mikeb@uhccux.uhcc.hawaii.edu (Mike Burger) (08/12/89)
Objective size (the second number in something like 7X50) deterimines brightness, as most know. There is a simple way to decide what is useful that applies to binoculars and scopes. The human eye can open up under dim conditions to about 7 mm. In daylight it is considerably smaller. What the binoculars or scope is doing is taking the larger objective lens, and optically shrinking down that cylinder of light to the size of a human pupil. This is what the light gathering power of a scope is all about. What goes in as say a 50 mm circle comes out as a much smaller one that fits into the opening in the human eye. The difference in the exit diameter of the light and the entrance diameter at the objective is simply the magnifying power of the scope. Thus for a 7X35 binoc, it goes in 35 mm and comes out 5 mm, a comfortable fit for a human eye in ordinary light to moderately dim light. In the evening when the eye can open to nearly 7 mm, the larger 7X50 type becomes an advantage since it goes in 50 mm and comes out 7.14, nearly perfect for the largest normal openning in the iris under dim light. In bright light the human iris is shut down to say 3 mm and the 7X50 and the 7X35 both more than fill the circle of the iris and the view is the same, though the weight is not. As you can see, with 20 power if you want maximum light gathering power that the human eye can use in dim light you want an objective lens 140 mm in diameter, FIVE and ONE HALF INCHES! Such a monster would not be very portable. Above about 10 power with say 10X80 binocs, the exit diameter of the light from any scope will all fit into the human eye for any reasonable diameter objective lens. Thus any increase in size will be a practical increase in brightness under any lighting conditions. 80 mm will be obviously better than 40 mm. Other considerations such as portability and quality of optics (sharpness) rapidly become more important as magnifying power exceeds seven or eight power. Be sure to also check out color correction and color fringes on all binocs and scopes. Width of field, how much is seen side to side at a given power, is largely determined by the quality and design of the eyepiece. This can also very widely on scopes and is very important. At 20 power even "wide angle" scopes see very narrow, hard to point fields.