In reply to ow arm:
For a distant object like the moon, the focal length of the lens does not change the exposure; the F number, iso and exposure length are sufficient.
If you had an F5.6 300mm lens it would be 53mm across its front element. An F5.6 1000mm lens would be 179mm across and would gather more light to compensate for the larger image and the overall exposure would look the same.
However, some lenses have many additional optical elements or thick lens parts which all attenuate the available light to some extent and cause a dimmer image for a given exposure. This can be measured with the T stop which is a modified (higher) F number.