42mm vs. 50mm Objective - Which is Best?
What's the Best Choice?
Is a 50mm or a 42mm objective better when it comes to riflescopes? Will a 50mm objective give me a brighter image and a wider field of view? These are questions we get all the time. Let’s walk through the advantages of the 42mm and the 50mm so you can outfit your rifle with the correct scope this season.
Just so we’re clear, the objective of a riflescope is the lens furthest away from your eye. In most cases, the objective lens is the largest part of the entire tube body and plays a critical part in the amount of light that enters the scope. The basic premise surrounding the objective diameter is: the larger it is, the more light is allowed into the scope and ultimately transmitted to your eye. And, the larger it is, the ability to see detail increases as there is a larger “sweet” spot in the lens itself.
A common misconception is that the 50mm objective provides a wider field of view then a 42mm objective. This is false, FOV is determined in the design of the eyepiece and has no baring on the FOV. A quick way to prove the FOV misnomer is to punch a hole in a business card size piece of paper and cover the objective. In looking through the scope, you will notice that the FOV does not change, even though you are limiting the size of the objective diameter, however the image will be much dimmer. This proves the importance of the objective for gathering light. Here is a quick video that demonstrates this theory with a binocular.
So, how do you know if 42mm or 50mm is the best fit for you? There are advantages to each objective size so it pays to understand the relationship between scope, gun and the type of hunting/shooting you would like to do. 5 Tips to help you make the right choice:
1: Best Mounting Position
Why is this important? Being aware of the size of the objective diameter will give you an idea as to how the scope is going to sit on your rifle.
Mounting a 50mm scope in most cases will require the use of high mounts and will result in mounting the scope higher off the receiver which can cause an issue with proper eye alignment and discomfort. A good shooter knows that having a low, consistent cheek weld improves accuracy.
Mounting a 42mm objective is going to allow for a lower profile. You’ll be able to achieve a consistent cheek-weld, fast and easy eye alignment, and you can buy and use standard medium height mounting rings.
JL explains the advantages of each and why he chooses one over the other when it comes to different hunting applications
2: Consider the Weight
A 50mm objective lens typically adds to the overall weight of the riflescope.
So, if you’re concerned with weight, bulkiness, and weldability you’d probably want to go with the 42mm objective. Especially if you’re planning that once in a lifetime sheep hunt which requires a lot hiking……You’ll appreciate keeping your gear as light as possible.
If you’re hunting whitetail from a tree stand or box blind and don’t mind carrying a few extra ounces, moving up to 50mm lens might be for you.
Weight Comparison of the TORIC Riflescope Line:
3: Glass Quality / Increased Resolution
One of the biggest reasons hunters look for larger objective lenses is because they think it will provide a brighter image. Please keep in mind that if inferior glass is used, all you have is a big poor quality lens at the end of your riflescope which defeats the purpose of going to 50mm.
Here are the facts. If high quality glass is incorporated, a larger lens will provide greater resolution (which is the power to resolve detail in various lighting conditions). Since image quality and brightness are directly related to glass quality, not the size of the lens, you should seek out an optics company that incorporates high transmission glass and superior coatings. This will insure maximum brightness in low- light situations and provide an overall exceptional visual experience.
Therefore, a 42mm scope with exceptional glass will outperform a 50mm objective scope with average glass in low light and provide an overall better image.
What about resolution…Can I see more detail? It is true that a larger 50mm lens will provide greater resolution due in part to a larger sweet spot (center of the lens) being greater in size than the sweet spot of a 42mm lens.
4: High Magnification
Is it true that a 50mm scope will allow the shooter to stay out longer and make shots in extremely low light better than a 42mm scope?
As the sun goes down, your effective hunting range gets shorter and shorter with a high powered optic. As you may have already discovered, you’ll need to bring the magnification down to get as much usable light as possible. However, there are advantages to a larger objective size when it comes to shooting at long range. Which leads us to the next point about exit pupil.
5: Light Gathering / Exit Pupil Advantages
What is an exit pupil and why is it important when choosing the correct objective size for my riflescope?
The exit pupil is that circle or beam of light that you can see in the eyepiece lens if you were about 10 inches away from it. This shows you how much light is being transmitted to your eye and it can ultimately give you an improved and brighter image as you turn down the magnification setting. You can visually see this when hunting in low light.
The exit pupil can be calculated by dividing the objective diameter (mm) by the magnification.
If you do the math you’ll see that a scope set on its lowest magnification setting will provide you with the largest beam of light. Conversely, the highest magnification setting will provide you with the smallest beam of light.
A 3-15x50 scope has an exit pupil of 16.6mm at 3 power to 3.33mm at 15 power (50 divided by 3, then 50 divided by 15).
As you can see at 3x, there’s a big difference in that window of light vs 15x. Does a larger window of light equal a brighter image in the dark? While it may seem like “the larger the better” concept would be true, there’s a limit how much your eye can take in. Here’s the facts:
The human pupil is only capable of expanding between 2mm in bright light to 7mm in total darkness (for healthy adults with great eyesight). So if you’ve got your scope on the lowest magnification and you’ve got an exit pupil of 16.67, or pretty much anything larger than 8 or 9 mm, all that light is a waste. It’s not profitable since your eyes won’t be able to use it at all.
For example, a 3-15x50 TORIC at 10x will give you an exit pupil (window of light) of 5mm. This 5mm exit pupil will be very good in low light situations for hunters who need to see detail at 10x. A 3-15x42 at 10x has a window of light (exit pupil) of 4.2mm – still great for hunters. But, as you start to crank up the magnification to 12x for example on the 3-15x50 = 4.1 and 12x on a 42mm scope = 3.5. Now it’s starting to get below 4mm which is tough to use in low light. So, for more flexibility with using higher powered optics especially in low light conditions, a 50mm objective lens is the best choice.
Another benefit to having a large exit pupil is that you’ll have greater flexibility to look through the scope and get on target in a hurry. You can quickly get on target for when that big buck steps unexpectedly into your line of sight. Precision shooters have the luxury of finding that dead on spot with a 2mm exit pupil, but hunters will appreciate the freedom of still getting dead on even if your eye isn’t perfectly aligned with the optical axis.
Conclusion – Advantages of a 42mm & 50mm Objective Lens
Advantages of 42mm Scopes
- Provides a lower mounting profile
- Enables a sufficient and comfortable cheek-weld
- Good exit pupil size for close to mid-range low light hunting
- Light-weight and compact, only weighing 20 ounces
- Smaller size allows better accessibility in and out of gun scabbards or ATV cases
Advantages of a 50mm
- Efficient for long range shooting
- Good exit pupil size for longer range shots in low light conditions
- Improved image quality and greater resolution in low light conditions
- Ability to use higher magnifications longer as available light diminishes
- Flexibility to find sweet spot faster with larger exit pupil
Get “Right Sized”
To determine which size objective lens is best for you, you need to evaluate what type of hunting or shooting you’re going to be involved in. If you’re looking for a more versatile and all-purpose riflescope that does its best in any light, terrain, and hunting situation, then you’d probably do best with a 42mm objective lens size.
But, for those who want to maximize any potential of light gathering capabilities with any light of day, a larger objective diameter lens will provide more light than you can use, plus some extra perks.