From Wiki: "The entrance pupil is typically about 4 mm in diameter,
although it can range from 2 mm (f/8.3) in a brightly lit place to 8 mm
(f/2.1) in the dark. The latter value decreases slowly with age; older
people's eyes sometimes dilate to not more than 5-6mm."
When designing night glasses the designers consider the fully dilated
pupil to be 7mm. Since the exit pupil diameter of an optical instrument
(like binoculars) is the diameter of the objective lens divided by the
magnification a pair of 7x50 binoculars produces an exit pupil of 50/7 =
7.14mm diameter. That produces a beam of light that won't fully fit
into the eye if it's not dark adapted. For us old farts a pair of
lighter weight 7x35 binoculars with a 5mm exit pupil may be just as
bright since our pupils may not open wide enough to use all the light
that 7x50s can produce.
Chuck Norcutt
On 10/24/2014 5:33 PM, bj@xxxxxxxxxxxxxx wrote:
For what it's worth, one can actually see the effect of aperture stopping down
by examining the behaviour of one's eyes as the intensity of light alters.
Our eyes have auto aperture adjustment which is presumably designed to
limit damage to the retina in strong light, and to ensure better survival
against
predators n dim light.
Carefully think about what you actually see in very good light. You should find
that in central field, objects are in sharp focus right from nearby to infinity,
because your iris has stopped down to f/64 or something like that.
Then do the same in dim light, and look at something about 3 metres ( 9 - 10
feet) distant and you should be able to detect that the DOF is quite shallow..
Because the iris of your eyes is wide open at F/1.4 or whatever it might be.
Cheers, Brian
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