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Basic light transmission question

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/11/2011 at 14:29
natman View Drop Down
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I'd like to clear up what is probably a pretty basic point of scope light transmission. As I see it all scopes transmit some large percentage of available light and lose some small percentage. Larger lenses, larger tubes, better coatings can cause a scope to lose a smaller percentage, but no conventional scope can transmit ALL the light, much less make the image brighter than it is to the naked eye. Larger & clearer, yes - brighter, no.

Am I on the right track?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/11/2011 at 15:27
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Correct. Every time light goes through a glass lens a small percentage of the light is reflected off the glass it is entering. A variable power riflescope has up to five or six lenses that the image (light) has to travel through just to get to your eye. If you lost at least 1% of the light at the surface of each of 5 lenses you have lost at least 5% of the light. Now multiple anti reflective coatings can decrease the amount of light being reflected on each lens surface. They also decrease the amount of glare in the imaged due to the anit reflective properties of the coatings. Fully Multi Coated is the way to go for sure.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/11/2011 at 16:20
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Originally posted by natman natman wrote:

I'd like to clear up what is probably a pretty basic point of scope light transmission. As I see it all scopes transmit some large percentage of available light and lose some small percentage. Larger lenses, larger tubes, better coatings can cause a scope to lose a smaller percentage, but no conventional scope can transmit ALL the light, much less make the image brighter than it is to the naked eye. Larger & clearer, yes - brighter, no.

Am I on the right track?
With everything but the tube size. Tube size has nothing to do with light transmission

Edited by Roy Finn - September/11/2011 at 16:20
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/11/2011 at 16:35
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I wrote a short series of articles on riflescope fundamentals some time ago and there is a section there on optical quality.  I have a discussion of light transmission there among other things.  Perhaps, you'll find it useful.


ILya
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/11/2011 at 17:29
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As far as whether or not a scope can make an "image brighter than it is to the naked eye," it is obvious to me that it can. Even with my worst low-light scope, a single-coated Leupold Vari-XII 1-4X20, I can see things after sunset through the scope that I can't see with my naked eye. A couple of my other scopes, with larger objectives and fully multicoated lenses, are far superior in that regard.

A 50mm objective lens has an area of about 1963 square mm. A 7mm eye pupil has an area of about 38 square mm. So the scope lets 50 times as much light in. If 90% of that light is transmitted, at 7X, with an exit pupil of 7mm, it would seem that 45 times as much light, or light energy, or photons, would reach the eye, compared to a naked eye. If that doesn't mean it's brighter, maybe it's just a matter of definitions and semantics.

As far as lens size, other than the objective lens:

[Nikon site FAQ: Answer ID 10688
"Do the 30mm scope tubes have oversized optics, or are they just 1 inch tube optics placed in a 30mm tube? "
"The 30mm scopes are actual 30mm scopes with optics that are larger than the standard 1” scopes. This does not gather or transmit any more light than a 1” scope, but it does provide a larger sweet spot to enhance resolution. "]
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2011 at 02:40
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Originally posted by Shenko Shenko wrote:


As far as whether or not a scope can make an "image brighter than it is to the naked eye," it is obvious to me that it can. Even with my worst low-light scope, a single-coated Leupold Vari-XII 1-4X20, I can see things after sunset through the scope that I can't see with my naked eye. A couple of my other scopes, with larger objectives and fully multicoated lenses, are far superior in that regard.

A 50mm objective lens has an area of about 1963 square mm. A 7mm eye pupil has an area of about 38 square mm. So the scope lets 50 times as much light in. If 90% of that light is transmitted, at 7X, with an exit pupil of 7mm, it would seem that 45 times as much light, or light energy, or photons, would reach the eye, compared to a naked eye. If that doesn't mean it's brighter, maybe it's just a matter of definitions and semantics.


I can see things with my scope during the day that I can't see without it, but that doesn't necessarily make them brighter.

If I understand the second paragraph correctly, there are transmission losses in all scopes, but the scope's larger lens puts more light in so despite the transmission losses the resulting image is indeed brighter than without the scope. I assume this is the source of the famous phrase "gathers light".

Originally posted by Shenko Shenko wrote:

As far as lens size, other than the objective lens:

[Nikon site FAQ: Answer ID 10688
"Do the 30mm scope tubes have oversized optics, or are they just 1 inch tube optics placed in a 30mm tube? "
"The 30mm scopes are actual 30mm scopes with optics that are larger than the standard 1” scopes. This does not gather or transmit any more light than a 1” scope, but it does provide a larger sweet spot to enhance resolution. "]


Point taken about the tube size. More resolution, not more transmission.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2011 at 07:45
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Originally posted by natman natman wrote:

Originally posted by Shenko Shenko wrote:


As far as whether or not a scope can make an "image brighter than it is to the naked eye," it is obvious to me that it can. Even with my worst low-light scope, a single-coated Leupold Vari-XII 1-4X20, I can see things after sunset through the scope that I can't see with my naked eye. A couple of my other scopes, with larger objectives and fully multicoated lenses, are far superior in that regard.

A 50mm objective lens has an area of about 1963 square mm. A 7mm eye pupil has an area of about 38 square mm. So the scope lets 50 times as much light in. If 90% of that light is transmitted, at 7X, with an exit pupil of 7mm, it would seem that 45 times as much light, or light energy, or photons, would reach the eye, compared to a naked eye. If that doesn't mean it's brighter, maybe it's just a matter of definitions and semantics.


I can see things with my scope during the day that I can't see without it, but that doesn't necessarily make them brighter.

If I understand the second paragraph correctly, there are transmission losses in all scopes, but the scope's larger lens puts more light in so despite the transmission losses the resulting image is indeed brighter than without the scope. I assume this is the source of the famous phrase "gathers light".

One further comment; while there may be some merit to the larger lens argument, there's no way a 50mm scope produces an image 45 times as bright as ambient. That's over 5 F-stops. If there was that much difference it would be blazingly obvious at night and you'd have to wear sunglasses to shoot during the day. There's something amiss in the math somewhere.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2011 at 10:48
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Maybe if you're talking about a magnifying glass, concentrating direct sunlight on one spot. But go read Koshkin's article. Also, to clarify a point on tube diameter: While Nikon may be right that a larger tube diameter increases the "sweet spot" that would only be an indirect benefit. The major reason for larger tube sizes is to allow for more internal adjustment in the erectors. Translation: more elevation.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/12/2011 at 20:23
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I don't see anything wrong with the math. A 50mm lens has about 51 times the area of a 7mm lens. The larger lens can gather, or collect, or get hit by, 51 times as much light. If most of that light isn't transmitted through the scope and out the eyepiece, where does it go?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 02:54
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Originally posted by Shenko Shenko wrote:

I don't see anything wrong with the math. A 50mm lens has about 51 times the area of a 7mm lens. The larger lens can gather, or collect, or get hit by, 51 times as much light. If most of that light isn't transmitted through the scope and out the eyepiece, where does it go?

I didn't mean that there was anything wrong with the arithmetic of calculating the area of the objective lens. But there's something not right with the conclusion, because looking through a 50mm scope does NOT result in an image 45 times brighter than ambient.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 09:53
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I was kinda hoping that the article I wrote on the subject would be helpful, but I guess not.

Do not get too hung up on the difference in the total amount of light that gets through the scope.  There are several additional things in play that make it not straightforward at all.  Most importantly, your eye adjusts to the amount of light it receives in order to extract the most information out of it.  If the scope delivers twice the amount of light to your eye, it does not mean that it delivers twice the information.  Also, image quality plays a huge role in perceived brightness.  If you look at two scopes side by side and one looks brighter than the other it does not necessarily mean that it brings more light to your eye.  It might actually have less light coming through, but if it has better resolution and contrast it will look brighter.

ILya


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 09:55
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I Googled this last night. People who write (who sound like they know what they're talking about) on sites like Telescope Science and TelescopeOptics.Net seem to consider this common knowledge.

Natman, I guess you can do your own research, or believe what you want.


Edited by Shenko - September/13/2011 at 09:56
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 11:04
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Originally posted by Shenko Shenko wrote:

I Googled this last night. People who write (who sound like they know what they're talking about) on sites like Telescope Science and TelescopeOptics.Net seem to consider this common knowledge.

Natman, I guess you can do your own research, or believe what you want.


I have done my own research and have a clearer picture of what's going on now.

You're right about a scope with a larger lens being brighter as far as it goes. However there's a lot more to it than just how big the objective lens is. Not all scopes with the same OL size perform the same. For starters, magnification has a huge effect. The ratio of OL size to magnification is called the exit pupil. A scope with a given OL size but higher magnification is going to have a smaller exit pupil.

How much the exit pupil matters depends on how it matches up with the size of the pupil of the eye that's looking through the scope.

Here's some quotes:

Quote It is impossible for any scope to "gather" light. It can only transmit existing light. And, regardless of advertising claims you may have heard, there is no riflescope made that can transmit 100% of available light.


http://www.schmidtbender.com/facts_light.shtml

Quote Scopes don't gather light, as most people think, although the term "light gathering ability" has become accepted jargon. Scopes transmit available light through the lenses to your eye, always losing a bit in the process. The best a scope can hope to offer in light transmission is about a theoretical 98%, which only the very finest (read expensive) scopes can hope to approach. Anything above 95% is considered great, and most scopes are around 90%, give or take a bit.

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Quote Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. An astronomical telescope requires a large pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller pupil since the object will be brightly illuminated. A set of 7×50 binoculars has an exit pupil just over 7 mm, which corresponds to the average pupil size of a youthful dark-adapted human eye in circumstances with no extraneous light. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). In daylight, when the eye's pupil is only 4 mm in diameter, over half the light will be blocked by the iris and will not reach the retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is sufficient to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use.


http://en.wikipedia.org/wiki/Exit_pupil

Edited by tahqua - September/13/2011 at 11:11
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 13:09
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Originally posted by Shenko Shenko wrote:


A 50mm objective lens has an area of about 1963 square mm. A 7mm eye pupil has an area of about 38 square mm. So the scope lets 50 times as much light in. If 90% of that light is transmitted, at 7X, with an exit pupil of 7mm, it would seem that 45 times as much light, or light energy, or photons, would reach the eye, compared to a naked eye. If that doesn't mean it's brighter, maybe it's just a matter of definitions and semantics........
The amount that the eye can handle here is the fly in the ointment. If the eye dilates to 7mm's then that is max regarding the optic that is providing it. After that then the only thing's that will determine how bright an image looks will be the % of light transmitted and at what magnification and objective lense size in order to maintain the 7mm exit pupil size.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 16:08
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Originally posted by Roy Finn Roy Finn wrote:

The amount that the eye can handle here is the fly in the ointment. If the eye dilates to 7mm's then that is max regarding the optic that is providing it. After that then the only thing's that will determine how bright an image looks will be the % of light transmitted and at what magnification and objective lense size in order to maintain the 7mm exit pupil size.


Let's see if I have this part right:

The ratio between the objective lens and and the magnification of a scope is called the exit pupil. For a given size of objective lens, a scope with higher magnification will have a smaller exit pupil.

Depending on how bright it is, the diameter of the human eye varies. If the exit pupil is larger or equal to the diameter of the eye looking through the scope both scopes will appear to be similar, since the eye will limit the amount of light it lets in. OTOH, if it's relatively dark and the pupil is wide open (~7mm), then a scope with an exit pupil less than 7mm will appear darker than one that has an exit pupil at or above 7mm.

Note to moderator: Sorry about the inappropriate link in my previous post. I didn't realize. Point taken and apologies to all.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 16:21
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There is a F.A.Q. area at the top of this site were the rules are. It can be informative, as is ILyas article.
http://www.opticstalk.com/exit-pupil-light-transmission-30mm-vs_topic5023.html
But, you are getting it right.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 16:41
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One final point: Is it correct to say that no matter how big the objective lens, no matter how big the exit pupil, no matter how wonderful the coatings, there's no way a conventional scope can produce an image that is brighter than ambient light? 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 16:49
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How could it make an image brighter?  The light traveling through the scope gets darker because of all the lenses it has to pass through. You posted quotes yourself that even the best scopes only allow 95%ish of the light through them.  Maybe if it was some kind of electronic it could do that. 

It can make an image look better because it gives you a closer image which makes the resolution better.  Which could make a person think it is brighter.  I have experienced that with my binos in really low light.  I could barely make out elk with my bare eye, but when I pull up the binos I could see them much better.  But it did not make anything brighter, it just made me closer to them so I could make out more detail. 

One advantage of having more exit pupil than your eye can handle is it gives you a much more forgiving eye position.  You can move around in the scope a little and still get a large complete image.


Edited by supertool73 - September/13/2011 at 16:49
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/13/2011 at 17:00
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You guys are on a roll. Thanks st 73 for saving me the typing time..
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/14/2011 at 03:15
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I came to the conclusion that a scope couldn't present a brighter image than ambient, but I just wanted to hear it directly. Thanks, guys, this is a great site. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/14/2011 at 10:07
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Originally posted by supertool73 supertool73 wrote:

How could it make an image brighter?  The light traveling through the scope gets darker because of all the lenses it has to pass through. You posted quotes yourself that even the best scopes only allow 95%ish of the light through them.  Maybe if it was some kind of electronic it could do that. 

It can make an image look better because it gives you a closer image which makes the resolution better.  Which could make a person think it is brighter.  I have experienced that with my binos in really low light.  I could barely make out elk with my bare eye, but when I pull up the binos I could see them much better.  But it did not make anything brighter, it just made me closer to them so I could make out more detail. 

One advantage of having more exit pupil than your eye can handle is it gives you a much more forgiving eye position.  You can move around in the scope a little and still get a large complete image.

Even a passive scope can and most certainly does make the image look brighter that it would with your naked eye as long as there is magnification involved.  Thinking about total amount of light in this case is patently incorrect.  What you want to think about is flux, i.e. the energy that passes through a particular area.  Alternatively, you can think about it in terms of energy density.

Energy density in the exit pupil is higher than energy density of the entrance pupil.

Now, there are other things you give up, and the whole concept of "brighter" is misrepresented in this whole thread, but if you are only talking about the amount of light getting into the eye, then yes, the scope increases it.

ILya
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