AR Sponsor
Posted: 1/23/2016 12:12:10 PM EDT
Probably a stupid question, but here goes. I have a Leupold Mark AR 3 to 9 X 40. I am leaning toward a Mark AR 1 to 4 X 20 for another rifle. How much of a reduction in low light transmission can I expect? I know the Objective lens at 40mm gathers in more light, but wouldn't both being a 1 inch tube result in both transmitting about the same amount of light to the Ocular lens? I maybe way off in my thinking, please educate me.
|
|
Objective lens diameter divided by magnification power equals exit pupil diameter. That determines the size of the beam available to enter your eye pupil. Assuming lens quslity is equal, you need about 6mm exit pupil size in very low light conditions to be able to fully activate the light sensing cells in your own eye (rods and cones). Some might say that 5mm is good enough. Anything less than that will start to look dim.
40 divided by 9 is a bit less than 5, so you might see your image start to dim out at dusk unless you dial back magnification to 8 or even 7, depending on your own eyes. 20 divided by 4 is 5. So exit pupil may be fine, but the image at dusk might be a bit brighter at 3x. But, will the 20mm objective be as "bright" at dusk, all other factors being equal? No, the larger objective simply captures more light. Whether your own pupil can use it is another issue. Having a 5mm or 6mm exit pupil at a higher magnification is an advantage because you can see more detail before the scope starts dimming out. Main tube diameter is irrelevant. That is a misunderstood red herring. By the time the light beam enters the main tube from the objective bell, the beam has been compressed to much smaller than one inch and stays smaller than one inch all tbe way through the tube. That's why 30mm, 34mm or 35mm main tubes do not make a scope brighter. Those designs are for different purposes, such as permitting greater elevation adjustment range or to allow thicker tube walls for scopes that must be used in combat or other conditions where the scope will be subjected to impact or abuse. Also, if the exit pupil is larger than your eye pupil, you can see the full image without your eye having to be exactly centered behind the eyepiece. This produces a more forgiving eyebox allowing some lateral movement without losing the image. That is an advantage. It also reduces eye strain, as you do not have to concentrate on keeping your eye exactly centered on the ocular. All of this assumes equal glass quality, lens coatings and internal reflection mitigation. Two scopes may ha e a different number of internal lenses or lens surfaces. Each one has parasitic loss. Less than 100% of the light passes through each lens. There is a cumulative loss. One of the best performing low light scopes I have is only a 2-7x36mm. But, it outperforms other scopes with larger objectives in low light because the lens quality and coatings chosen are superb. As you might guess, it is European, an Austrian Kahles. In most European countries hunting is allowed after dark and the best optic companies have been catering to such hunters for many decades. There are so many factors at play that it is difficult to generalize. The most important factor is lens quality and the related lens coatings. Those coatings are also not color neutral. Some favor transmission of some parts of the color spectrum over others. This can make contrast better under specific conditions, depending on the intended use. Having said this, if low light performance is important, a 20mm objective is likely to disappoint. But 1-4x scopes have very useful purposes and meet other important needs that may have higher priority than image quality at dawn or dusk. |
AR Sponsor