So a f/1.8 on a 1" sensor would still gather more light than a f/3.5 on an APS-C?

Bob A L wrote:

Great Bustard wrote:

Aside from DOF and noise, why do we care about the f-number at all? It's not exposure time, because whatever exposure time the 1" sensor can use at f/1.8, the APS-C camera can use at f/3.5.

I don't understand. We care about f-number because, the size of the hole to let light through (f-number)

The f-number is not the size of the hole. f/1.8 on FF 50mm is not the same size of hole as f/1.8 on 35mm APS-C.

and the shutter speed are the only way we can control the amount of light getting to the sensor.

And other than noise, why do you care how much light gets to the sensor?

And we use f-numbers because physical size of the hole must vary according to focal length to allow the same amount of light through and f-numbers are derived from calculations that create the same amount of light reaching the sensor at f1.8 regardless of focal length,

That's not true, either.

If the lenses on a FF camera and on an APS-C camera are both at the same f-number, and you have the same scene luminance and same shutter speed, then the FF sensor will receive two and a quarter times as much light as the APS-C sensor.

where the physical size of the aperture will be much different on a 5mm lens than a 500mm lens. And aperture physical size is based on focal length,

No, it is based on diameter, which can be expressed in terms of focal length by using an f-number.

not sensor size.

But (assuming the lens has a big enough image circle) sensor size determines how much light falls on a sensor for a given exposure.