Scope light gathering

[johnwoitalla]

In looking up scope evaluations on both forums and videos, I have seen repeated mention about how well whatever particular scope is being discussed-gathers light. My understanding of lenses and the physics used to describe them is exceedingly weak, but as far as I remember, the amount of light a scope, or any lens for that matter, gathers light is entirely determined by the size of the objective lens. Regardless of the kind of glass, the coatings on the surfaces of the individual lenses, or the price point of the scope, the amount of light it gathers, I think(correct me if I'm wrong), is entirely determined by the front lens. Therefore, light gathering doesn't really need to be an issue in the evaluation. If you want to know how much light the lens gathers, just look at the size of the front element. That is the only thing that determines how much light enters the scope. And as far as what percent of the gathered light is actually transmitted to the shooter's eye, it seems to me, that without highly controlled, scientific measurement, a true ability to discern the difference between say, 90% transmission and 92% for example, cannot be observed by the human eye/brain. I think we shooters are very prone to confirmation bias and prejudice with regard to how much we may have paid for a scope and/or the reputation the name brand has that is associated with a scope. I would love to get your feedback on this subject.

 

[dizzums]

youre right to an extent .. they shoot you alot of fluff out that you dont really know what it means just to sell stuff ... but a larger lens on LOW power mag generally lightens your view in a low light situatuon and i definitely appreciate my BUDGET 4x gamo scope in the late evening lol .. lights ole fluffy nuts right up lol ... but yeah the while 'gathering' stuff is bullsht, the phenomena is inherent in 'all' scopes depending on several factors ...one of which is not tube diameter, which is a 'light transmission' selling point also .. has little to do with it .. a bigger tube is a much more robust build though imo, especially in how the adjustments work ...

 

[Brent]

The configuration inside the erector tube will effect light transmission. The quality of glass will also effect it. So in theory you could have a front objective lens that is smaller with a better set up configured inside the tube and better quality glass and still have better light transmission than my lesser quality larger objective lens. Bigger is not better if you have to give up quality to get it.

 

[Hal4son]

The mechanics of a variable power scope rob some of the light transmission to your eye. Higher magnifications will reduce the light transferred through the ocular lens.

As stated above- quality glass, configuration and objective size all play a part in the "brightness" of your viewing image.

 

[stoti]

Scopes are very subjective, that’s for sure! Everything above is correct, I just wanted to add a bit. The exit pupil of a scope is the objective diameter divided by the power of the optic, or with variable optics, whatever power it’s set to. However, after light is gathered by the objective lens, a tiny bit is lost as it travels through each of the internal lenses. As others have already said, the quality of the lenses, the coatings, the alignment and of course, the size of the objective all contribute to the amount and quality of light and picture. This is a very simple description, optics can be a bit daunting. If possible, it’s a great idea to test as many scopes as possible side by side. It’s the best way to really compare scopes and see what’s, what and if they live up to the hype. Shooters are very lucky, there are so many great optics available these days, in every price range.

Stoti

 

[CyberDyneSystems]

Maximum Light gathering is indeed restricted/dictated by the front lens element diameter or aperture. Add in an equation with the magnification, (ie: lower magnification = more light getting through, and vice versa)

Yes, it's physics and those parameters dictate what amount of light is POSSIBLE to be transmitted. Sort of a "perfect world"

Well the world aint perfect, and neither is glass. So once you have that base number, then any number of other factors can prevent that perfect world amount of light from actually getting to your eye. Also amount vs. quality. If the brightest scope happens to be the one with the most CA, least sharpness, etc. your image is still going be nasty. Light gathering is just one aspect of "good glass". Clarity of image detail, chromatic aberration (CA) light fall off, (darkening at the edges) etc. all play a part. These issues also reduce the light, compounding their effect subjectively.

 

[Revoman]

A rhetorical question please:

If there is no 'coned' ocular lens such as a 50mm reducing to a 30mm tube and it's just the 30mm tube with an ocular lens, why is that not the best light gathering?

Is it because the larger diameter ocular lens can gather light more effectively and send it down the tube?

Inquiring minds need to know1

mike

 

[FredAZ]

All the light funnels through your pupil at a diameter of 7mm max.

How the scope gathers that light at the front and reduces the stream to that diameter differs from scope to scope and does effect net transmission.

Glass size, type and especially coatings all make a big difference in transmission especially across the spectrum.

No manufacturer that I know of spec's their transmission numbers.

Camera lens makers (a place you think it would matter greatly) sometimes publish a "T" number - they are always not as good as the "F" number -- that's the measurement you want.

 

[Deerstalker]

I believe that a 50 mm scope has a 51 times more light gathering than the human eye. That is without calculating any internal losses in the scope components.

Formula is:

D = objective diameter (mm)

d = eye pupil diameter (mm)

Example:

d=7mm

D=50mm 

https://www.optics-trade.eu/blog/light-gathering-power/ Of course old folks like me, 82+, probably have an exit pupil less than 5!

 

[AirSupply]

Just from a small amount of experience looking through a high quality scope the glass makes a difference. Maybe it’s not just it’s ability to gather the light but other qualities that allow you to see with better definition in low light. 
I don’t know exactly why but if you have 2 scopes with the same size objective lens and magnification the quality of the glass can make a lot of difference to what you can see. 
It can also make a big difference when there is glare when shooting towards the sun. Without a sun shade good glass gives you a better sight picture. 
On low magnification in very low light I can “only” see some things through the scope. It seems to have better light gathering than my eyes. Not that there that great!

 

[FredAZ]

The physics of the glass sizing give you a maximum.

Gathered light is easily squandered along the path and that's what impossible to tell without testing.

It's all about the glass quality and coatings.

 

[johnwoitalla]

When a person(s) thinks they are detecting differences between the sight pictures of two different scopes, those reported differences are subject to a myriad of human failings. Let's discuss two (fictional) scopes, both are 4-16x40. If that group knows that scope A was made in country X and retails for $150, and scope B retails for $1,500 and was made in country Z, right off the bat, prejudice is introduced into the equation. If we asked a group of shooters to compare them and they use no controls, no reliable measurements, just let the people make their observations- those observations will be tainted and virtually worthless as reliable evidence for anything. If one of the categories of the observation was "brightness," don't you think that most people would say that scope B was brighter, that it gathered light better simply because it costs so much more and was made in a country with a reputation for manufacturing high quality products? But without strict controls and reliable measurement, those reports would be worthless. All of the physics explaining the performance of the scopes is, at this point, moot. If no controls were cooked into the comparisons, if no reliable measurements were made, none of their opinions would be worth a grain of salt. And that is the upshot of my thread. There is so much more to say than I care to write. And if I did, I'm sure most of ya'll would get too bored with it to read the entire piece anyway...so "Good night, happy shooting, and have fun with your toys!"

 

[knifemaker]

All the light funnels through your pupil at a diameter of 7mm max.

How the scope gathers that light at the front and reduces the stream to that diameter differs from scope to scope and does effect net transmission.

Glass size, type and especially coatings all make a big difference in transmission especially across the spectrum.

No manufacturer that I know of spec's their transmission numbers.

Camera lens makers (a place you think it would matter greatly) sometimes publish a "T" number - they are always not as good as the "F" number -- that's the measurement you want.

Zeiss, Schmitt & Bender, Swarovski, and manly other high quality scope manufacturers do indeed list the sight transmission numbers. What you want to see is the light transmission % with all internal lens covered. Not just the objective lens. I use all of these scopes, and the low light and color rendition is phenomenal. I tried the top of the line Hawk once for a day. It went right back. Terrible!

EO TEC Vudu, and my top pic, Valdada. Woha!

For a Low cost scope that performs well, the middle class Athlons and the top class of Arken, Track Vector. Are all exceptional. 

KnifeMaker

 

[knifemaker]

I believe that a 50 mm scope has a 51 times more light gathering than the human eye. That is without calculating any internal losses in the scope components.

Formula is:

//www.w3.org/2000/svg%22%20viewBox=%220%200%20210%20140%22%3E%3C/svg%3E

D = objective diameter (mm)

d = eye pupil diameter (mm)

Example:

d=7mm

D=50mm 

https://www.optics-trade.eu/blog/light-gathering-power/ Of course old folks like me, 82+, probably have an exit pupil less than 5!

Sadly, this has nothing to do with light transmission or image quality. Even a coke bottle lens in a paper towel tube would give the same number. It is mostly meaningless, but to the average shooter, looks impressive. It is strictly a mechanical equation, and has nothing to do with the quality of the lens, coatings, or alignment internally of the internal complnents of the optic. 

KnifeMaker

 

[knifemaker]

When a person(s) thinks they are detecting differences between the sight pictures of two different scopes, those reported differences are subject to a myriad of human failings. Let's discuss two (fictional) scopes, both are 4-16x40. If that group knows that scope A was made in country X and retails for $150, and scope B retails for $1,500 and was made in country Z, right off the bat, prejudice is introduced into the equation. If we asked a group of shooters to compare them and they use no controls, no reliable measurements, just let the people make their observations- those observations will be tainted and virtually worthless as reliable evidence for anything. If one of the categories of the observation was "brightness," don't you think that most people would say that scope B was brighter, that it gathered light better simply because it costs so much more and was made in a country with a reputation for manufacturing high quality products? But without strict controls and reliable measurement, those reports would be worthless. All of the physics explaining the performance of the scopes is, at this point, moot. If no controls were cooked into the comparisons, if no reliable measurements were made, none of their opinions would be worth a grain of salt. And that is the upshot of my thread. There is so much more to say than I care to write. And if I did, I'm sure most of ya'll would get too bored with it to read the entire piece anyway...so "Good night, happy shooting, and have fun with your toys!"

Actually, Germany has an optical testing an standards board that does test all facets of optics performance. It is hard to find, but they test optics from all over the world with very sophisticated equipment and test annually. . It does not depend on opinion or perception. I wish we had such a test available here in the U.S.. 

For what it is worth, Schott is the top glass in the world today and has been for a very long time. Their glass and coatings are in many of the to scoped available today. The Schott glass works is a division of Zeiss Optics. 

It is a fascinating read.

there is one company that make top tier glass for their own scopes and that is Swarovski and their other division Kahles. 

 

[johnwoitalla]

Knifemaker,

My understanding is that Meopta makes, grinds, coats all(or most) of their own glass. Their products appear to be on par with most of the big Deutschlanders. They are a Czechoslovakian based company. I believe JSB and CZ are also Czech companies…which make high quality, affordable ammo and guns. The Czechs are really giving the Germans a run for the money-IMHO. 

 

[FredAZ]

Schott makes the raw glass used in many scopes, not just Zeiss/Swaro/Kahles.

Grinding, polishing and coatings are as important, if not more, than the raw glass. The who/what/where these processes are done are more of a mystery.

MeOpta does make their own lenses - one of the very few who go from "sand" to finished products.

 

[NickT]

A question: If all other things are equal, will a fixed magnification scope outperform a variable magnification?

I had always heard this was the case because a fixed magnification scope has less components in the optical path and is therefore more efficient. But, I was casually comparing two scopes from Meopta and that does not seem to prove true.

Meopta touts their R2 8x56 as "Brightness Above all" while the R2 2.5-15x56 is "Our most versatile hunter". Both have 56mm objectives and presumably excellent (R2 quality) glass, but since the 8x56 is a fixed magnification and is described as "Brightness Above all", clearly the 8x56 is going to have better transmission specifications... But it doesn't, the specs are identical.

What am I missing here? Any thoughts or experiences that could shed some light?



https://www.meoptasportsoptics.eu/en/produkt/meostar-r2-8x56-rd-1149/

https://www.meoptasportsoptics.eu/en/produkt/meostar-r2-2-5-15x56-rd/mr-pa-13398/

 

[CyberDyneSystems]

A question: If all other things are equal, will a fixed magnification scope outperform a variable magnification?

I had always heard this was the case because a fixed magnification scope has less components in the optical path and is therefore more efficient. But, I was casually comparing two scopes from Meopta and that does not seem to prove true.

Meopta touts their R2 8x56 as "Brightness Above all" while the R2 2.5-15x56 is "Our most versatile hunter". Both have 56mm objectives and presumably excellent (R2 quality) glass, but since the 8x56 is a fixed magnification and is described as "Brightness Above all", clearly the 8x56 is going to have better transmission specifications... But it doesn't, the specs are identical.

What am I missing here? Any thoughts or experiences that could shed some light?



https://www.meoptasportsoptics.eu/en/produkt/meostar-r2-8x56-rd-1149/

https://www.meoptasportsoptics.eu/en/produkt/meostar-r2-2-5-15x56-rd/mr-pa-13398/

This is another "all else being equal" thing. There was a time that this was in fact the "rule" and for good reasons. Prime (single magnification" lenses are simpler with fewer optics, so it's easier to make them with better image quality. Same has been true of photographic lenses for decades. Zoom lenses always had more design obstacles between them being "perfect".

Things have changed, keep on changing and the technology keeps on improving, and tech behind zoom lenses has come a loooong way in the last decade. It's harder to do it. and usually costs more, but today zoom lenses can be as good as prime lenses in many cases.

 

[NickT]

Thanks for the reply. That is interesting and impressive. I guess that (at least partially) explains why those premium variable scopes cost what they do.

After reading your post, I went back to the Meopta site to see if I could find some specifications that would make more sense of this. Something that then occurred to me: I think I was looking at the wrong specifications. I was previously comparing the Daylight Transmission and Twilight Transmission values for these two scopes, which are identical between the two models I was comparing.

After giving it a little more thought, it seems like these values only tell part of the story. These transmission specifications seem to describe the "efficiency" of the glass; what percentage of the light entering the scope will make it to your eye. But, these values do not describe image brightness (in absolute terms) because they don't tell how much light initially enters the scope. By this, I mean, there could be two scopes with the same transmission specifications but different objective diameters that should have different light gathering characteristics.

It sounds like this is where Twilight Factor might come into play. As I read, Twilight Factor is the square root of (Objective Diameter * Magnification). Something about this value doesn't make sense to me. It certainly sounds like a larger Twilight Factor should be better since a larger Objective Diameter leads to a larger Twilight Factor. But why would higher Magnification make for a better Twilight Factor?

My intuitions tell me, if I wanted to make an image appear more bright at night, I would turn down the Magnification on the scope.... My logic here is that lower Magnifications have larger Field of View so, lower Magnifications would capture light over a larger field, sending more total light to my eye. I must be missing something here.

As I am imagining it, there should be some specification that is essentially the Objective Diameter multiplied by a Transmission factor. Or, even better, if they would simply state the light intensity of the image in the scope at some predefined ambient light level (and perhaps distance). This would seem to characterize the light gathering of a given design. (If any scope manufacturers steal this idea, you definitely—at a minimum—owe me a kick ass scope!)

If anyone wants to clarify any of this, I would be interested to hear.

Thanks,
Nick