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Focal Ratio = image brightness.
Focal Length = image size.
So, two scopes with the same focal ratio will produce images that are the same brightness with a given EP or camera.
Two scopes with the same focal length will produce images that are the same size with a given EP or camera.
The two properties have to be considered seperately. Anytime you want to consider brightness, use the F/ratio. When you consider image size, use the F/length.
regards, Bird
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