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Are some 6mm exit pupils brighter than others?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 06:19
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I understand exit pupil but would these 2 scopes appear to have identical brightness?

1-4 x 24mm set on 4x = 6mm exit pupil

2-8 x 42mm set on 7x = 6mm exit pupil


Thanks.







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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 06:52
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Glass qaulity, optical design also play a huge role.

I have read people who have both a trijicon 3-9x and a 2.5-10x say the later seems brighter at all magnifications.

iMO if your biggest need is brightness in low light, get the best glass u can afford and choose a large objective.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 09:45
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As said above, there is much more to it than the numbers. A cheap scope with bad glass will not be bright.

There are great low light scopes, if that is the priority.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 10:53
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My bad - I meant to add in my comparison that both scopes would have identical quality glass and coatings.












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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 11:23
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Brightness is a terrible way to describe rifle scopes.  The way image brightness appears to a human eye is fairly subjective and really depends on the scene.  Comparing perceived brightness of two scopes at different magnification is kinda like comparing apples with coconuts.  The way your brain processes the image depends on how well it can see detail.  Something with stronger edge definition will often look brighter.

A scope with wider FOV will usually look brighter to the eye, all else being equal.

A scope with higher contrast will usually look brighter.

ILya


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 13:20
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The 42mm objective has 3 times the surface area of the 24mm. If you are in dark conditions, and the eye is adapted to the dark and the pupil of the eye is dilated to 6mm, 3 times as much light will be transmitted to the eye by the larger scope.
That means that you will be able to see a deer when it is darker out.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 14:14
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And age doesnt help you because as you get older your eye may not dialate to 6mm. The important thing is that you have an understanding that the scope you have be it great or be it so so will be at its brightest when set at a power that allows a 6mm exit eye pupil.  True quality glass does make a big difference.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 15:34
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Originally posted by Shenko Shenko wrote:

The 42mm objective has 3 times the surface area of the 24mm. If you are in dark conditions, and the eye is adapted to the dark and the pupil of the eye is dilated to 6mm, 3 times as much light will be transmitted to the eye by the larger scope.
That means that you will be able to see a deer when it is darker out.

That is largely true, but not 100% accurate, since the scope set to higher magnification is collecting light from a smaller FOV.  While it has a larger objective lens, the energy density incident on that lens is lower.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 16:34
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Ilya, thanks for getting back on this one.

"the energy density incident on that lens is lower"
I do not understand how the light energy density at the objective lens can be affected by changing the magnification.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 17:52
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Originally posted by Shenko Shenko wrote:

Ilya, thanks for getting back on this one.

"the energy density incident on that lens is lower"
I do not understand how the light energy density at the objective lens can be affected by changing the magnification.

When magnification goes up, your field of view goes down.  When the field of view goes down, it subtends a smaller area.  If you subtend a smaller area you have less available light to pass onto the eye.

A cleaner example of this effect is with photographic lenses.  When you talk about bright or dark lenses, you talk in terms of F/#, not in terms of collecting aperture.  F/# is simple the ratio of the focal length (which  controls the FOV) to the diameter of the limiting aperture.  If you compare lenses with the same focal lengths (same FOVs), the one with the larger limiting aperture will be brighter.  However, once the focal lengths are different, you have to look at the F/# to determine what will give you the greatest energy density at the focal plane of the lens.

With riflescopes, it is a little more involved since there are three optical systems in them and depending on the magnification and other settings the location of the limiting aperture is different.

I am too lazy to go through the math here, but basically you have to account for the solid angle subtended by the riflescope.

For the simplicity of the argument lets say we are looking at two riflescopes with 6mm exit pupils.  One is a 24mm objective with 4x magnification and another is a 48mm objective with a 8x magnification.

Solid angle in a small angle approximation scales approximately the same as area (it is technically the area of the surface of the subtended spherical segment), so when you go up to twice the objective lens diameter, you get four times the area.  However, the scope is now looking at a 3.96 times smaller solid angle, so most of what you gained by increasing the objective you lost due to lower flux.

Now, that also assumes that we are looking at lambertian surfaces, which is typically incorrect in the real world, so you get more light from things in front of view than from the ones toward the edge of the FOV.

On top of that your eye is better at making sense of what is straight ahead of you, and magnifying the details generally makes it look clearer/brighter to you simply because of how we process information.

Ultimately, you gain more from a larger objective lens, than you loose from narrower FOV, but it is not a straightforward linear relationship.

ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 17:58
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 17:59
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Eduardo = pupil who exited...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 18:07
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 18:12
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 19:09
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I totally agree!

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/03/2016 at 19:12
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Koshkin, I am going to ask that you do all the math here, please; not because I doubt you, but I shan't contribute to your laziness.

I agree with everything this far. With the math outlined, we shall see.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/05/2016 at 08:00
tpcollins View Drop Down
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Thanks for the explanation Koshkin . . . I was actually able to understand it somewhat.









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