An exit pupil
of 4.0 is OK for general-purpose use (given good-quality glass).
My best overall views of M101 on most nights were at 253 x with a 3.5-mm exit pupil
Things like increased bulk, length, and weight; reduced field-of-view, exit pupil
and low-light performance.
Consider the following: Relative brightness and twilight factors for five binocular models EXIT PUPIL
RELATIVE BRIGHTNESS TWILIGHT FACTOR 7X35mm 5mm 25 15.7 10X50mm 5mm 25 22.4 7X50 7.1mm 50.4 18.7 8X20 2.5mm 6.3 12.6 8X42 5.25mm 27.6 18.3
Fixed power rifle scopes come in an almost limitless combination of powers and objective diameters, but all use the same method of determining the size of the exit pupil
. As you can see from the illustration above, the 4X32 rifle scope, which is rightfully one of the most popular models available, gives you an exit pupil
of 8 millimeters.
SPECIFICATIONS Magnification: 4x to 16x Objective lens diameter: 42mm Eye relief: 4 to 3.7 inches Exit pupil
: 10.5 to 2.6mm Tube diameter: 1 inch Overall length: 13.5 inches Weight: 18 ounces Adjustment increments: .25 MOA Adjustment range: 36 MOA Field of view: 23.9 to 6 feet at 100 yards Waterproof: Up to 1 meter for 10 minutes, nitrogen gas purged MSRP: $579.95
For visual observing through a telescope, the effective pupil size will be determined by your instrument's "exit pupil
" or by the entrance pupil of your eye, whichever is smaller.
As is prevalent with large objective, "fast" optical systems, the secondary spectrum often dominates near the edge of the exit pupil
. However, it should also be viewable while looking at a high-contrast "white and black" target when moving your head away from--and toward--the exit pupil
along the axis of the scope.
With a variable magnification scope you can power down to increase field of view and/ or to enlarge the exit pupil
to brighten the image.
Binoculars in the 6x and 7x range, with an exit pupil
of at least 5mm -- 6x30, 7x35 -- are ideal for twilight hunting.
Another important aspect of an eyepiece is its eye relief--the distance that the exit pupil
is located above the eye lens.