"The larger the second number on a binocular (size of the objective), the brighter the image" is one of the most commonly quoted binocular concepts you will hear. I can't even begin to count all the times I've heard sales people making the image brightness pitch to customers. The bigger the better mentality seems to work overtime on this one. Guess that explains why I sometimes need to get a customer to downsize their binocular choice at times. It also explains why I sometimes encounter folks out in the field struggling with the weight of giant binoculars around their neck. Yes, I actually met a guy out birding once, with a Vixen Giant Waterproof 12 x 80 BCF Binocular around his neck. saw a guy with When I asked why he was carrying such a giant binocular, he insisted that he needed it to get the necessary image brightness for birding in the woods. Fact or myth?

The human eye, of course, can open and close its pupil to control the amount of light that reaches the retina. The diameter of this opening is called the entrance pupil and its size varies with the amount of light in the observer's environment. On a bright sunny day, the entrance pupil may be as small as 1 or 2mm to protect the retina from too much light. Under totally dark conditions, on the other hand, the entrance pupil may open as wide as 7mm (average widest for most humans) to capture as much light as possible, though this maximum shrinks with age. By the time most of us hit our fifties, our maximum entrance pupil may only be 5mm.

On the other side of the equation, we can calculate the size of the beam of light that leaves a binocular's eyepiece. This is called the exit pupil and is simply calculated by dividing a binocular's first number into a binocular's second number. Thus an 8x20 has an exit pupil of 2.5mm and an 8x56 has an exit pupil of 7mm.

When we match up entrance pupil and exit pupil, we begin to understand that bigger does not always mean brighter in a binocular. If you are using a binocular that produces a 7mm exit pupil and your entrance pupil is only at 5mm, your eye cannot use all the light being delivered by the binocular - there is 2mm of exit pupil not entering your eye. In other words, 2mm of light is being wasted. The only time a 7mm exit pupil binocular, such as the excellent Zeiss 8x56 T* FL Victory Binocular will allow you to take advantage of all the light it delivers is when the entrance pupils of your eyes are open to a full 7mm and the only time that will happen is after at least thirty minutes of exposure to total darkenss - twilight doesn't count, here; we are talking total darkness, away from city lights or other light sources. If your in your fifties or older, as we have seen, it may never happen.

For most situations where a binocular is used, then, a binocular with a 4mm or 5mm exit pupil may be all you will ever need, even at twilight or near dark conditions. A better choice in that Zeiss would have been the Zeiss Victory 8x42 T* FL Binocular. It would have produced all the image brightness the most demanding birder would ever need. If you are observing under normal daylight conditions, even a compact binocular with a small an exit pupil of 2mm or 2.5mm will still deliver enough image brightness. Hard to argue with a good compact, such as the Nikon Premier LX compact 8x20 binocular, when size is an issue.

Don't get me wrong; there are some solid optical reasons for going to a large objective (second binocular number), but image brightness is not automatically one of them. Bigger is not atomatically better for image brightness. Myth busted