Home Forums Hunting & Shooting Hunting Optics Quality optics or Objective Bell Size

Simple physics, RD. A 6X puts you closer to the object viewed. The intensity of light varies by the square of the distance from the object.
Another way to look at it would be consider what it means in a rifle scope. A 4X requires a scope with a 20mm objective to produce a 5mm exit pupil. But a 6X requires a 30mm objective. Both squeeze the light that hits them down to a 5mm beam that strikes the eye. There is a whole lot more light striking a 30mm objective than striking a 20mm objective.
BTW, the actual test data I've seen puts a single coated variable like the older VariXII at 84-86%. Barsness has yet to report on anything by anybody that tests over 94%.
Further, in one of his latest comments on the subject, he stated that anything that tests 85% or more are plenty bright enough to work in any low light situation. That's because it's magnification first, not glass and coatings. E

A 6x will give you a "50%" larger image than a 4X. That's not physics, simle math will suffice. With two scopes (4X and 6X) of equal exit pupil, how does this "50%" increase in light transmission occur? Do you understand my question? Scope (a) a 4X transmits 60% total light. Scope (b), identical glass quality and coatings is a 6X. Does scope "b" transmit 90% total light? This is what you have been stating all along, correct?

No. A 6X puts you closer to the object viewed. Because you are that much closer, in effect, the object viewed appears 50% brighter.
Another way to look at it. Try to watch something as it gets dark. The darker it gets, the harder it is to see. But, if you move yourself closer to it, you see it much better. Same principle. E

Well personally, I'll opt for the fully multi-coated 4X scope and sharpen my stalking skills. <img src="/ubbthreads/images/graemlins/grin.gif" alt="" />

A few comments.

First of all, all this arguing about how much total light gets to the eye is not very important at all. Human eye is remarkably good in utilizing even miniscule amounts of light. You do want to keep the exit pupil of the scope reasonably big, so that the eye only sees the picture coming through the scope and concentrates on that. Here I am ssuming that we are discussing scope of equal or similar exit pupils.

I have elaborated on this a few times before, but I think it all fell on deaf ears. There is brightness (sometimes confused with transmission) and there is apparent brightness.

Brightness refers ctrictly to the total amount of light that gets through the scope.
Light transmission is the ratio of the light that gets out of the scope and the light that gets into the scope. For example, a light transmission ratio of 90% means that

(Light Out / Light In ) * 100% = 90%

The amount of light that is incident on the scope (we assume that all of the objective lens is utilized and there is not vignetting) is proportional to the area of the objective lens. For example, a 30mm objective lens is subjected to 125% more light than a 20mm objective lens. Since the area of a lens is proportional to a square of the radius of it, here is a brief table of how much light is incident on lenses of different size with respect to a 20mm lens

Lens Diameter, mm brightness ratio, % (with 20mm lens being 100%)
20 100%
24 144%
30 225%
32 256%
36 324%
42 441%
44 484%
50 625%
52 676%

Apparent brightness, is basically how bright the image appears to your brain. How bright the image appears to your brain does not have a whole lot to do with the actual amount of light that gets to the eye. If the image is clearly resolved, it will appear bright to your brain. If the image is not very clearly resolved, it will appear comparatively dim. The amount of light that gets to the eye has very little to do with how well the image will be resolved.

Magnification has a lot to do with how well you can resolve details, since the details are magnified a little more. However, it is not as simple as it sounds because when you magnify the image you also magnify a lot of the imperfections and flaws in the scope optics and make various opicall aberrations play a much more visible role in sidtorting the image. Still, with the same optical quality and the same exit pupil, you�ll obviously see better through a 6x scope than through a 4x scope. However, not as much better as you would think since to diminish the effect of flaws and aberrations a higher magnification scope actually has to be of a higher optical quality. Once the magnification gets usfficiently high, of course, you also run into a problem of shaking (bouncing crosshairs) in almost any field position. That degrades apparent image quality as well. Another additional factor is that with higher magnification you are looking at a smaller part of the scene. Although with higher magnification and larger objective lens you may be theoretically capable of letting more light through the scope, you may end up having less total ight incident on the front lens due to less light being available since you are looking at a smaller part of the object.

When I say optical quality, I mean the quality of the polish of the lense and the precision of the grinding of the lenses. That has the most effect on how well the image is resolved.

Another significant factor in the image resolution/quality is coatings. Multiple reflections between different surfaces inside the scope wreak havoc in image quality. This is where high quality coatings are most important.

Some personal experiences that may relate: back when I had both, I could see better (resolve more detail) with a IOR 4x24 than with Leupold M8 6x42, despite Leupold�s higher magnification (to forewarn the next question I am quite familiar with focusing scopes).

ILya

Thanks for pointing that out again. Some may misread tests like THIS thinking, for example, the 50mm Lupy was actually "brighter" than the Euros with the Hubble-sized objectives.

Is there an english translation of that article anywhere?

ILya

koshkin,

I posted a link to that article on snipers hide and one of the guys said he could translate it but it would a take a few days. I don't know if he ever did anything with it though.

JonA,

That is a good point. In addition, there are two types of transmission tests you could perform. One tests the quality of design and efficiency of coatings. This entails making sure that the same ammount of light enters all scopes tested and measuring the light coming out. This is the type of test done in the link. The other possible test will test the objective size as well as design and coatings. This could be done, for example, by taking the scopes outside and comparing light readings between scopes. It is not possible to find a % transmission for the second type of test, only percent difference between scopes.

-polarcow

The second test you are talking about, polarcow, is not straightforward either, because the power density across the objective lens can be different depending on the the field of view of the scope, luminousity of the scene looked at, etc.

ILya

koshkin,

The luminosity of the scene is easily overcome by choosing the same scene (i.e. the side of a building) and taking many measurements and averaging them. It could even be done inside for maximum consistancy, against a large white screen or wall. The point of this test is to see what effect the entire objective size, optical design, materials, coatings and everything have on a scope. Basically, it is comparing the number of photons exiting one scope compared to another on a given power. For example, if a scope transmits more light than another (function of coatings/materials), but the other scope has a larger objective, which one is actually brighter(function of coatings/materials, objective size etc.)?

The key is that with different size objectives, there will be a different number of photons entering in the second type of test whereas with the first (spectrometer) it is crucial that the same number enter all scopes.

-polarcow