I have been having a discussion elsewhere on the merits (or otherwise) of a full frame sensor compared to smaller ones such as aps-c or 4/3.

One of the arguments put forward against 4/3 is that when a lens is say F2 it is not the same as using an F2 lens on FF in that less light hits the sensor.

That is an F2 lens on 4/3 means you get a 1/4 of the light on a 1/4 of the surface area. So lenses like the 35-100 F2 when used on 4/3 does not deliver an equivalent package to a 70-200 F2 lens on FF in terms of low light capability.

I am struggling with this concept as it doesn't seem right to me. For a lens to be F2 it simply means the diameter of the maximum aperture has to be wide enough in relation to the focal length i.e. for a 30mm lens to be an F2 lens the diameter of the maximum aperture would need to be 15mm.

That being so then if the available light dictates an exposure of 1/250 @ F2 @ ISO 100 then setting that on a 4/3 camera will give the same result as setting the same shutter and aperture value on a FF camera for the same ISO. You don't have to set the 4/3 camera to a slower shutter speed. Right?

Can some 4/3 guru confirm my thinking or explain why 4/3 is less sensitive than a FF camera?

Dave