# Tough Question On Scope: Objective & Exit Pupil

Discussion in 'Long Range Scopes and Other Optics' started by iSnipe, Jul 27, 2010.

1. ### iSnipeWell-Known Member

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67
Joined:
Nov 23, 2008
Hey Guys!

Ok, got a question here that I once knew but now forgot. I don't know the exact terms,
so I'm sort of guessing...

My question has to do with the objective side of the scope and the exit pupil, I think.
It has something do with regardless of the size of the objective lense(bell), and what
power one is using, only so much light can go through to the eye. It had some thing
to do with 5.

On a 3-9x40mm scope, I think the setting that allows the most optimum or maximum
light through is the 8 setting. Reasoning: Dividing 40 by 8 = 5.

Here's another example: Using a 3-10x50mm, 10 would be the maximum light allowance
setting because 50 divided by 10 = 5.

I'm not sure if I have this right, but I'm somewhere close, I think.

What I'm trying to figure out, is how to figure out or determine what scope(40mm,50mm)
to get that allows me the best light transfer based on a certain setting... not scope brand.
If I recall correctly, it was once told to me that you get to a certain point and it's no longer
helpful because the eye can only soak in so much light.(or something to that effect)

One may think to get the largest bell or object lense(mm) and problem solved,
but it's not that easy.

I'm going to buy a new scope. It will either be a 3-9x40mm or 3-9x50mm. I don't
necessarily need the widest view, like what the 50mm will do, but trying to figure
what I wrote above.

Do you know what I mean?

iSnipe

Last edited: Jul 27, 2010
2. ### Jon JackoviakWell-Known Member

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385
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Dec 5, 2003
Well you have the calculations correct, but you have the theory pretty close too. To calculate the exit pupil, you take the objective divided by the power setting. (3-9x40 on 8 power is an exit pupil of 5: 40/8 = 5). But that would not be the maximum mm's of light to travel through the scope. The largest exit pupil on this particular scope would be to have the power set at 3. (40/3 = 13.3). I think where you come up with the 5 is a young healthy eye can only intake about 7mm's of light. As you get older that number starts to go down. An older person may only be able to intake 5mm's of light. So the younger you are the more light your eye can use.

3. ### iSnipeWell-Known Member

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67
Joined:
Nov 23, 2008
JJ,

Not sure if that's what I was looking for, but it may be. I'll have to figure that one out.
Everything you said, sounds right, except the 7 part. Maybe where I go the previous
info from, was a person "ball-parking" a figure. Your reasoning makes sense as well.

=============================

To JJ and all others:

Besides a larger field of view with the 50mm, does it allow more light to travel to the
eye so one can see easier in low light conditions... like at morning, dusk and the

To me just setting a 3-9x40 scope to 3 was not the simple answer I was looking for.
It seemed more than that. ??

Any other input is appreciated. Thanks again JJ.

iSnipe

4. ### Jon JackoviakWell-Known Member

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385
Joined:
Dec 5, 2003
Sorry about the confusion. That question is not a simple as you ask. A 50mm scope will allow more light through the scope than a 40mm on the same power.

50/8 = 6.25mm
40/8 = 5.00mm

So if you have both scopes on 8 power the 50mm objective will allow 25% more light than the 40mm.

Now to get the same amount of light through the 40mm, you can just turn down the power to about 6.5 power.

Last edited: Jul 27, 2010
5. ### FEENIXWell-Known Member

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11,064
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Dec 20, 2008
(C&P)
The Advantages Of A Large Exit Pupil
By Hugh Birnbaum
(Optics - The Advantages of a Large Exit Pupil)

A significant characteristic of an optical viewing instrument is the diameter of the exit pupil. The exit pupil is the circular patch of image-forming light the instrument presents to your eye. If you point a riflescope toward a brightly lighted wall or a patch of clear sky (but not at the sun!) and position your eye about 10 inches from the eyepiece, along the optical axis, you will see a bright disc of light in the center of the field. That disc is the exit pupil. The larger it is, the brighter the viewing will seem, because more of your eye will be bathed in light.

All other conditions remaining constant, changing the magnification to 6X reduces the exit pupil to 3.3mm.

You can calculate the size of a scope's exit pupil by dividing the effective objective diameter in millimeters by the magnification. For a 4X 32mm hunting scope, divide the 32mm objective size by 4 and you find that the exit pupil is a generous 8mm in diameter. With a 6.5-20X 50mm target/varmint variable scope, the exit pupil ranges from a large 7.7mm at 6.5X to a smallish 2.5mm at 20X. In a low-light situation, all other factors being equal, a lower magnification setting will provide seemingly brighter viewing than a higher one.

It is tempting to conclude that the largest obtainable exit pupil is the most desirable. But that's not always the case. The catch is that the pupil of a normal human eye opens to a maximum diameter of 5mm to 7mm, depending on the individual, even in extremely dark surroundings. Exit-pupil diameters that exceed about 7mm deliver more light than your eye can accept.

On the other hand, large exit pupils offer advantages beyond simply flooding your eye with light. With riflescopes, one of the blessings of a large exit pupil is greater freedom to position the eye with respect to the optical axis. When you must mount the rifle quickly for a now-or-never shot, you don't have the luxury of placing your eye behind the ocular with the exquisite precision a target shooter can lavish on finding the sweet spot of a 2mm or smaller exit pupil. You'll be truly grateful for the chance a large exit pupil gives you to see what you need to see, even if your eye is less than ideally located.

Looking through a 1.5-6X 20mm variable-power riflescope set to 1.5X reveals a sizeable 13.3mm exit pupil (crosshair is too out of focus to show in photo).

Another benefit of a suitably large exit pupil is the ability to see the scene clearly, without eyestrain. If you compare two scopes of equal optical quality, the one with the larger exit pupil will probably strike you as more preferable. Both will give you the same visual information, but one will make your eye work less.

Having praised generous exit pupils, I must confess that the riflescopes I use most often are fixed-power target models with exit pupils from about 1.1mm to 1.6mm. In daylight ranging from heavy overcast to bright sun they do what I need done.

One cautionary note regarding the exit pupil. There is no correlation between the size of a scope's exit pupil and overall optical quality. Some superb scopes have small exit pupils and some real dogs have very large ones. Exit-pupil calculation is a useful tool in selecting scopes, but it isn't the whole toolbox. You should consider everything and then ultimately believe your own eyes.

I find the above article very helpful as I was researching what factor an exit pupil plays and must confess I never paid attention to the exit pupil - I just trusted my poor old eyes.

Good luck!

Ed

6. ### iSnipeWell-Known Member

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67
Joined:
Nov 23, 2008
Gentleman!

Between the replies, I believe I received the very information I was looking for. The posts are educational and makes sense. Although it's not exactly what I recall from the old info I received a few years ago, it sounds about right and is the information I can use when choosing additional scopes.

I understand the math in each of the posts and did some figuring to make sure I understand... and I think I got it, so I'm good.

I appreciate the time you guys have taken to help me out. It does take some effort and dedication to take one's valuable time helping others. I know because often I go out and try to help others as well when I can.

Thank you very much!

iSnipe