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Actually I have just realised an important proviso to my earlier assertion about brightness. If a scope produces a larger exit pupil, some light may be lost if the exit pupil is larger than the pupil of the observers eye. So if I was using a 40mm ep and a scope of f10 the exit pupil would be 40/10 or 4mm. If I went to a scope exactly identical but at f5 the exit pupil would be much larger at 40/5 or 8mm. At my age my pupil can maybe dilate to about 4 or 5mm max so potentially I am wasting light with the faster scope so in this case the "faster" scope would actually produce a dimmer overall image.
I think!!
Malcolm
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