars.  Does this mean I need new binoculars?


Can someone explain what all this means?







Posted By: Dolphin
Date Posted: November/11/2007 at 06:59
I am not familiar with this particular way of measuring exit pupil, as it can be easily calculated by dividing the objective size by the magnification.  Another way to measure low light performance is to calculate the twilight performance, which is the square root of the magnification times the objective size.  Now all of these figures are relative, as if you compare these numbers from chinese made crap to your excellent optics, they do not mean alot.  And no you do not need to replace your binoculars.  Because of the larger amount of magnification, then the figures when calculated come out in favor of your scope.  However, the same priciple applies, if you want to collect more light and are going to be using your binoculars in low light conditions, then you want to have larger objective sizes, but keep in mind that you also want to maintain the quality that you already have.

D. Overton

Posted By: smitty
Date Posted: November/11/2007 at 10:45
What I think they are saying is to use the diameter of the projection instead of the objective size on the label to calculate the exit pupil. From what I remember in
the article, the diameter of the projection may be smaller at say, 9x that is is at 3x, for a variable power scope. Some scopes with 50mm objective may not use the
full 50mm because their internals don't allow full use of it.

I was hoping more of you would try this with your scopes, especially a variable scope to see if the diameter of the projection remains the same thru the different
magnification settings. I only have the K4 right now and I did this with both of my binocs.

Based on how these numbers relate to each other, it seems logical that a single scope on lower magnification would be brighter than at higher magnification. This is
good to know if hunting at dawn or dusk. I have a friend who hunts pigs and is often complaining that he can't see the animal thru his scope and it's a 50mm.
Maybe he has it set at 10x and if it was set on 5x, he'd see it more clearly?

I'm suspicous about some scopes not being as good as their price would imply. If there was testing that shows us the scopes performance, we could make better

For the term "relative brightness", what does it relate to?


Posted By: koshkin
Date Posted: November/11/2007 at 14:44
Most scopes use almost the whole objective lens.  With variable scopes, typically the whole (or close to it) objective lens ends up being used at high magnification, and only a portion of the objective lens is used at low magnification.

The term relative brightness is just about useless for  describing scopes and binoculars used for hunting, so I would not agonize about it too much.  The best usage for the term I have seen so far has been to use it to sucker people into buying stuff they have no need for.  It does mean a little more for astronomy applications, but pretty useless for sporting optics.

The origin for the term is indeed scientific: the amount of light that gets through the scope (let's call it radiant flux) is proportional to the area of the exit pupil, which in turn is proportional to the diameter of it.  However, in terms of how well a scope performs, that is only a fraction of the story, so I would not dwell on it.  There are a lot of other parameters that are important.

As a general rule of thumb, for low light, use high quality scopes and binoculars with exit pupil of 6mm or greater. 

For your friend with a 50mm scope, he may be able to see more if he dials down on magnification a little bit.  Going down to 6x or 7x is probably a good start.  If that does not help him see better, it is time to get a better scope (you did not say what his scope is).


Posted By: koshkin
Date Posted: November/11/2007 at 14:45
Forgot to add, twilight factor is also best used as a sales tool.  Not much other use for it.


Posted By: silver
Date Posted: November/11/2007 at 17:04



This was a demonstration, not a test.  Now, if they had placed a LED into a scope cap and placed a color light meter at the other end...

"If we weren't all crazy we, We would go insane."   Jimmie Buffet

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