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Re: [OM] Binary DoF

Subject: Re: [OM] Binary DoF
From: Jez Cunningham <jez@xxxxxxxxxxxxxx>
Date: Sat, 25 Oct 2014 09:50:59 +0100
Which is why I recently bought some nice Zeiss 10x42's  !!

On 25 October 2014 02:46, Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
wrote:

> 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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