>>3406838I’m not DISPUTING it, I’m EXPLAINING it. Read the comment I was replying to. He was saying that they both let in the same amount of light because they both had a 75mm aperture.
>>3406859This is just flat out wrong. I had to look up some terms to make sure, so I’ll save everyone else some time.
Illuminance: amount of luminous flux per unit area
Luminous flux: the perceived power of the light coming out of a source. SI unit is the lumen.
So, the whole point of f/numbers is that they abstract physical aperture size and focal length into one number that’s the same across lenses. Ie, an f/4 lens is outputting the same amount of light (same luminous flux) regardless of the what the f/ (focal length) in that equation is.
For example, a 200mm f/4 has a 50mm aperture, and a 50mm f/1 has a 50mm aperture, but the 50/1 is putting out a shit-ton more lumens onto whatever it’s projecting in.
Where illuminance comes into play is that larger sensors tend to have larger photosites, so the illuminance (ie, lumens per unit area for a given luminous flux) is the same but over a larger area, meaning more photons get caught. THAT is what affects the signal to noise ratio. Or alternately, more pixels with the same surface area, which ends up working the same way in practice because the noisy pixels are smaller and each individual noisy boy is a smaller percentage of the overall photo, but that has nothing to do with light values.
But in no situation ever will a lens with a larger physical aperture size but higher f/number put out more lumens or give greater illuminance than a lens with smaller physical aperture size and lower f/number. That’s the whole point of f/numbers.
