Home Forums Hunting & Shooting Hunting Optics 50MM obj scope brighter, given same exit pupil size, as a 42mm?

Lens quality bring equal, and given the power of a 50mm objective scope set to give the same exit pupil size as a 42mm objective scope, is the perceived brightness the same -- or is the 50mm brighter?

You're likely to perceive it as brighter since you're using about 20% more magnification with the same exit pupil.

Yes. You'd have higher magnification which increases your twilight factor.

A simple answer is that when comparing like optics, a
50 mm scope or binocular is brighter than a 42mm.

The extra magnification and certain lens coating tints may fool you into thinking it's brighter, but all scopes transmit the same amount of light for a given exit pupil diameter.

Based on this statement you are not taking into account the quality of lens or coating. Is that correct?

How does that work?

8x56 --> 7mm exit pupil
6x42 --> 7mm exit pupil

pi * (56/2)^2 = 2463 sq. mm objective area
pi * (42/2)^2 = 1385 sq. mm objective area

2463/1385 = 1.78 --> 78% more photons land on the 56mm lens of the 8x scope and get focused into the same 7mm beam size.

Why would this not increase the intensity of the beam of light that shines on the viewers eye? And would it not follow that "more light" strikes the eye? Same area beam cross section x higher energy density per unit area = "more light" in the beam.

mathman,

Thanks for the very interesting and new way of looking at this. I've never heard this argument considered in this way before. I don't know if the "math" (pun intended) really works out like you've posted but, it sounds reasonable and it certainly gives some food for thought.

But are you magnifying the light? I don't think so but I could be wrong.

Think about it like a funnel.The light coming out of the small end of a funnel is no greater no matter how large the big end.If it were, a large ended funnel would work like a flashlight with a beam coming out the small end.

Bad analogy. A funnel acts as a aperture to light. Lenses focus the light and increase the intensity of the beam. Otherwise you couldn't burn things with a magnifying glass.

I agree photon flux or number of photons is what we mortals call brightness. Good AR coatings reduce the number of photons that bounce off the objective and thus do not contribute to forming an image, larger aperture increases the surface area of the objective allowing a greater number of photons to come in contact with it. Why do the astronomers want bigger and bigger apertures if this is not so? The brightness of the image you see is dependent on the number of photons that hit the rods and cones in your eye, more photons into the objective equals more into the focal point so the formed image is brighter. Someone tell me how a 24 mm objective set at 4X and a 50 mm objective set at 4x with the identical glass and AR coatings on all surfaces have the same "brightness".

This can occur when low levels of light (low reflectivity target)enter the scope. You might perceive equal brightness, but more likely both are low and the difference is hard to see.

A practical answer to the OP's queston:

I have a pair of 4.5-14 conquests. One 44mm objective, one 50mm objective. Sometimes you can see a difference, but not always. I cannot always see a difference in brightness between the 44 mm unit and my Zeiss 85 mm spotter which should be pretty comparable coatings wise. Under certain conditions, the spotter is much, much brighter. Looking at a full moon, the conquests are sort of equal, but the spotter is so bright it's actually painful. Looking at a deer under tress or against brush in low light, the 44 mm conquest, the 50 mm conquest and the spotter are much closer together in perceived brightness.

For the people who think cheap optics are just as good, I have a Leupold Wind River 65 mm spotter that is nowhere as good as either conquest. The conquests are brighter and will resolve bullet holes better.

brighter? maybe. But can you really see the difference. I had heard that it doesn't matter what the objective lens is as your pupil can open just so far....anything more and your eye just can't see it. Any truth to this?

Even if that is the case,I wouldn't think the difference between a a 40mm and a 56mm or even 72mm would be enough to register with your vision.After all even a magnifying glass can't effectivly magnify ambient light enough that you can see the difference.

I still don't see a scope as working in that manner but I don't know enough about it to argue the point.It is interesting though.

Mailmanmark
I've read that a healthy eye can only utilize the light from about a 7mm exit pupil.For older folks it is even less.That being the case,it's only at high magnification that larger objectives become useful.

I think we've been over this one before.

Several quantities are involved: number of photons, density of photons in the beam cross section, and the relative size of the beam and the aperture in your eye.

In the example I hashed out above I was careful to select the magnifications and objective sizes to produce the same size exit pupil, and this 7mm exit pupil shaft of light could all get past a fully dilated pupil in your eye.

In your example of 4x24mm and 4x50mm scopes you have not controlled the exit pupil variable. Say the eye in question can dilate to 6mm. The shaft of light coming out of the 4x scope is as wide as will fit into the eye. The 50mm scope is producing a huge 12.5mm exit pupil, but most of the light in the beam isn't getting into the eye. The extra photons which landed on the bigger lens are just illuminating the iris with a 6.5mm wide annulus of light that fails to reach the sensor, the retina.

You'd need to turn up the magnification for the 50mm scope to get more photons to the retina.

Yeah, and some "antique" eyes may not fully realize a 7 mm pupil aperture...

DF

Not sure I agree with your statement here. Are you talking the percentage of light a scope can effectively transmit or the concentration of light energy ?

google twilight factor

Yeah, good old twilight factor.

This is a concept that originated in Europe, which takes into account both exit pupil and magnification. It suggests that extra magnification allows us to see better, as long as the exit pupil is at least 6mm-7mm. Essentially, extra magnification places us closer to the target, just like walking closer.

This leaves lens quality out of the equation, but does provide a pretty good idea of potential dim-light performance--and is exactly why a larger objective lens does provide a better view in dim light: It allows a 6mm-7mm exit pupil while optically placing us closer to the target. This is also why a 6x scope with 7mm exit pupil doesn't provide the same dim-light detail as an 8x or 10x scope with a 7mm exit pupil.

An extra-large objective on scopes less than 7x, however, doesn't provide any "extra" light to the eye, and tends to increase parallax problems.

The OP stated "lens quality being equal" so I didn't feel it necessary to repeat it. But yes, I'm talking about scopes of equal optical quality and that would include coatings.