f2.8 vs. f4 set at f8

Tango

I never cease in stupid, so here is another question Ive been wondering about.

Will I get the same amout of light through a f2.8 lens compared to a f4 lens both using aperture f8? or will the 2.8 give me more exposure set at f8 because its a faster glass?

(specifically starlight light.)
(more specifically Canon 16mm-35 2.8 set at f8, vs. a Canon 17mm-40 F4
set to f8)

divamum

AaronNelson wrote:

I never cease in stupid, so here is another question Ive been wondering about.

Will I get the same amout of light through a f2.8 lens compared to a f4 lens both using aperture f8? or will the 2.8 give me more exposure set at f8 because its a faster glass?

(specifically starlight light.)
(more specifically Canon 16mm-35 2.8 set at f8, vs. a Canon 17mm-40 F4
set to f8)

I'm sure somebody will correct me if I'm wrong, but my understand is that the larger aperture can help focusing regardless of what setting is chosen, but that the actual amount of light hitting the sensor will be the same in both lenses at a given aperture.

However, I'm not entirely sure of that (that's that SCIENCE stuff, which is not necessarily my forte!), so I'll be interested in seeing The Pundits have to say too!

raptorcaptor

At f8 the amount of light is the same. This is true for all f stops.
Divamum is correct that the faster glass will help with focusing, as the the lens will only stop down to the set aperture when the shutter is pressed. Until then it is wide open.

jogle

there will be very slight differences between the lenses but it's down to lens design as much as maximum aperture. F stops measure just the aperture width as a ratio of the focal length. Cinema lenses are actually measured by the amount of light transmission, they call it T-stops

Grainbelt

raptorcaptor wrote:

Divamum is correct that the faster glass will help with focusing, as the the lens will only stop down to the set aperture when the shutter is pressed.

love those bright viewfinders.