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Exit pupil

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 10:56
TexasPhotog View Drop Down
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How much is too much? 

I'm told as we age the pupils in our eyes lose their ability to open up as readily and can only take in so much light.  I realize lens coatings and other things play a part in light transmission too, but I'm wondering how much exit pupil is really usable on a scopes or binoculars (assuming it's good glass that has a very high transmission percentage)?  Might a 50 year old might be unable to use the light a 6x42 scope can transmit and might be just as well of with a smaller 6x36 for example?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 11:06
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Two issues at play here. 
 
One is the size your pupil can dilate to take advantage of existing light (generally taken to be about 7mm in the young to 5mm or so for us older folks).  The other is how fixed your eyes have to remain behind the exit pupil in order to see the entire image without it blacking out.  This is a comfort/ eyestrain issue.
 
Your 6x42 example might provide more available light than we can use but is probably a very comfortable optic to view through for extended periods.  The 6x36 will probably give you all the light you can use in a lighter optic.  Different needs for different purposes.  If weight is not a factor and you can achieve a good head position the 6x42 sounds ideal.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 12:03
TexasPhotog View Drop Down
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Thanks, Dogger.  That's interesting.  Could you elaborate on what you mean by "how fixed your eyes have to remain behind the exit pupil"?  Do you mean keeping in the eye relief box - going forward and backward from the scope? Or side to side movement?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 12:15
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Lateral movement with a big exit pupil is more forgiving before you get into blacking out of the image.  Your eye doesn't have to be quite so precisely aligned with the centre.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 12:24
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Literally it is a band of light coming through the scope, if it is larger than your pupil then you can move your pupil around a little and still have a good clear image.  Once part of your pupil moves out of that bank of light you start getting black edges and lose sight through the optic.

So even though it may be more than you need to see clearly, it is always a nice thing to have that extra exit pupil so you can have a little movment.

An easy way to test it is to get behind a variable powered scope and on the low end eye placement is very forgiving.  But as you turn it up to 9 or 10x you have to be much more precisely behind the scope.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/18/2009 at 13:30
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Got it.  Thanks guys!
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