Here is one I found.
Exit Pupil
The size of the column of light that leaves the eyepiece of a scope (usually measured in millimeters). The larger the exit pupil, the brighter the image. To determine the size of the exit pupil, divide the objective lens diameter by the power of the scope. IE; a 4x32 scope would have a 8mm exit pupil. 32/4=8.
The maximum the human eye can perceive is 7mm in total darkness. As you get older you pupil will only dilate to 6mm then 5mm.
The 3-9x40 and 3-9x50 will both transmit the exact same amount of light to your eye, IF you place them on the correct power.
50mm scopes are not brighter than 40mm scopes. It is all relative to what power the scope is on. It is easier to explain using a fixed power scope for now. People up to around 40 can perceive around a*7mm exit pupil from the scope, because that is the maximum diameter of their pupil in low light or darkness. So ideally you would want a scope that transmits a 7mm exit pupil, the only problem is how do you know what its transmitting (its easy). Just divide the objective lens size by the power and the answer is the exit pupil or the amount of light that is coming out of the back of the scope. ie: a 6x42 Leupold transmits a perfect 7mm exit pupil (that's why they make scopes and binoculars in strange numbers like 8x56, 6x42, and 9x63. The formula is a little different for a variable scope, you would take the objective lens size and divide it by 7 to determine what power to put your scope on for a 7mm exit pupil (the most your eye can handle). ie: a 3-9x40 would need to be set on 5.7x to produce the desired 7mm exit pupil. A 3-9x50 would need to be set on*7.1x to get a 7mm exit pupil. You see, these two scopes (40mm and 50mm) are just as bright, they just do it on different powers. A larger objective allows you to use your scope on a higher power and still have perfect light gathering. A 3-12x56 transmits perfect light on 8x. A 1.5-5x20 does it on 2.9x.
*
First decide what power you need for what you are doing, then multiply that by 7 and the answer will be the size of objective lens you will need.
All of these formulas for producing the same light with different powered scopes are only relevant if you are comparing scopes from the same manufacturer and the same models. ie: Leupold VX-II. You can't get a BSA 3-9x50 and set it on 7x and expect it to be just as bright as a Leupold 3-9x50, because it is the glass and coatings that primarily determine the scopes ability to transmit light to your eye efficiently.
*
Basically,*a 7mm exit pupil is a 7mm exit pupil regardless of the objective that helped produce it. All things being equal a 20mm scope set on 2.86x will be just as bright as a 56mm scope set on 8x.
When your pupil is constricted in the*day time*a scope with a large*exit pupil*is great for quick shots because the exit pupil is larger than your pupil and you can get a full picture faster.
Exit Pupil
The size of the column of light that leaves the eyepiece of a scope (usually measured in millimeters). The larger the exit pupil, the brighter the image. To determine the size of the exit pupil, divide the objective lens diameter by the power of the scope. IE; a 4x32 scope would have a 8mm exit pupil. 32/4=8.
The maximum the human eye can perceive is 7mm in total darkness. As you get older you pupil will only dilate to 6mm then 5mm.
The 3-9x40 and 3-9x50 will both transmit the exact same amount of light to your eye, IF you place them on the correct power.
50mm scopes are not brighter than 40mm scopes. It is all relative to what power the scope is on. It is easier to explain using a fixed power scope for now. People up to around 40 can perceive around a*7mm exit pupil from the scope, because that is the maximum diameter of their pupil in low light or darkness. So ideally you would want a scope that transmits a 7mm exit pupil, the only problem is how do you know what its transmitting (its easy). Just divide the objective lens size by the power and the answer is the exit pupil or the amount of light that is coming out of the back of the scope. ie: a 6x42 Leupold transmits a perfect 7mm exit pupil (that's why they make scopes and binoculars in strange numbers like 8x56, 6x42, and 9x63. The formula is a little different for a variable scope, you would take the objective lens size and divide it by 7 to determine what power to put your scope on for a 7mm exit pupil (the most your eye can handle). ie: a 3-9x40 would need to be set on 5.7x to produce the desired 7mm exit pupil. A 3-9x50 would need to be set on*7.1x to get a 7mm exit pupil. You see, these two scopes (40mm and 50mm) are just as bright, they just do it on different powers. A larger objective allows you to use your scope on a higher power and still have perfect light gathering. A 3-12x56 transmits perfect light on 8x. A 1.5-5x20 does it on 2.9x.
*
First decide what power you need for what you are doing, then multiply that by 7 and the answer will be the size of objective lens you will need.
All of these formulas for producing the same light with different powered scopes are only relevant if you are comparing scopes from the same manufacturer and the same models. ie: Leupold VX-II. You can't get a BSA 3-9x50 and set it on 7x and expect it to be just as bright as a Leupold 3-9x50, because it is the glass and coatings that primarily determine the scopes ability to transmit light to your eye efficiently.
*
Basically,*a 7mm exit pupil is a 7mm exit pupil regardless of the objective that helped produce it. All things being equal a 20mm scope set on 2.86x will be just as bright as a 56mm scope set on 8x.
When your pupil is constricted in the*day time*a scope with a large*exit pupil*is great for quick shots because the exit pupil is larger than your pupil and you can get a full picture faster.
