Help me understand something about light transmission, scopes/scope accessories

[HeavyAssault]

Why does it seem that no one paints the inside of the sun shade?? Seems as it in (any color other than black) would help in the light gathering.

This is more about the sun shade portion/accessory, not about the scope objective bell itself.

 

[Rocketvapor]

ONLY light that comes from the target/field of view straight to the active portion of the objective lens gets focused and passed on to the eye.
Light from outside the field of view that is cut off by the sun shade or bounces off the shade and hits the objective is mostly cut off inside the scope.
BUT,
this out of field light can illuminate any slight defects (no glass is perfect) and bounce around and reduce contrast.

 

[HeavyAssault]

Okay.....so the concept is to prevent any "unwated" light to be viewed thru the scope creating a visual problem for the shooter. Does that sound right?

 

[Guthwine]

As far as I understand it, it is because the lenses in the scope do not magnify the image but they increase your viewing angle which makes the object appear larger to your eye. So only the light in this certain angle is wanted and light coming in from another direction will actually decrease the image quality. Allthough I am by no means an expert, so someone please correct me if I am wrong.

 

[Rocketvapor]

With photographic lenses that have a field on view that is wide (less than 1X) a lens shade needs to be larger than the lens objective to prevent vignetting (shadowing of the image).
Telephoto lenses do not need a large diameter lens shade. The light coming in to a telephoto camera lens, binocular, spotting scope, rifle scope with magnification will be at an angle almost parallel to the axis of the objective lens. A very little bit of the light that hits the outer edge of the objective from the edge of the field of view will illuminate the active area of the objective lens. Often internal light stops and the eyepiece vignet the full image that hits the objective.

Look up the field of view for YOUR scope. That will be feet/meters @ 100 yards.
For variable scopes the field of view decreases with increasing magnification.

What is YOUR field of view with no lenses or shades. Your natural eye field of view.

 

[Rocketvapor]

There's a lot of info available on the web.
Better to read published articles, some with drawings, than a quickly composed, error prone post from me

Look at the front of a "50mm" rifle scope. Like a 6-24X50mm.
The objective active area is 50mm wide.
Now look at the normal 3 or 4 inch long sun shield. It's I.D. is likely 58 to 60mm wide.
Just slightly larger than the objective. Most have a anti-reflective internal surface like concentric rings that look like threads.

 

[HeavyAssault]

I was thinking more along the line of "we need light to see", light reflects off of lighter colored surfaces, lighter surfaces under lower light conditions will be seen easier, etc etc...having the inner area of a sun shade painted in a lighter color will help with visibility under low light conditions....

 

[srt-4_uk]

Pull out your camera app on your cell phone and point it at something. Now take a flashlight and aim it directly at the lens without it being in your picture. That should give you a better understanding what a sunshade is for.

 

[HeavyAssault]

You didn't read my post above.

 

[Guthwine]

I was thinking more along the line of "we need light to see", light reflects off of lighter colored surfaces, lighter surfaces under lower light conditions will be seen easier, etc etc...having the inner area of a sun shade painted in a lighter color will help with visibility under low light conditions....

I think the problem with light coming in at extreme angles is that you can't bend far enough to direct correctly it into your eye. So it just end up as interference.

 

[HeavyAssault]

Yea...I get that idea. Light needs to come thru the objective. Too much in the wrong area is trash.

 

[srt-4_uk]

You didn't read my post above.

I did. I was just giving you an example that you could try at home. I'm a visual learner myself so being able to see it is a lot easier than somebody trying to explain it. Painting the inside of a sun shade white lessons it's effectiveness.

 

[Rocketvapor]

Hey, might work.
Line the inside of your sunshade with aluminum foil.
Shiny side to the inside.
Now sneak outside this evening when the light is low.
Since small changes in intensity is hard for the eye to discern point your scope towards the rear of your car or truck, maybe 50 yards away. DO NOT have the rear lights in your field of view.
Have someone step on the brakes and illuminate the stop lights.
Move your aluminum foil sun shade closer to the stop lights.
Do you see red glow before the lights are actually in view?
That's light that you do NOT want to see.
Outside the field of view.

 

[HeavyAssault]

Okay....First off no one said "Paint it white"..."Line it with aluminum foil"...."Look directly at the sun"...No duhhhh.

I'm thinking in lines of low light (dusk/dawn/twilight) conditions.

 

[Rocketvapor]

If the off axis captured light is DIM it will not interfere MUCH.
But if the off axis light is brighter, just a little, than the fiew of view, it WILL LIKELY DEGRADE the image. Light that bounces off the sun shield WILL NOT FOCUS. Trying to focus on something
IN THE SHADOWS will be degraded.
ANY LIGHT THAT DOES NOT FOCUS TO THE SAME POINT AS THE SCOPE OPTICS WILL DECREASE CONTRAST AND SHARPNESS.
WORSE THAN CHEAP GLASS.
PERIOD.

 

[HeavyAssault]

Okay got it.

 

[spife7980]

I was thinking more along the line of "we need light to see", light reflects off of lighter colored surfaces, lighter surfaces under lower light conditions will be seen easier, etc etc...having the inner area of a sun shade painted in a lighter color will help with visibility under low light conditions....

What you are trying to see is what needs to be lit up...

 

[HeavyAssault]

Thank you.

 

[Rocketvapor]

Sorry if my reply sounded harsh but do some reading on scope sun shields, vignetting, off axis illumination.

 

[HeavyAssault]

I get it. I hear you say sun shade. I'm not questioning about bright day light. Nor about painting the inside white. More about low light conditions, painted a brown color, maybe dark gray, or whatever color to help gather the light that IS present to the objective. I wasn't trying to concern myself with all the off axis light that isn't directly reflecting into the OBJ. Light travels in weird ways, it reflects, it can be gathered an used to "see".

I see guys painting steel white .....why......so they can see it no matter what Gucci brand scope they are using. Paint it dirt brown and they can't see anything with the same Gucci scope. Hmmmmmmm

We need light to see .....so (within reason) we need to gather what we can to make out the object. The object doesn't even need "direct lighting". I've seen PLENTY (no pun) with my own eyes, under VERY low light/moon light conditions.

It's just a question I had to ask. Not that I expected to have some earth shattering idea, just was trying to figure out why people don't do it for low lighting conditions.

 

[srt-4_uk]

People paint steel white for contrast, not for light reflection. Black painted steel against snow can be seen without gucci scopes.

 

[HeavyAssault]

The way I understand contrast:
In visual perception of the real world, contrast is determined by the difference in the colour and brightness of the object and other objects within the same field of view.

So painting steel white against the snow they are going to have a VERY hard time seeing the target. Nonetheless to SEE the color and brightness you need light. Close your eyes tight and tell me what colors you can see......ZERO colors cause there's no LIGHT entering your eyes.

Just remember there are five fundamental characteristics of light: Direction, Intensity, Color, Contrast, and Hardness.

 

[Rocketvapor]

Contrast in a scope scene is just how black the blacks are and how bright the non-black objects are. If off axis light adds just a tinny bit of light to the black areas, contrast goes down.

 

[HeavyAssault]

Thanks guys. I get the "why we don't do that"....LOL

 

[Rocketvapor]

Here's my Ultra low light scope
(just kidding )
Notice how the off axis light from the porch light messed up the picture.

 

[HeavyAssault]

Sure, but you can dim down the light to a point to which it has no effect, then further to a point to where is helps, then to a point where there is no visual effect.

 

[Rocketvapor]

Sure, but you can dim down the light to a point to which it has no effect, then further to a point to where is helps, then to a point where there is no visual effect.

I could just turn off the porch light.
At 100 yards you are stuck with what's there. No light (dark) outside the field of view, no interference.
The large reflector in my picture captures about 8 times the light that would normally come in to the objective. Gathers lots of light, then sent down the 4 inch long sun shield to straighten it out and send it into the objective lens.

The very first sentence of this thread should be sent to a scope manufacturer.
"Why does it seem that no one paints the inside of the sun shade??"

You might make some money off the idea.

 

[OneMoreNoMore]

Keith Baker discussed it with David Baker on the Just F’n Send It podcast and brought up some good points on what his fix was how the light effected their dope. @BLKWLFK9

I think Baker was trying to remember snells law but regardless what both shooters witnessed when shooting from full sun to overcast was pretty drastic. I would think anything reflective in the sunshade would counteract what the sunshade is trying to do by limiting how much light is being transmitted through the lenses.

 

[Baron23]

Now, I'm more than willing to be corrected...and it seems to me that @Rocketvapor has some photonics/photography skills and knowledge....but my understanding is that you cannot "gather" light. Oh, you can focus it from a larger area to a smaller one and increase its intensity (power/area) but you cannot attract or gather photons...well, not normally.....perhaps not without a black hole or some other advance technology which is absolutely NOT germane this discussion.

The objective and intervening lens will focus and concentrate ONLY that light that enters it and yes, in normal physics photons travel in a straight line.

And yes, I'm discounting the small bit of photons that come in at an angle, bounce back and forth across the sun shade/tube and in the process cause modal dispersion (I worked in the optical telecom industry for a good while...this is dispersion from multi-path such as photons bouncing around off of shades and tubes). I don't think its significant to the total light intensity and as mentioned early in this thread, the reason they are black inside is to absorb as many of these aberrant photons as reasonably possible.

As for low light performance....I found the discussion by the engineer from Leupold on the Modern Sniper web seminars to be very interesting. His statement was that the only way to make a brighter scope is to use lenses that have a higher light transmissivity. He went further and stated that in almost all cases of scope manf's stating their light transmissivity percentage it is ONLY for the objective and not end to end through the objective, Barlow lens, whatever is provided for erector lenses, reticle glass, and ocular lens (and no, I have zero expertise in the internals of scopes...but this ain't a bad starting list). So, if they say "oh, we have 92% transmissivity" it most likely ain't.

So, better glass that is made into better lenses seems to be the key to brighter picture.

hahaha...ok, so someone with real expertise in scopes and similar, please do correct me as I'm sure I have some bloopers in here.

 

[HeavyAssault]

Cool. I'll have to read up on Snells Law.

Now be careful. Reflective is a subjective term. I'm not thinking in terms as reflective as in a mirror. To me a sun shade is doing what @Rocketvapor is talking about. Blocking that light that one would say is "unwanted" as to it causes problems.

Where I'm thinking using a sunshade with (insert color) along the inside area to help even smaller amounts of light that you want into the objective. Even with that maybe the inside area could be 1" ring or a 2" ring inside the sun shade.

I would think using (hence painting) the inside of the sunshade is more friendly than painting the objective bell of a scope. At least if I figured out it doesn't help I can paint the interior area of the sun shade back to a black color. No one would know.

Going one step further one may find that (insert color shade) is better than another. Say "Battleship Gray" isn't helpful, but MAS Gray works great. Maybe a 2" MAS Gray area is better than a 1" Earth Brown. This is just my thinking.

 

[Baron23]

Now be careful. Reflective is a subjective term. I'm not thinking in terms as reflective as in a mirror. To me a sun shade is doing what @Rocketvapor is talking about. Blocking that light that one would say is "unwanted" as to it causes problems.

Well...and again, I'm more than willing to be schooled by someone with more than my little bit of expertise....but yes, the primary purpose of the sun shade it to prevent flaring and glare from light sources not inline with the scope.

But, I do think he's right that black is used inside and inside of the tubes to absorb some percentage of those small amount of off angle photons that would otherwise reflect back and forth as they traverse the length of the scope and the best "color" for that is indeed black.

I suspect you will have some challenges in determining whether a painted ring increases total light intensity without some expensive test equipment as subjective determinations are often unconsciously influenced by our desires and hopes. I have no idea what it would take to set up a high qual light meter with sufficient resolution to see any difference for less than a shitload of money (love that well defined tech term....shitload! haha).

Cheers

 

[HeavyAssault]

With my short amount of reading guided by the posts here I do come across a theme.....There's plenty of theory...not a lot of law. LOL

Snells Law seems to read more about what happens as light passes thru a medium. I'm thinking more along "gathering" the light prior to the lens.

Gathering may be the wrong term, but it's the concept.

I found this interesting:

"It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium."

So if light needs a medium to be reflected, what medium works best??

 

[Baron23]

Snells Law seems to read more about what happens as light passes thru a medium.

Absolutely and that was my thought when I followed the link. Sort of like why its hard to spear a fish from above the water as your spear looks bent right where it transitions from air to water.

My understanding is that pure white is the most reflective as "white" by definition is the reflection of all colors across the spectrum. But, its sort of a theoretical "white" as I don't think there is a perfect reflector.

Just like black is, by definition (well, a pure black body which is only sort of theoretical....carbon soot...lamp black...is 98% absorbent) the absorption of all colors across the spectrum.

As a practical matter....probably some sort of white or silver smooth tape? Smooth because no matter how "white" it is, any sort of texture at all will disperse photons and not reflect them the way you want.

Still inviting someone to school me as I do feel I'm pretty dang far out on the limb of my knowledge and skills.

Cheers

 

[PDXGS]

Ah jeez.....
Think about it this way.... Light, bounced off the target and the area around the target needs to get to your retina. Any other light is useless in forming that image and can distort or reduced the clarity of that image. Including what light bounces off the sun shade interior that doesn't come from the target and its surrounding environment... The field of view of your scope.
Each lens in the scope has coatings that help to prevent that other, non-target origin , scattered, excess light, from entering the optical pathway to the focal point on your eye.
Better/higher quality glass can be thought of as being more efficient at transmitting the light that makes the image of your target. Shit glass =dim image. Shit glass will also bring with it imperfections and areas of varying density that can cause image distortion.
Also, as the white light that makes up your image passes through each element it tends to split... Remember prisms?
It's really challenging to deliver a non-scattered, non color-separated image to the focal point of your eye. More lenses make the image better but also cut down on the brightness that reaches your eye... So the challenge is to use the best glass, fewest elements, best coatings, most consistent grinding and polishing methods and best lens mounting and lens train alignment methods that you can afford. All in the right prescription.
Then there's zoom mechanisms , aspheric designs and shit like plastic lens materials and ultra black coatings for internal components.... But all of that's for another night.

 

[HeavyAssault]

Light, bounced off the target and the area around the target need to get to your retina. Any other light is useless in forming that image and can distort or reduced the clarity of that image.

THIS^^^

Exactly. So as that "Light, bounced off the target and the area around the target need to get to your retina." intensity (or whatever) is reduced we need more of that specific light to "see".
Yes, we don't want all the "other light". So what can be do to make sure the "good light" is getting to us or at least we have done the most we can do? I know I'm getting down that rabbit hole. We can't make super larger tubes, super larger objectives, super larger whatever....there's a point of diminishing returns. Now we think of NVDs but it's a whole different animal. I'm just thinking out loud.

Isn't that how/where we got to "starlight" scopes??? Hence:
These early design night vision devices were passive, and did not emit any light of their own, relying on incoming light. This was then amplified using a photocathode and an image intensifier tube in order to significantly increase image contrast. As they emit no light of any kind (IR/UV/etc) they are not detectable, however this also means that they are of limited use when there is no ambient light, which is why they are usually termed "starlight" scopes. Clear nights with star or moon light are optimal, while a cloudy night with no moon far from a city renders them ineffective.

 

[PDXGS]

Taking advantage of one photon and multiplying its effects through a photo multiplier is very different than getting a complex, white light image through a scope.
A starlight scope does gather ambient light... But the image quality isn't good, at all. Also the focal depth is really whacky due to the large lens... That image needs to be reduced too a size your brain can process, through your retina...Bigger tubes/lenses don't matter as much , or anywhere near as much, as the glass and coatings quality and number of lens elements. That image wants to fly apart when it goes through each lens... The glass quality,curvature, coatings etc reduce that image breakup and place it where it needs to be as it enters the next lens.

 

[Rocketvapor]

High end scopes are now using high index lenses to reduce the number of surfaces the light has to pass thru. ED glass. Old camera lenses sometimes used thorium/radium doped glass.
Heavier glass with a higher index of refraction. I have a couple. After a few years they have to be de-yellowed with UV light.


Anyway there is a way to TEST your hypothesis without an optical bench. A digital camera that has the capability to create a histogram of the incoming light. You can adapt the camera to the eyepiece of the scope. Focus on a darkesh scene and have some off axis light that you can move and adjust. The histogram will show you more light coming in. That is how good the lens stops and antireflective coatings, and the 'BLACK' interior prevent this unwanted light.
Off axis light makes the scene appear brighter. More light is brighter but the desired image is washed out by this unwanted light. Take my word for it it is unwanted.

My red light example can easily be duplicated with a digital camera at the eyepiece end of a rifle scope. Find a darkish area and set up a dozen rabbits in a row. Dead rabbits, frozen rabbits, or even paper rabbits Make the row wider than your field of view at mid power on your scope.
Illuminate the rabbits with a green light. Use a Red light to illuminate the rabbits outside your field of view. Check the overall histogram of the camera output and look for off axis red light.
In this experiment Green is the desired image, Red just degrades it.
The sun and sometimes the moon can be big issues and easily noticed. Lesser light pollution might not be as noticeable but it does reduce image quality.

Most scopes, camera lenses are manufactured with marketable specifications.
A 6-24X50mm scope will likely LOOK better at less than maximum magnification.
The 50mm objective is 'good enough' out to the edge to be 'good enough' at maximum magnification. Center of lens sharpness is almost always better than edge sharpness.
Even with $30,000 camera lenses. Good Enough is good enough. Spend more get a little better Good Enough

There may be slightly better ways of designing scope optics, often with better types of glass, glass shapes, but for now black is good to help out the optic designs where they fall short.

Take a look into aperture reducers for large objectives and what they do for image sharpness and depth of field.

Find some on-line ray tracing diagrams showing a telescope (rifle scope). Straight lines from the object or total field of view to the objective lens sets the aperture of the scope. Nothing outside the aperture is desired. A 50mm objective rifle scope will have a shield about 58-60mm in diameter. This will limit viewing angle to just a few degrees. Stack two 4 inch long sun shields and you will Vignette as you decrease power on the scope. This cuts into the field of view. A dim area at the outside edges of the scope. By reducing the TOTAL amount of light entering the scope (losses at the edge) the remaining area appears brighter. Why? The eye adjusts to the total scene. Removing edge lighting and the eye adjusts to the remaining light.
It just might look brighter with the narrower illuminated spot zone.
The camera histogram will show less light but the center will be brighter than the edge.

 

[ArTeeKay]

So having read this thread, I don't believe anyone directly addressed the following: The only desirable photons are the ones being reflected directly by the objects you can see through your scope.

Since we aren't trying to look at the inside of our scope, or the inside of our objective scope bell, or the inside of our sunshade, as we don't want photons reflected off of those things and focused on the retina of our eye by the ocular lens. If light is reflecting in any way off of your sunshade or inside of your objective bell, no matter the color of the thing (white, black, mas grey, coyote tan), what you will be seeing as a result of that light is the objective bell, or your sun shade, or as previous posters relayed about extreme angle light sources, the source of the light itself. Since we don't want to see those things, we want to minimize to the greatest extent possible the light coming from those things, hence the flat black finish and texture of your sun shade.

 

[alamo5000]

Why does it seem that no one paints the inside of the sun shade?? Seems as it in (any color other than black) would help in the light gathering.

This is more about the sun shade portion/accessory, not about the scope objective bell itself.

I'm not an expert in spotting optics by any means. I will rely on what I've learned taking photos though.

The color of anything is not about the "amount of light" but rather about what part of the spectrum you see.

Grass is green because it absorbs all other colors of the spectrum however with green it reflects that part of the light. Because your eyes differentiate between the various parts of the spectrum of light hence when you see grass it appears green.

White on the other hand is good at reflecting light all together. It still doesn't do anything about the AMOUNT of light present.

In photography you have a thing called T stops. (This is in comparison to F stops or aperture). T stops is the actual amount of light that goes into the front of an optical device and can be transmitted out the back of it. Whether it's to a camera sensor or your eye it's still the same principles. It's basically a measure of how efficient a given lens is at letting light through it.

The color of a sun shade won't increase the volume of light but it can cause glare or blinding shiny spots on the image (or your eye).

 

[koshkin]

I apologize in that I have not read the entire thread. It got a little voluminous.

Short answer: the sunshade should be coated with something that reflects as LITTLE light is possible. Trying to coat with something that reflects more light is counter productive. Getting as much light as possible into a scope from all possible directions makes things worse, not better.

Generally, light transmission in modern riflescopes is only useful for marketing purposes.

Long answer: I wrote this quite a long time ago, so a lot of it needs a refresh, but the technical considerations have not changed. There is a section there on optical quality, light transmission, etc:

Riflescope Fundamentals » OpticsThoughts

If that does not clear things up, I do a Youtube livecast every couple of weeks, with the bulk of it being Q&A. That is a good opportunity to ask questions like this in real time.

ILya

 

[PDXGS]

ILya's the man... I'm dredging up things I learned (poorly) back in the last century whole working for a semiconductor manufacturing device company whose name starts with NIK-,ends with -ON. And then for a custom 360degree imaging/optical lens manufacturing startup.
I had no business being in those businesses!