Dear all,

Can't figure that. I know the formula is f = Focal Length / Entrance pupil, but, it is not at all intuitive for me that if you have light from a point at infinity falling on two lenses A and B, both with the same entrance pupil, both converging that light to points: fa at 150mm from the lens and and fb at 300 mm from the lens, for me, it is the same amount of light on both images. How come that lens A is one f stop brighter and will need half of the exposure time compared to B?

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