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as said above, the rules are different for extended objects (like planets) where the light spreads out over a larger area as you increarse focal length, vs stars (or any point source) where the light is always a point source no matter what the focal length.
Generally speaking the focal ratio will give you the image brightness, but I think this only applies to extended objects.
Likewise focal length gives you image size, but again only applies to extended objects, ie you can see that a star is always a point source no matter what the focal length (within reason!).
cheers, Bird
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