on Light Gathering Ability
At dusk, the average person's pupil dilates to 5.5
millimeters, so optics used for early morning or late afternoon
wildlife viewing should have at least a 5mm exit pupil.
To determine the exit pupil size, divide the size of the
front lens by the magnification.
For 7 x 35 binoculars, for example, the exit pupil is 5mm
minus 35 divided by 7 equals 5. Increasing the size of the objective lens
to capture more light often forces the optics designer to also increase the size
and weight of the instrument.
In daylight you can get by with as little as a 2mm exit pupil,
and that makes light-weight mini-binoculars and scopes an option.
It is possible to buy binoculars with an exit pupil larger
than 5mm. However, since the human eye dilates to only about
7mm in total darkness, these binoculars
may gather more light than the average person really needs. When optics
makers push up the exit pupil size,
they also must increase the price, since they use more ground glass
for the front lens.
Field of view
list the field of view in feet, giving the width
of scene a binocular will show 1,000 feet away. Most
birders prefer wide-angle glasses, since they allow
you to take in more of the landscape and keep track of passing species.
Others want to be able to observe as much detail
Generally, the higher the magnification, the narrower
the field of view.
The final technical considerations in making a selection are in Part
3, such as the focusing mechanism and price.
to Part 1
to Binoculars & Scopes Home