Why are AR 3G scopes small in the front?

[Frankly]

How come all the 3-gun tactical style scopes are small diameter up front and larger at the eyepiece? Versus hunting and long range scopes that reverse this? 

 

Should't I want a larger front to gather more light? And a larger back to give me better viewing? So whether I'm in close quarter combat or shooting precision rifle, shouldn't my scope be fat at both ends? 

[Hi-Power Jack]

I wondered about that too, when I first started looking into 3-gun scopes.

 

Not sure, but I believe it's because they're 24 mm as opposed to 30+ mm, in 

an effort to cut down the size and weight 

 

We'll see if I'm right on this as soon as an expert comes along.     

[Frankly]

I’m sure the Hubble space telescope designer is lurking.

[Hi-Power Jack]

[Jfitz427]

That’s a good question. I always wondered that myself. 

[davsco]

guessing at lower power (max of 4, 6 or now going to 8x), don't need all that extra glass (cost and weight) for light vs the 16, 22x etc on longer range scopes.

 

look at a nf nxs 5.5-22x56, has an exit pupil of 2.55 at full mag.  vs a vortex 1-6x24, which has an exit pupil of 4, so even with a much smaller objective it has more light transmission.

 

[Localizer]

On 1/2/2019 at 11:15 AM, Frankly said:

How come all the 3-gun tactical style scopes are small diameter up front and larger at the eyepiece? Versus hunting and long range scopes that reverse this? 

 

Should't I want a larger front to gather more light? And a larger back to give me better viewing? So whether I'm in close quarter combat or shooting precision rifle, shouldn't my scope be fat at both ends? 

While i’m no scope expert, I think it’s because:

 

1. They are relatively low power scopes. (1x-2x -4x-8x).

2. The low magnification does not require a big objective lens for a good/usable exit pupil...exit pupil is calculated by dividing the objective lens by the magnification. Thus, a 1-4x scope has an exit pupil of 24mm-6mm.  Compare this to a typical 3-9x40mm hunting scope.  Exit pupil is 13.3mm - 4.4mm.

3. Keeps weight and sizedown...weight & size (not necessarily cost) is primary driver to acquire low power scopes.

 

I think the market will start to demand bigger objective lenses on low power scopes with 8x magnification. 3mm exit pupil makes the scope hard to use in a dynamic environment, effectively limiting the capabilities of the scope.

[Harpo]

+1 on the "Low power = small objective" theory.

And I personally think it would be fun to try a 1.5-8x40,  if nothing else to try low-light use at minimum magnification.

[Localizer]

I have two low power scopes; a Nightforce NXS 1-4 & a March 1-4.5.  The March scope, with its parallax adjustment enables me to almost discern lines on an NRA SR-1 target at 600yards at our competitions.  We are limited to top end of 4.5power though.

 

The ideal low power scope would have in order of importance:

 

1. 8x top end.

2. Fist focal plane reticle.

3. parallax adjustment. 

4. At least 5mm exit pupil (40mm objective)

5. MSRP < $2,000

 

The USO 1.5-8x comes close...but it’s a bit heavy for this application...and not a true1x at low end.

[jon49erfan]

On 1/2/2019 at 8:37 PM, davsco said:

guessing at lower power (max of 4, 6 or now going to 8x), don't need all that extra glass (cost and weight) for light vs the 16, 22x etc on longer range scopes.

 

look at a nf nxs 5.5-22x56, has an exit pupil of 2.55 at full mag.  vs a vortex 1-6x24, which has an exit pupil of 4, so even with a much smaller objective it has more light transmission.

 

Exit pupil and light transmission are not related. 

[Jakobi]

Exit pupil is really the only driver of objective size. At 8x 40mm would be awesome but ~25mm is pretty serviceable. 32mm would be perfect.

[Blockader]

This has also been a mystery to me. Every 2-7 scope has at least a 32mm obj, so I assumed it had to do with improving the 1x aspect. 

[Hi-Power Jack]

10 hours ago, jon49erfan said:

Exit pupil and light transmission are not related. 

 

7 hours ago, Jakobi said:

Exit pupil is really the only driver of objective size 

 

I'm confused by the terminology.

 

I always believed ??  that a 6x by 40 mm would be brighter than a 6x by 32mm ???

 

Am I incorrect ?

 

Then, in English, what affects the brightness of the scope (light gathering ability) ?

[Localizer]

Let me try explaining this again.

 

Exit pupil is the diameter of the image in the eyepiece (part of the scope you look through).

 

Mathematically, exit pupil = objective lens diameter / magnification

 

Therefore, a 1-8x24mm scope,

exit pupil at 1x = 24/1 = 24mm

exit pupil at 4x = 24/4 = 6mm

exit pupil at 8x = 24/8 = 3mm.

 

from a practical point of view, our eyes sees the area occupied by the image size. That is to say, we perceive the surface area of the circle.

 

therefore, a 6mm image is not 2x the size of a 3mm image but actually 4 times the size.

 

area of 6mm image = pix6x6/4 = 28.3mm2

area of a 3mm image = pix3x3/4 = 7.1mm2

 

The 3mm exit pupil scope will be difficult to “get behind” and use in all but benchrest style shooting.

 

In conclusion, size matters...the bigger the objective lens the more useful the top end magnification.

 

On the topic of light gathering & brighter image, these are two different topics.

 

objective lens diameter matteres as it aids in light gathering...a bigger objective lens will gather more light.

 

light transmission and image brightness have to do with the quality of the glass, and coatings thereto...it is therefore possible to have a scope with a smaller objective lens have a brighter image than a scope with a larger objective lens...when comparing high end and lower end scopes.

 

hope that helps.

Edited January 31, 2019 by Localizer
Editing

[Blockader]

That all makes sense, but still begs the question, why not have a

1-6x32? Am I missing something? 

[Hi-Power Jack]

22 minutes ago, Blockader said:

 why not have a

1-6x32?  

 

I think it's weight ??? 

[minnesota1]

I've wondered that as well.  It seems like all 1-6 or 1-4's have the small tubes all the way across.  I use some for hunting and would like a little bigger bell on the end to suck in more light later in the evening.  That being said, many use the low power scopes for 22's and small rifles that would look weird with a larger scope and it would make it a lot harder for young or smaller shooters to look through them if they were elevated to accommodate the larger bell on the end.  Just some thoughts.

 

Plus, they wouldn't look Tactical!!!  LOL

[Jakobi]

Leupold put together a pretty good article on common scope design misunderstandings.

https://www.leupold.com/leupold-core/core-knowledge/articles/top-5-optics-myths-explained

Edited January 31, 2019 by Jakobi

[Localizer]

1 hour ago, Hi-Power Jack said:

 

I think it's weight ??? 

Nope. Group think, large organization momentum etc.

 

if enough customers indicate there’s a market for larger objectives on low power scopes, they’ll all jump in.

 

i personally want as much objective lens as I can get. 

 

For me, the optimal low-power scope is a 1-8x42mm.

 

Nightforce can use the existing NXS 2.5-10x42mm model as a jumping point to minimize tooling.

[Hi-Power Jack]

There are no 1x variable scopes that have large objective lenses ?

[Blockader]

I also don't see how it could be weight. My 1-6x24 Razor is 25 ounces, the new model is I think 22 oz. If they made a version of the new lighter one that was 1-6x32 but that took it back up to 25 ounces that is the one I would choose. I think others would too. 

[polymerfeelsweirdman]

I don't do optics anymore so I'm a little rusty on my terminology but in telescope design you want your objective lens to be the limiting lens in your system other wise you end up with problems like vignetting. I Ihink I could sketch this better than I can describe it but this is some geometrical optics stuff.

 

In a nutshell and as others have stated, if you made the objective larger and not changing the radius of curvature or things like that you would at best increase the exit pupil size without increasing brightness- think of it as power per unit area. You get more total power aka light through, but you are also increase the area that is illuminated aka the exit pupil and as a result will just have an exit pupil bigger than your pupil which is not that helpful (might be a bigger "eyebox")