Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 43,701
Re: Whoops!

flakin stephen wrote:

In fairness, the OP's questions was about lenses. You could have a 40mm f2.8 lens designed for APS-C, 35mm, or medium format. Even though the density of light on the sensor would be the same, each would transmit a different total amount of light, as the area it will illuminate is designed to be larger for the larger formats.

Absolutely everything else could be exactly the same (f stop, aperture size, noodle points). The variables are all to do with sensor size, so are irrelevant to a question about lenses. Surely measuring photons by the proxy of noise levels (or vice-versa) is a minority interest ?

We are not "measuring photons by the proxy of noise levels (or vice-versa)" -- we are saying that the more light that makes up a photo, the less noisy it will be, and that the same exposure on different formats results in more light falling on the sensor of the larger format.  Even more specifically, the same total amount of light falls on the sensor for all systems for a given DOF and shutter speed.

If you're going to say, "Well, duh!", then congratulations -- you already knew! 

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