So what is an objective lens on a scope on how does it affect you? The objective lens is positioned to the front of the optic. An objective lens is the lens closest to the object being viewed.
When you are choosing an optic you may see a set of numbers that read like so 3-9x 40mm. This isn’t a math problem, but a look at the scopes specs.The 3-9x represents the magnification levels the scope can reach and the 40mms represents the objective lens diameter. When we are looking at rifle objective lenses we are measuring in mms.
Objective Lens & Light Gathering
The purpose of an objective lens, and where size comes into play, is when we talk about light gathering. An objective lens gathers light to give you a bright and vivid picture. The bigger the lens, the more light you are capable of transmitting through the scope. However, there are some mitigating factors to consider. To understand these we have to talk about how you eye’s work.
The human eye detects light through our pupils, which can shrink and grow as necessary to see. In the dead of night, someone with perfect eyes has an exit pupil diameter of 7mms, this wider exit pupil allows you to see better in the dark and gives you natural night vision. During the brightest parts of the day yours, pupils are at 2mms to avoid being incredibly uncomfortable throughout the day. For you objective lens to be at its maximum level of light gathering efficiency the exit pupil of the scope should match the exit pupil of your eye.
Magnification affects the exit pupil of the scope as well as the objective lenses size. There is actually a very simple formula you can use to determine the exit pupil size of a scope.
It goes like this, Scope Diameters in millimeters ÷ Magnification level = Exit pupil size. Let’s say you are hunting in the evening and the sun is beginning to go down. Your object lens is 40mms, and you want an exit pupil size of 8. Just a little above normal to make things a little brighter. You scope is a 2 to 10 power scope. So you set the magnification to 5. 40 ÷ 5 = 8 mm exit pupil.
With this in mind, you can imagine that some objective lenses are going to be large and powerful enough to gather enough light to be quite uncomfortable unless a proportional amount of magnification is attached to the rifle. This works both ways, if you have a lot of magnification with a small objective lens you may find your sight picture being dim and dark. Bigger is not always better.
Why Large Objective Lens Is Not Necessary Better One?
Outside of what our eyes are capable of seeing we have a number of different challenges with large objective lenses. A large objective lens is typically anything over 42 mms. This means 50 mm and up are considered large objective lenses. The first issue is cost, they will be more expensive when put against a comparable scope of similar quality. They will also be heavier, which makes off hand shooting more difficult.
Another issue is large objective lenses are mounted higher and farther from the barrel. This can decrease the overall accuracy and precision of the scope, as well as degrade the ability to make broad adjustments up and down. Because the scope is mounted higher you run the risk of losing a good and proper cheek weld without a special adapter.
Benefits of Larger Objective Lenses
Large objective lens scopes have their place, and they are relatively useful for long distance shooting at a fixed target. They are also good when hunting or shooting at near night, or during the night. Like everything, there is a certain level of compromise involved with choosing an objective lens.
With most shooters I suggest starting with something in the 40 or 42mm range. You’d be hard pressed to find and overall better size for most applications. You can, of course, go smaller with scopes designed for close range shooting, or low magnification shooting. Going larger is a tricky proposition, and often, in that case, the juice isn’t worth the squeeze for average shooter.