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What gets lost with a 20/24mm objective?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2012 at 08:29
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Optics Journeyman
Optics Journeyman


Joined: January/12/2009
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I just bought my first AR15 and eventually I'll probably put a compact  1-4 power tactical scope with a 20-24mm objective. My two varmint rifles have 44mm objectives, deer rifle is 40mm, ML is 36mm, and my slug gun is a 32mm.
 
What do I lose in a 20-24mm objective - FoV, light gathering . . . ?  With that small of an objective my first thought would be they'd be like looking thru a pin hole. But Leupold make a tactical 1-3x14mm for $1100 so I would assume you ought to be able to see something thru it for that kind of money.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2012 at 12:08
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 A scope with a 7mm exit eye pupil is as bright as it gets.  Part depends on how large you eye will dialate the pupil in low light as we get older it may only be 6mm or so.  With optics how bright the optic appears in low light is (Sze of the Ojective) divided by the (Power of the Scope).
6x42 scopes have                ( 42 )                         divided by       ( 6 )   =  7mm exit eye pupil
 
so think of multiples of 7 example  3x7=21 so a 21 mm objective wont suffer at all at 3x.
A 20mm objective at 4x would be 20 divided by 4 = 5mm exit eye pupil so still fairly useable.
Want it brighter dial down the power and it will brighten the image.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/14/2012 at 13:23
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Optics Apprentice
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1-4X scopes have a very wide field of view at low magnification. All the scope companies include FOV on their specs, so it's easy to check.

A smaller objective lets in less light. A variable scope gives the brightest image (provides a useful image in the lowest light conditions) when the exit pupil of the scope is the same size as the entrance pupil of the eye.

For example, given an eye that dilates to a maximum of about 6mm (like mine) a 20mm, 32mm, and 44mm scope of equal quality, set at equal magnification, will work about equally well in good light. Set at 3X magnification, they will all provide a useful image at the same minimum light level. The 32 and the 44, set at 5X, will still provide a useful image when it is too dark to use the 20mm scope. At 5X, the 32 and the 44 will go dark at the same time, but when the 44 is turned to 7X, it will still provide a useable image.

There is a calculator for this at scopecalc.com.
I have tested this with 25mm, 32mm, and 50mm scopes of comparable quality myself.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/16/2012 at 21:11
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Optics Master
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The coatings matter more than the objective size.
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