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Relative Brightness vs. Twilight Factor

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2008 at 07:44
magshooter1 View Drop Down
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Can anyone tell me what the difference in these two references is.  Some manufacturers use Relative Brightness and some use Twilight Factor to describe their Binoculars.  I have even seen one that used Geom. Twilight Factor, which I assume is Geometric Twilight Factor.  If anyone can clear this up it would sure be a big help.  Thanks, Whacko
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/03/2008 at 09:21
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Now a days, alot of that stuff does not mean as much as the quality of the lens.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/05/2008 at 14:52
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Both are dated terms that were supposed to tell the buyer how bright a bino would be. Relative brightness is derived from squaring the exit pupil size (8x40 has a 5mm EP therefore a 25 RB factor). Twilight factor factor is derived from multiplying the power and objective sizes and determining the square root (8x40=320, sr would be appr. 17.9) This was used back in the day prior to lense coating technology that we have today. According to those factors, a 10x42 would be brighter than a 7x50. I can tell you that with quality being equal, there is no way a 10x42 is brighter than a 7x50. Neither of these factors take into consideration the quality of the glass used, quality of the coatings used or stray light management systems. In my opinion, they are basically worthless.

Roy

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/05/2008 at 19:07
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Chris,

This little book will explain in detail and is worth reading.

Just click on this: How To Choose Binoculars

Edited by Bird Watcher - September/06/2008 at 14:33
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: September/06/2008 at 21:56
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Don't purchase binoculars just by those numbers. There are many other far important factors: lens coating, prism coating, field curvature, color fidelity.

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