I NEED PROOF

[razmuz]

Got a big deer camp bet to prove.  Where can I get written proof that a 3X9 scope gathers more light on the three point setting than the nine point setting?  Or am I wrong?  All things being equal.  Thanks.

[Dave in WV]

You can email Leupold or other scope makers and ask them. BTW, you are correct. Divide the objective lens diameter by the magnification to get the EXIT PUPIL size. The larger the exit pupil, the more light the scope can gather.

[CJ]

You can also look through it at dusk or dawn and turn the power ring. I usually turn down to lower powers and streeetch it a few more minutes. With quality scopes you will see a difference. When you get older and get into really big objectives diminishing returns kicks in though. When the exit pupil of the scope is bigger than your pupil will open in low light the rest isnt usable.

[Graybeard]

Sure hope you didn't bet the farm cuz you lose. Sorry.

Point one. Scopes DO NOT GATHER light. They transmit light. See my long explaination of this in the thread at: http://www.graybeardoutdoors.com/phpbb2/viewtopic.php?t=20064

Point two. The exact same amount of light enters the objective lens and is transmitted thru the scope regardless of the power setting. The only variables in how much of the light that enters the objective that is transmitted is the quality of the optics and coatings and the size of the lens in between the objective and your eye. This doesn't change with power setting.

Point three. What you thinking of is that the exit pupil is larger at lower power. That is because the exit pupil is the magnification divided into the objective diameter in mm. Pure and simple. That don't make it brighter.

Point four. There are several supposed numbers or ratios used to determine what scope is brighter. Usually which is used by a specific manufacturer is based on how they handled the necessary compromises with their scopes. Twilight factor is one fairly uniformly recognized measure that most seem to agree on. I honestly don't. But it goes up as power goes up.

If you have a quality scope with excellent optics and multi coating you'll find that you can in fact see much better at higher powers than lower powers. This supports those who like the twilight factor as a measure of optical brightness. It does have some merit even I acknowledge that. If you have a poor quality scope or a very high power scope that at the max magnification takes the exit pupil below 4 then it will appear brighter and you will think you see more at the lower powers that keep the exit pupil up to 5 or more.

Sorry to break it to ya but proof ya ain't gonna find.

GB

[Zachary]

If you have a quality scope with excellent optics and multi coating you'll find that you can in fact see much better at higher powers than lower powers.
GB

I certainly agree with point as I have discovered while in the field.  The following is my most recent experience:

I was hunting hogs in the Texas hill country about 3 weeks ago.  The moon was kinda bright, and the animals were feeding mostly at night.  In Texas, you can hunt hogs at night if you want to.  Well, I was waiting for a hog to come by.  Unfortunately, only deer came by - feeding on the corn that I had spread on the ground about 100 yards from me.  I had my Nikon Monarch 3.5x-10x-50mm Illuminated set on about 4x or so (it was getting very dark so I couldn't read the readings).  Anyway, as GB said, yes, when you divide the size of the objective by the power setting, you get a specific number.  This number is greater at lower power settings, and thus smaller at higher power settings.  However, I could see the deer much better at the max power setting - 10x, than I did at the lower power setting.  

The above is just my most recent experience.  I have tested this on many other of my scopes in the past, and the same results occur.

Zachary

[Mueller Optics]

The size of the objective lenses determines the amount of light which reaches the eye. The larger the objective lens, the greater the brightness and resolving power (sharpness) of the image seen through the optic. Therefore, an optic with a large objective lens is best for low light conditions, e.g., early morning or late afternoon (dawn and dusk) hours and overcast days. R.B.I. (relative brightness index), twilight factor and R.L.E. (relative light efficiency) are common indices used in the optic industry. However, brightness is only one criteria to be considered when purchasing optics but not the most important. The following factors are worth considering: magnification, objective diameter, the type and quality of the lens, type of lens coating and prism quality.

Optical Coatings
Brightness can also be improved through the use of optical coatings. The optical elements are coated to reduce internal light loss and glare, which in turn ensures even light transmission, resulting in greater image sharpness and contrast. Choosing a good optic with a good lens coating will translate to greater satisfaction with the product you ultimately select. Lens coatings range in quality as follows: Coated - fully coated – multi coated – fully multi coated. Multi-coated means one or more surfaces of one or more lenses have been coated with multiple film. Fully multi-coated means all air-to-glass surfaces have multiple films. Both multi-coated and fully multi-coated are very good choices. Fully multi-coated lenses give the best light transmission and brightness images and therefore is the most desirable.
President Mueller Optics