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Posted: 10/28/2008 7:01:31 PM EDT
| I've read about finding the exit pupil on scopes and how a low mag. and large optic gathers more light for low light viewing. My question is on a var. magnafication scope does your sight picture actually fade darker if you crank it up in low light? |
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I've read about finding the exit pupil on scopes and how a low mag. and large optic gathers more light for low light viewing. My question is on a var. magnafication scope does your sight picture actually fade darker if you crank it up in low light? Technically, glass does not gather light. That's marketing talk. Photons strike the glass, and this sets off a chain of events and reactions described in any Quantum Physics book. If you need to see through a scope in the middle of the night, you'll need to be prepared to make compromises as, well, to quality. No easy way to write that. Lenses are needed to correct image production. The more lenses, the higher the quality of the image produced. However, the more lenses, the less amount of light that will get through the lens system. On average, and as a general rule of thumb, a lens surface will reflect 2% of the light that strikes it. A lens has two surfaces, and that means each lens reflects, again as a general rule, 4% of the light energy striking it. The more lenses in the system, the more the loss of light energy. To the other extreme, the fewer lenses involved in a lens system, the more light energy will retransmit through the system. But you also have more error. More error in image production through the axis of the system; more error in image production off axis; and the more error in reproducing colors. (Axis refers to the center of the physical lens center of the lens system. This is where your eye should be.) The best advice is to good quality scopes with large exit pupils. (Exit pupil defined as objective lens diameter divided by power.) As a general rule, if you're a target shooter, you'll want a small exit pupil. If you need to take quick snap shots, you'll need a large exit pupil. The reason for a small exit pupil for target shooting is that it forces your eye to be in the center axis of the scope. Lining up your eye straight down the center axis minimizes parallax error. It also takes up time to line up your eye correctly. For snap shooting, you have to be quick. Your eye will end up off the center axis inducing parallax error. But for snap shots, you're not interested in the perfect shot, only getting the shot off and being close enough. It used to be that fixed powers had fewer lenses than variable scopes. This isn't really the case anymore. There are variable scopes out there that are even very popular on this board that have 6 to 7 lenses while the scopes I deal with will have 12 to 13 lenses in the system. Somewhere in here hopefully there's some information you'll find useful. |
| Thanks. Not having owned very many var. powered scopes I was just wondering about low light viewing with them. My Dad use to own a pair of 7X50 binoculars that where alot better than my 10x50 at viewing at dusk. Doing the math I saw the exit pupil was larger on my Dad's. I was wondering say if you had a 4-12x50 scope on the lowest setting you would have a large exit pupil. If you moved the power up will your actual sight picture get darker? |
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Quoted:
Thanks. Not having owned very many var. powered scopes I was just wondering about low light viewing with them. My Dad use to own a pair of 7X50 binoculars that where alot better than my 10x50 at viewing at dusk. Doing the math I saw the exit pupil was larger on my Dad's. I was wondering say if you had a 4-12x50 scope on the lowest setting you would have a large exit pupil. If you moved the power up will your actual sight picture get darker? there is a limit to what exit pupil your eye can take advantage of, so (it is said) a number larger the 7 is of no advantage, But in the case of a variable at some point you cross the line as you increase power and the magnification begins to cause darkening of the image. So for example if you scope is 4-16x50 you might be as clear at 8x as you are at 4x, but around 10 you may see it starting to get dark. |
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Thanks. Not having owned very many var. powered scopes I was just wondering about low light viewing with them. My Dad use to own a pair of 7X50 binoculars that where alot better than my 10x50 at viewing at dusk. Doing the math I saw the exit pupil was larger on my Dad's. I was wondering say if you had a 4-12x50 scope on the lowest setting you would have a large exit pupil. If you moved the power up will your actual sight picture get darker? You bring up a great point about binoculars. The best binoculars in the world (as measured by a combination technical instruments and human reviewers) normally have a retransmission rates of 60% to 70% while the cheapo Chinese knock-offs have rates into the 80s (%.) The simple explanation for this is the difference in the prisms. Higher end binoculars use much larger prisms than cheaper binoculars, and the larger the prism, the more light gets "eaten up." Again, this is a trade off between light retransmission against image production and trying to reduce on-axis, off-axis and improved color reproduction. (In Optics we also talk about "how much of the lens is used" in a lensing system. Unfortunately, I don't have a good analogy to describe this concept. So until I get smart enough one day, the more light energy that has to go through a lens or prism, the more distorted the image produced will be if the lens or prism is small in relation to the amount of energy. This is why this part is in parenthesis. I'm just going to apologize for it right now.) Getting back to the 4-12X 50mm scope example, at 4X exit pupil is just over 12mm, and at 12X, exit pupil is just over 4mm. As a general rule, 5 mm has been chosen as the average pupil dilation of an adult. (As you get older, the pupils don't open as large as when you're young––-Kids' pupils can open up to 7 mm.) As cckw wrote above, there are limits to how much light can get through your pupils. (If there's too much light, your pupils will contract making the image look darker.) Just by gauging the exit pupil numbers, once the numbers get below 5 mm, you can expect a "not as bright" image compared to exit pupil numbers 5 mm or more. The other thing to consider is how many lenses are in the system as I wrote in the post above. Everything in Optics is a trade off. If you want to design for more of one characteristic, it will come at the cost in reduction of other desirable characteristics. It's a zero sum game. But it's also your money, so get what you want. |
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Quoted:
Quoted:
Thanks. Not having owned very many var. powered scopes I was just wondering about low light viewing with them. My Dad use to own a pair of 7X50 binoculars that where alot better than my 10x50 at viewing at dusk. Doing the math I saw the exit pupil was larger on my Dad's. I was wondering say if you had a 4-12x50 scope on the lowest setting you would have a large exit pupil. If you moved the power up will your actual sight picture get darker? You bring up a great point about binoculars. The best binoculars in the world (as measured by a combination technical instruments and human reviewers) normally have a retransmission rates of 60% to 70% while the cheapo Chinese knock-offs have rates into the 80s (%.) The simple explanation for this is the difference in the prisms. Higher end binoculars use much larger prisms than cheaper binoculars, and the larger the prism, the more light gets "eaten up." Again, this is a trade off between light retransmission against image production and trying to reduce on-axis, off-axis and improved color reproduction. (In Optics we also talk about "how much of the lens is used" in a lensing system. Unfortunately, I don't have a good analogy to describe this concept. So until I get smart enough one day, the more light energy that has to go through a lens or prism, the more distorted the image produced will be if the lens or prism is small in relation to the amount of energy. This is why this part is in parenthesis. I'm just going to apologize for it right now.) Getting back to the 4-12X 50mm scope example, at 4X exit pupil is just over 12mm, and at 12X, exit pupil is just over 4mm. As a general rule, 5 mm has been chosen as the average pupil dilation of an adult. (As you get older, the pupils don't open as large as when you're young––-Kids' pupils can open up to 7 mm.) As cckw wrote above, there are limits to how much light can get through your pupils. (If there's too much light, your pupils will contract making the image look darker.) Just by gauging the exit pupil numbers, once the numbers get below 5 mm, you can expect a "not as bright" image compared to exit pupil numbers 5 mm or more. The other thing to consider is how many lenses are in the system as I wrote in the post above. Everything in Optics is a trade off. If you want to design for more of one characteristic, it will come at the cost in reduction of other desirable characteristics. It's a zero sum game. But it's also your money, so get what you want. Very good explaination. I love the part about 30mm tubes allowing more light than a 1" tube, LOL. It ain't like a water hose sonny. Bill |
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