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Objective size question

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Unread 08-01-2013, 07:03 PM
Bronze Member
Join Date: May 2013
Posts: 34
Objective size question

Hey fellas, I need to draw on those more experienced than myself. I'm in the process of piecing together a new build. I'm building a 300 win mag that I'm designing to be a medium weight long hunter. I had a manners sl stock that I chose to not use on a different project and am now gonna use it with a #4 bartlein and hopefully a stiller action. My primary question is about scope objectives. I don't own a single scope with an objective diameter larger than 42 mm. I only own one other rifle that was purpose built for longrange shooting and that one even has a vx111 longrange with a 40mm objective. So far I haven't seen a true need for a 50 mm or larger objective. What opinions do you guys have? I truly like my scopes to ride low but I'm not totally deadest on the 40-42 mm objectives. Any opinions and info would be appreciated.
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Unread 08-01-2013, 07:30 PM
Platinum Member
Join Date: Jan 2008
Location: West Central Idaho
Posts: 1,219
Re: Objective size question

For a long long time I peeked through a scope with a 40mm objective lens and never had a problem early morning or late afternoon where I had trouble seeing my target. Purchased a new Vortex HS and it has a 50mm lens. Hate the size but am putting up with it. I'm with you about the low and sleek scope. Some of the new 44mm scopes like the Vortex HST would be a cat's meow but anything larger is just too intrusive for my style of hunting.

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Unread 08-02-2013, 08:27 AM
Platinum Member
Join Date: May 2011
Location: Ventura CA
Posts: 1,106
The difference will be during the first and last minutes of legal hunting time, when your eye increases its pupil to the largest size. The 50 mm objective will allow you to use a higher magnification, without loosing brightness. The specific benefit of 50 over 42 mm depends somewhat on your age, because the eye's pupil size decreases with age.

For example a person in his 30s probably has a 7 mm pupil in low light. A 50 mm scope can be turned up to ~7X mag and be as bright as the naked eye. Above that mag the image looses brightness fast. A 42 mm scope can only be turned up to 6X mag without loosing brightness.

If both scopes are set to 6X the brightness will be the same, but the 50 mm scope will have a larger exit pupil and therefore will be easier to align to your eye in low light.

This of course assumes all other factors are the same, such as lens transmission (similar anti-reflection coatings). Tube size is not a factor in scope brightness.

Last edited by bruce_ventura; 08-02-2013 at 09:15 AM.
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