Chuck Norcutt wrote:
> The first thing is to understand the size convention which is to list
> magnification and objective lens diameter. For example, the common 7x35 (and
> there's a reason why it's common) means magnification 7X and an objective
> (front) lens diameter of 35mm. These two numbers are important since
> together they determine the size of the exit pupil.
>
<big snip>
Nice explanation of issues I was too busy/lazy to spell out. The only
problem is that it leaves out at least a couple of related issues.
A problem with exit pupils beyond what you have said is that they also
affect ease of use. With a small exit pupil, careful alignment of both
optical paths and irises is required. Some people find this troublesome,
complaining about difficulty getting them adjusted so the image is not
partially occluded in one eye or the other. Particularly a problem for
some with folks with glasses and some vision problems.
The whole objective/magnification/exit pupil business interacts with
resolution. Assuming equal optical quality and good vision, the 7x50 in
the beach example is capable of resolving more detail than the 7x35.
Just as with camera lenses, and all optical systems, resolution is a
function of aperture. This factor is important in birding, which is why
you often see serious birders hauling around bigger, heavier binocs than
might otherwise seem needed.
Moose
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