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Geometric Brightness vs Twilight Perf

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: July/27/2005 at 12:19
mouflon View Drop Down
Optics GrassHopper
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I am trying to make sense of the table I created below to analyze light transmission in various S&B scopes.  I used the formulas shared by Chris last year in his Rifle Scope School post.

 

Scope Mag Min Mag Max Objec-tive EP Min EP Max EP Avg Geo Bright Twilight Min Twilight Max Twilight Avg
S&B 2.5-10x56 2.5 10 56 22.40 5.60 14.00 196.00 11.83 23.66 18.71
S&B 3-12x42 3 12 42 14.00 3.50 8.75 76.56 11.22 22.45 17.75
S&B 3-12x50 3 12 50 16.67 4.17 10.42 108.51 12.25 24.49 19.36
S&B 4-16x50  
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/01/2005 at 13:05
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Optics GrassHopper
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I am bumping this in the hope that some of the optics wizards on this forum can help my understanding a little.

 

Many thanks,

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: August/01/2005 at 21:55
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Dark Lord of Optics

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As the formulas suggest Geometric Brightness (GB) does not take into account magnification. 

 

You can rework formulas above to correlate Twilight Performance (TP) and Geometric Brightness (GB):

 

TP=Magnification * SQRT(SQRT(GB))

 

Which basically says that the object may appear slightly less bright, but if it is more magnified to your eye you might see it a little better.  However, higher magnification really diminishes field of view.

 

Anyhow, if you want to compare the performance of several scopes you should really compare them at the same magnification.  At the same magnification, a scope with a larger objective lens will always have better twilight performance.

 

You have to first figure out what magnifications you want to use and work off of that.

 

Ilya

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