Objective Lens Diameter: Factors to Decide How Large

[JungleShooter]

 

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. Simple math 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. [edited]



Table for recommended exit pupil size

The table as PDF file is here: 

View attachment 1555276085_792020475cb3a135ada6f8.27145938_Scopes. Recommended Exit Pupil Size. 02.pdf





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:
https://www.airgunnation.com/topic/scope-model-comparison-with-specifications-3-16x-300-500/

You find the factors influencing the magnification not in the thread itself but in the PDF document linked at the end of that post.


Happy shooting!

Matthias

 

[CampFussell]

Thanks for the post Matthias. Very informative

I use my Sightron SIII 10-50x60 on 30 power. Guess I need to dial it back to 20 or 24 power to get optimum eye box? But heck I've been using 30 power for almost 3 yrs now and pretty used to it?

Thanks again

Fuss

 

[JungleShooter]

Fuss,

if it works for you at 30x, GREAT! Reducing magnification increases your eye box.

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." So, with time and patience a smaller eyebox can be accomodated for.

It's when time and/or patience are lacking that a small eye box is more of an issue -- for instance in hurried shooting -- read hunting.

Now, for field target I understand that there are time limits, but they aren't as pressing as on a stalking hunt....

 

[CampFussell]

JungleShooter,

Yes, well said.....New shooters seem to really have a problem finding the eye box on my scope at 30 power. Most the time I will go ahead and turn the magnification down to 10 power when friends come over to shoot , and when they are comfortable with that we will gradually work them up to the 30 power. Once they can read the numbers and clearly see the 2 mm ten ring on a bench rest card they see why I use 30 power and rarely want to go back to 10 power.

Another thing that helps speed up the location of the eye box is a rubber eye cup mounted on the eye piece. If it's on properly and hasn't been bumped around, you are very quickly locked in on the eye box just as you touch your brow up against the rubber eye cup . I use a Bisley rubber eye cup when shooting field target to make target acquisition faster.

 

[JungleShooter]

Great idea, the rubber eye cap, I knew about them, but they don't seem very popular, and so I ignored them. If I ever go high magni, I will be sure to get one of those.

Sweet rig you have there! Nice! One of those ultra-special guns... -- do tell: What FPE could you get with some adjusting but without swapping out parts. How much cc of air does it hold? Brand and model?

Matthias

 

[CampFussell]

Mathias

If you ever need a rubber eye cap be sure and get a "Bisley" it's by far better than the rubber eye cap that came with my Sightron.

https://www.precisionairgunsandsupplies.com/ProductDetails.asp?ProductCode=bisley+eyecup

The rifle pictured is my .177 Thomas FT. It was built by Mike Niksch in 2016. It's favorite ammo is the 13.4 gr JSB Monsters at roughly 820 fps to keep it under the 20 foot pound limit for field target.

http://www.thomasrifles.com/home.html

Can't remember the cc of the air tube, but get's over 100 shots from a 200 Bar shot down to 100 Bar.

https://www.airgunnation.com/topic/130-shots-on-one-fill-26-five-shot-groups-at-28-yds-with-my-thomas-ft/

Another great thing about the Thomas is it can be adjusted to different power levels by the quick change out of a power jet.

https://youtu.be/VGeWbDZtmeY

They are loud without the shrouded barrel. I love the shroud on mine and it hasn't affected accuracy one bit. I tried mounting a Huggett off the end the barrel but accuracy went to crap? Really messed with the barrel harmonics?

https://youtu.be/YvqTMvBXpW4

Here's Mike shooting a 25 meter BR card

https://youtu.be/squbXsM0-Gg

It's the only air rifle I have that I managed to shoot a 250 score card with. The closest I have got to it is a 247 with my .177 Cricket.

Fuss

 

[JungleShooter]

Very cool, Fuss, thanks for your answers and the links.

100+ shots at 20FPE, great! Both thumbs up!

 

[ackuric]

This is a good guide but could be more comprehensive, you should clarify the range is technically best between 2-4 MM in light conditions and 3-8 mm in dark...3mm isn't a bad neutral recommendation but I personally use 2.5~ for light conditions and 5~ for dark...99.999% of my shooting is in 'light condition' and my young eyes do just fine...the 2.8+ on your chart really should be in the green IMO...I'd simply round up anything over 2.5 to 3 and put it in the green...but again thats just my young eyes and in the conditions I shoot...



Nonetheless good guide!



The formula is Scope objective diameter / 'Your eye pupils diameter @ the shooting condition' = ideal max magnification...


So for my 50mm objective scope, I do 50/2.5 = 20 for light condition, and 50/5 = 10x for dark. 



-Matt

 

[JungleShooter]

Updated the original post and table.

 

[DuncanHynes]

Most of this I concur. Like you made the point that a quality 44mm may very well be brighter than a cheaper 50+...

I did obtain some time back a 44mm Sig Tango6 18x FFP scope. I thought it would be a lighter option than some 56mm mambo jambo on an already heavy rig. I saw wild spec differences as the models were being phased out. Surely, 31 ounces is a misprint. NOPE. haha. It is so dense I am certain there is lead and mercury in this thing. But the absolute clearest sight picture I have ever seen. ED glass is astounding. I do like 42 and 44mm objectives but the features I find are always in a 50mm setup. (The Tango 6 only parallax to 25 yards.)

 

[JungleShooter]

Duncan,

do I understand you correctly, you do like the 42 and 44mm objectives, but the features that you want in a scope usually come only with scopes that are 50mm?

If that is so, I believe it -- as a matter of fact just this week I found out a similar either-or polarization: I researched scopes with a bottom end of 2x magnification, and a top end of 9x, 10x, or 12x. Because I wanted something much lighter than a 18x/20x/24x top end magnification scope with their heavy 50mm objectives.

Same situation -- most of these magnification ranges (at least in the 200-500 dollar bracket) are lacking a host of features and specs that I cannot do without:

The most disappointing lack of a feature was the minimum parallax distance -- 50 yards, 100 yards..., with these scopes they clearly didn't want to make any money from airgunners....

I think turrets consistent with the reticle is a basic and logical thing: either MOA/MOA, or MIL/MIL. But nope, many mix and match those, but that's really not matching, but mis-matching... (UTG and Nikko are prime offenders in that area)

Then I gave up on getting FFP, few had it. But if you manufacturers are making holdovers more complicated with SFP, at least give us turrets that click! But in this magnification range most turrets are capped, so to me that's the manufacturer's way of saying to the shooter: Don't click this scope much, it's not made to do that. (Sightron, MTC, Athlon, Vortex, are some of those who disappoint here)

Well, I found a few scopes that qualified to these requirements. But then they often didn't have side parallax (Hawke!), or no IR.

All those things I find fairly easy for scopes with 50mm objective lenses.... Go figure! ???

 

[Swami]

Where did you end up with this, I am in a similar situation. There is always something missing from the ideal scope setup for me. My ideal is.

40 mm objective

10yrd paralax min

Mil/mil

Good half mildot recticle 

Max mag 16x 

Ideally ffp

Not heavy 

Side focus and good clear glass.

Any suggestions welcome!

 

[FredAZ]

A front objective is just that, the front lens and is NOT indicative of the light-transmissivity of the unit in question - if the internals are tiny, it will be dark.

So you can not buy on size alone.

Anything larger than what matches the "system" is extra weight at best, or added glare and aberations on the bad end....

 

[nced]

"most of these magnification ranges (at least in the 200-500 dollar bracket) are lacking a host of features and specs that I cannot do without"
Last week I returned my 5-15x50 Hawke Panorama under warranty and learned that it was messed up inside, however that scope has been discontinued and it was suggested that I accepted a free replacement of a 4-16x44 Vantage with side focus. I accepted the replacement and before even mounting it on my HW95 the scope was boxed up and returned today. The main reason for returning was that the new 4-16x44 Vantage would sharply focus down no closer than 15 yards whereas the older Panorama focused sharply down to 10 yards at 15x. Matter of fact, the whole current Hawke Vantage scope ALL have a 15 yard minimum focus distance be it AO or SF!I tested the scope at 10 yards and at 16x the image was so fuzzy that it would be impossible to aim at a 10 yard 3/8" diameter killzone. The Vantage also didn't have a "lighted reticle feature" like the Panorama. I don't use the "illumination feature" very often, however it's a necessity (for my 72 year old eyes) when aiming at a dark painted field target set in a shaded lane.

I searched the Hawke web site for a replacement scope with features similar to the 5-15x50 Panorama in the $300 (list price) price range scope. I found the 4-12x50 Hawke AirMax at about $250 (list price) but the magnification is only 4-12x, and there is no reticle illumination. The closest available Hawke to the 5-15x50 Panorama is the AirMax 30 Compact 4-16x44 except it has a 30mm tube and a $380ish list price. There is also a 30mm tubed Hawke 4-16x50 AirMax 30 SF with my required features, however it's list price is $450ish!