Scopes: Objective Lens Diameter: Factors to Decide How Large It Should Be
SCOPES: Objective Lens Diameter:
Factors to decide how large the diameter should be
Some time ago this question boggle my mind as I was making a decision on a new scope that was going to cost me more than the gun. How large of an objective lens diameter do I really need – since I don’t want to spend more than necessary.
So, here are some considerations that I gathered from the forums (Thanks for all the help!). If you have any additions or corrections, let me know, I’m still learning....!
Advantages of a larger objective diameter
- #1 Brighter Scope Image
The larger the diameter the more light enters the scope and therefore the larger the exit pupil, resulting in a brighter scope image (all things like glass quality etc. being equal).
However, consider that the human pupil can only open up (dilate) from about 4mm to 8mm (with 8mm after 30min in the darkness, with young, near perfect eyes). People in their 40s and 50s can only get to about 5mm to 6mm. Therefore, more exit pupil than what the human pupil can open up to does not give more light to the eye.
Note also that a lower quality scope with a large diameter can(!) be less bright than a much higher quality scope with a smaller objective. The best is to compare them side by side at your local gun shop or gun range.
Now, there is another reason that favors a larger objective diameter:
- #2 More tolerant as to WHERE the shooter’s eye needs to be
The smaller the diameter and the larger the magnification, the smaller the eye box, i.e., the area where the shooter’s eye has to be in order to see the full scope image. If it is smaller, it will take more time and patience by the shooter to find the eye box and maintain the eye inside of it until the shot is fired.
When doing unhurried shooting (e.g., benchrest) the shooter has the time and can patiently adjust his eye position to find and maintain it inside the eye box. But this time (or patience) might be lacking when doing hurried shooting – read hunting or shooting in certain competitions.
Explanation: The eye box or eye relief tolerance depends on the size of the exit pupil. Calculation: objective lens diameter / magnification = exit pupil. Cf. chart below. The exit pupil should not be too small. Bob Sterne recommends 3mm, with 2mm being marginal.
Table for the recommended exit pupil size:
The table as PDF file is here:
Disadvantages of a larger objective diameter
- #1 More expensive scope
Bigger lenses are usually more expensive, but then, I want the performance, so maybe “Buy once, cry once” is not a bad slogan.
- #2 Heavier scope
Now, if I’m already lugging around an 9-pound rifle, is 0.2 pounds more going to make such a big difference?
For example, the 4-12x Hawke Panorama comes with 40mm (19.1oz) and 50mm (22.6oz) objectives. That’s a difference of 0.2 pounds for the 50mm objective.... – Just one can of Coke already weighs 13.9 oz – that’s four times as much....
- #3 Taller scope
A taller scope must clear the barrel, and sometimes that requires higher scope mounts. Before buying measure or ask the seller or on the forums for advice.
Factors that influence the scope magnification or the “power” of the scope
How much magnification a scope should have is a related but different issue. It very much depends on the type of shooting that one wants to do.
Some of these issues I discuss in the following thread:
You find the factors influencing the magnification not in the thread itself but in the PDF document linked at the end of that post.
In optics you get what you pay for. A good 40mm can outperform a poor 50mm. Some of the older, cheap used-to-be-large objective scopes were built on tubes designed for smaller objectives. The too-narrow tubes actually cut out part of the tramsitted light, essentially turning them back into smaller objectives. And lens quality is paramount. Before buying a big objective scope, it might be good to think why you want one. A bigger objective gathers more photons resulting in higher resolution. For long distance shooting that's a plus. For short range work, not so much. (To see the gain in resolution, all things being equal, square the diameter of the lens. 40mmx40mm=1600. 50mmx50mm=2500 or a gain of 56%.)
(For the record, I made a comment that referenced pi since we're dealing with circles, but realized it still came out to 56 % since pi is in both equations, so I deleted my comment--sorry to any who read it for the confusion).
Safe and Happy Shooting!
Ed, the Airgun TuneMeister
Here is the thing,forget all the mumble jumble,you rarely need a OJ lens more than 44mm...40 will do 90% of the time.....unless you have a eye problem ..or shooting 200 yds in dim light.....and I agree with Red....and do not forget bigger scope usually + more weight...I "well known USA scope maker is what you need...better optics cost more ,butt with them you will a smaller lighter scope.....
Another thing I See,some people got the money to spend on too expensive optics,one someone says they have a budget of say $120 on a scope too many say get this or that and they are all over $200,tell me how that is helping someone? if you do not know a good scope in their price range, why do you tell them to get a scope over twice their budget?
I get what I can afford and try to be specific on my needs and thank those that can figure that out.
Another thing it is a "pellet gun",I bit 90% of those shooters rarely shoot over 60yds...this scope deal to me is becoming ,"mine is bigger than yours"....screw that...it is practice that makes a good shooter,not the size of his scope!ha ha....I mean what I say,butt I hope you see some humor n what I say....and truth hurts....some.
Couple of odd-ball things I've found over the years.
A really well made small lens is actually better than a poorly made large objective lens.
Hunting, you might be concerned with crappy light....most of us shoot targets(paper or metal) in at least decent lighting.
Don't change what is working. If it's working now, changing to an "up graded"scope is more about pride of ownership (ego) than results.
Based on over 45 years heavily involved in airguns, objective diameter is just one of several places that field target (as it's played now, not its original intent) has had negative effects on airgun design and use.
Yes, if you aren't allowed a range finder, a large objective, high power scope will give you an advantage...in field target competition. However that's almost the only situation where the advantages outweigh the disadvantages...a high mounted, heavy scope that ruins the balance of the gun, can't cope with targets that move, can't be maneuver in heavy cover, are awkward to carry...and on and on. Add a stock that's been influenced by field target styling cues (deep forearm, heavy, lots of drop at the butt) and the gun is almost unusable under real world field conditions.
For plinking, pest shooting, small game, an objective of 32-40 mm will almost always make the gun easier to shoot than something bigger. Sure, spend up a little for better glass, but you hit diminishing returns very quickly. A $1000 scope is nowhere near ten times as good as a $100 scope. You'd be doing well to get twice the performance for ten times the price. Think about that.