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Old 04-06-2021, 10:59 AM
JA
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JA is offline
 
Join Date: Oct 2016
Location: Melbourne, Australia
Posts: 3,052
Quote:
Originally Posted by roughy View Post
Whilst there are quite a few tools to calculate the difference in FoV, is there a formula to calculate the percentage difference in brightness between these to focal ratios?
Sensors that will be used are KAF8300 (5.4 microns) and MN34230 (3.8 microns).
Thanks
Mark
Hi Mark,

Yes, it has to do with the way the f-Number is defined and the increase in light per unit area/field, but by way of a worked example:
in your question of an f/7 vs f/6.5 optic, the f/6.5 would be ~16% brighter.

(7/6.5)^2 = 1.15978

In other words the f/6.5 optic is 1.15978 times (~16%) brighter than the the f/7 optic.

Or if you prefer,
if the f/7 brightness were 100 units, then the f/6.5 brightness would be 115.978 units or ~ 16% brighter.
Best
JA

Also note that if the image circle differs between the 2 optics the considerations are more complicated, especially if there is any vignetting.

Last edited by JA; 04-06-2021 at 11:20 AM.
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