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

Discussion in 'Digital SLR' started by Anoop Saxena, Apr 18, 2009.

1. ### Anoop SaxenaGuest

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.

Regards,
Anoop.

Anoop Saxena, Apr 18, 2009

2. ### David J TaylorGuest

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

David J Taylor, Apr 18, 2009

3. ### Chris MalcolmGuest

The angle of acceptance of light of the lens, otherwise known as the
width of the field of view, and linearly proportional to focal length.

Chris Malcolm, Apr 18, 2009
4. ### OGGuest

At the focal plane the (4x as much) light is spread out over a (4x) larger
area for the 100mm lens.

OG, Apr 18, 2009