> The fastest autofocus primes I see are f/2.8 and f/4, which are roughly equivalent in speed to an f/1.6 or f/2.3 respectively. <
Can you elaborate on that? What do you mean by "equivalent in speed?"
Can you elaborate on that? What do you mean by "equivalent in speed?"
I'm aware that sensor size doesn't change depth of field, however if you compensate the focal length to produce the same field of view on both sensors, you then need to compensate the aperture to arrive at the same depth of field.
So a 50mm f1.4 on a full frame sensor would be roughly equivalent to an 85mm f/2.4 on one of these digital medium format sensors.
And an f/4 for any size format will be gathering the same amount of light per sensor area. It's just that the sensor area is larger on larger formats, so the total amount of light (but not light per pixel!) is more. This is why telescopes have such wide apertures; they're limited by total amount of light.
Please note that the f-number doesn't indicate total light hitting the sensor. It indicates total light per unit area at the sensor. Thus, for the same f-number, a larger sensor will gather more light.
All of this info can be found in the wikipedia page: https://en.wikipedia.org/wiki/F-number