How do different lens of varying focal length let same amount of light at a given stop?

Anoop Saxena wrote:
Hi,
Consider two lens, 50mm and 100mm. At f2, the aperture diameter
is 25mm and 50mm respectively. So how can they let the same amount of
light? Am I missing anything here.

Thanks in advance.
Regards,
Anoop.

Anoop,

Same amount of light per unit area on the focal plane. Same "brightness",
if you like.

Given a simple scaling up of an optical design, the focal plane coverage
of the larger lens will be four times the area (i.e. twice the linear
dimension).

David