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View Full Version : Scopes with skinny ends, what am I missing?



Petamocto
07-02-2016, 10:21 AM
This is going to be the most entry-level question I've ever asked, but I'm not embarrassed to admit my ignorance.

Why is it that some scopes / optics (particularly combat-like optics) do not flare out into bigger objectives?

It doesn't seem to just be a cost-saving measure, because a lot of these with skinny ends are even more expensive than conventional scopes.

For example, take these from Vortex and Leupold:

http://www.vortexoptics.com/uploads/series_rfl_strike-eagle.jpg

https://www.leupold.com/wp-content/uploads/2016/06/_VXHOG_1-4x20_web-530x123.jpg

All of my common sense is telling me that more light coming in = better, so obviously I am missing something.

It doesn't seem to have anything to do with magnification, they have a similar magnification than ACOGs which do flare out.

Why would someone want less light on purpose, if the answer isn't that it costs way less?

Please educate me, thank you.

lone-wolf
07-02-2016, 10:30 AM
I have no idea, but with a small objective there's one less thing to worry about when trying to mount your scope as low as possible.

I'm wondering if field of view & magnification plays a part in it. At 1x, with an objective larger than the tube, wouldn't that have a fish eye effect?

blacksmithden
07-02-2016, 10:35 AM
It all has to do with light gathering and magnification. The more you magnify an image, the less light there is to spread out. If you want to magnify the hell out of something, you want a big objective lense to gather as much light as possible so the image that you see isn't dark. Scopes with small objective lenses will have lower or no magnification. High magnification scopes will have larger objective lenses.

Foxer
07-02-2016, 10:51 AM
Well BSD and LW have basically got it. What you're looking at is low powered scopes - usually 1 - 4 power. A large lens allows for two things - more light to be collected AND of course a larger field of view. That's necessary if you're going out to 6 or 9 power because the further the magnification is the less light there is to play with AND the smaller the field of view gets, so you need a wide objective lens to make sure the field of view is wide enough to find your target and bright enough to see it.

At 4 power, none of that is really necessary. And as far as light transmission goes, after a certain point the human eye can't process additional light so for the short range 1-4's there's absolutely no value in increasing the lens to increase light because it won't help much (provided the glass is good). And as 1-4's are usually intended for brush guns for close encounters (But giving them the ability to reach out and touch someone if needed ) a low mounting profile and light weight are key features

I've had a leupie 1-4 like that for a number of years, shot it off my 7600 for a while, I do like that slim profile on 'fast' guns.

Petamocto
07-02-2016, 10:59 AM
...At 4 power, none of that is really necessary...

Okay, interesting. I was going to ask why ACOGs flare out then if there's no benefit, but upon looking at their specs, they don't really flare out as much as they seem to.

On their common models like the TA11 (3.5x) and TA31 (4x), they're actually only a 35mm or 32mm objective; it's the case shape and flaring sun shade that makes it seem like it's flaring more like a conventional scope than it actually is.

Rory McCanuck
07-02-2016, 10:59 AM
It has to do with pupil size.
Your eye's pupils will only expand up to about 7mm in total darkness, more like 5mm in actual low-light shooting conditions.
If the scope has a 20mm objective, and it's at 4x, it will concentrate everything down to a 5mm picture (20/4=5), the same as your pupils.
If it has a 50mm objective, and it's at 5x, it will concentrate everything down to a 10mm picture, but your eyes cannot use all the 'extra'.
The excess picture gets blocked by your iris, negating the extra $250 you spent on the scope ;D

Petamocto
07-02-2016, 11:05 AM
It has to do with pupil size.

I call BS, because your pupil can still benefit from a much better picture that's put in front of it to see.

When I look in a dim room I see things poorly lit, but when I look through an expensive huge camera lens, that lens puts an image in front of my eye that is much better-lit and clearer than what my eye is capable of seeing on my own.

I can't count the amount of times I've taken a picture and said "Holy cow, this is much better than I remember it looking". It's the opposite effect with a crappy phone camera; your eyes think it's lit okay, but the pictures they take indoors are terrible because the lens is 2mm.

My eyes are still only capable of seeing the same amount of light by themselves, but that doesn't mean a huge objective can't make it easier for your eyes to see things, even at 1x magnification.

Foxer
07-02-2016, 11:29 AM
I call BS, because your pupil can still benefit from a much better picture that's put in front of it to see.Yes but only to a point. Pupil size limits the amount of light that can be concentrated and still actually get into your eyeball to be processed by the brain. And for a VERY healthy YOUNG person, the max size you have to work with is 7mm. And as mentioned, for most of us it's more like 5 mm or less as we get older and the eye muscles lose a little of their oomph. (which is also why you tend to need glasses as you get older for reading and such).

So what we're talking about is the ratio between the objective lens and the actual hole in your eye that we can focus light thru. And we can change the one, but not the other.

Remember - the farther you magnify out to, the more that ratio opens up so that's why it IS valuable to have a wider objective on a scope that can magnify out to 6 or 9 times or more, but why at shorter magnifications that's not really valuable.

Here's a little blurb on it from leupold -

Exit Pupil

Variable-power scopes also offer a light management advantage over fixed power scopes. As you change magnification, you also change the exit pupil, a measure of the light passing through the scope to your eye. Exit pupil is derived by dividing the diameter of the objective lens by the magnification of the optical device. The human eye can dilate from about 7mm for a young person in total darkness to 2mm in bright sunlight. Ideally, the exit pupil of the scope should match or slightly exceed the dilation of the eye’s pupil so that the eye receives as much light as possible.

An example of exit pupil: if a hunter goes out with a 3.5-10x40mm scope early in the morning, he can dial his scope down to 5x, and he will receive an 8mm exit pupil (40mm divided by 5x equals 8mm exit pupil). Later, when the sun is high and bright, he can turn his scope up to 10x and still receive a 4mm exit pupil (40mm divided by 10x equals a 4mm exit pupil), which is excellent for bright conditions. As evening approaches, he can turn the scope back down to 8x or so to receive a 5mm exit pupil (40mm divided by 8x equals a 5mm exit pupil) and still have plenty of light to make a shot at the end of legal shooting hours.

Rory McCanuck
07-02-2016, 11:39 AM
I call BS.
Then why are you asking instead of telling?

Foxer
07-02-2016, 12:17 PM
So just to put that in perspective, a 1-4 scope with a 20 mm objective lens (skinny lens as you say) at max magnification will offer a 5 mm exit pupil (20 / 4 = 5). Which for most people is about as much as you can use. At 3 power it's just under 7 - the most even a young man with healthy eyes can use. Having a larger objective lens would not really help AT THAT MAGNIFICATION - but it does help the farther out you go in magnification so even 2-7's tend to have a bit of a bell on the end as they tend to go into the 30's on the objective lens to get enough light at 7xmag.

Now - just to make it MORE complex - remember, we're talking about the size of the hole in the eye that we can get light thru. That's kind of fixed by the mathimatics of the way scopes work - you can't really just cram more light into a smaller area.

BUT - the QUALITY and AMOUNT Of light transmitted thru the scope CAN change by using better optics and coatings. So - the OTHER factor in how bright and clear a scope can be is of course what the lenses themselves allow thru to be packaged inside that 5mm beam of light that you've got to work with. It's almost impossible to make a lens which is perfect at multiple magnifications, which is why it's easier to make a fixed scope that is bright and clear. So - manufactuers tend to set theirs up so that the 'perfect' conditions are somewhere close to the middle of the road - maybe around 6-7 power for a 3-9. There is a TINY bit of distortion issues even in good quality scopes if you jack it to the lowest or highest setting - and the highest setting tends to be set up to allow the 'minimum' light necessary in. This is great for sunny days, but not so great hunting at dawn or dusk.

So, you've just learned what a lot of experienced hunters know and what a lot of newbs make a mistake doing - at low light or very low light you are often NOT doing yourself a favour trying to jack the scope to its max power. Start off with it around it's 'mid' point (or less if you're expecting close shots) and if an animal is so far away that you can't hit it with that magnification you probably have time to dial up to full mag. I splashed my longest shot moose at just over 250 yards at 5 power, and had no trouble making a good double lung heart shot, but i see a number of newer shooters setting their scopes at close to max as their standard setting, and that doesn't always give you the best actual results.

Booletsnotreactwell
07-26-2016, 10:56 PM
There's a shit load of stuff when it comes to optics, as much as guns. Seeing as you brought up the ACOG and LPV's I'm guessing you're in the market for an LPV but want to be sure it doesn't trade something off in performance over the ACOG.

Go down to the spec sheets for each of those products and compare what you have vs what you want to have. If the LPV of choice exceeds whatever you currently have in specifications then don't worry about it.


So if the LPV has greater FOV at 100m, greater eye relief, greater magnification, less critical eye relief, etc, you're good to go.


As to why ACOG's flare out, there's many possible angles to this but my take is that with the technology as it applies to glass quality, coatings, etc, having the objective built that way was probably the only way to get the ACOG to work back when it was first conceived. Scopes have improved tremendously in 20 years so it's a possibility such features are no longer necessary. The ACOG having an overly large objective for the given magnification could be for no other reason than being an old design which it is.

Foxer
07-26-2016, 11:10 PM
There's a shit load of stuff when it comes to optics, as much as guns.That's true. And arguably of the two the optics are more important. A mediocre gun with good optics might still shoot well, but a great gun with bad optics is going to shoot badly. It pays to know enough to match your optics choice with your gun and your needs.

BrotherRockeye
07-27-2016, 03:46 AM
it's so you don't have to mess with your rear iron ;)

Booletsnotreactwell
07-28-2016, 06:50 AM
That's true. And arguably of the two the optics are more important. A mediocre gun with good optics might still shoot well, but a great gun with bad optics is going to shoot badly. It pays to know enough to match your optics choice with your gun and your needs.


I spent a few days reading material across the net before I purchased my first optic, maybe 40hrs worth of reading material total before I felt I had enough knowledge to make an informed decision. Too often guys will drop big cash on a Zeiss or something and while it's nice glass the features are completely outta whack with what they're trying to accomplish.

Foxer
07-28-2016, 08:07 AM
I spent a few days reading material across the net before I purchased my first optic, maybe 40hrs worth of reading material total before I felt I had enough knowledge to make an informed decision. Too often guys will drop big cash on a Zeiss or something and while it's nice glass the features are completely outta whack with what they're trying to accomplish.
True. It's gotta be a good match.

Justice
08-01-2016, 10:42 AM
20mm objectives are mostly about weight and length. Great big scopes on stuff like lever actions, shotguns and some semi-autos aren't needed and wreck the balance.
For example, the Leupold VX HOG 1-4X(9.5" long) weighs 8.1 ounces(230 grams) vs the 9.9 ounces(281 grams) for a VX-1 2-7X 33mm (11.3" long). That 1.8 ounces isn't much until you've lugged the thing up a mountain or used one in thick bush.
And camera lenses and telescopic sights ain't the same thing.