Originally posted by Photobill Does not a lens have a maximum opening? This opening would only allow in a maximum amount of light. Yes deferent sizes of sensor's can absorbs deferent amounts of light from the same lens but the exact same amount of light passes through the aperture???
Camera manufacturers know that photographers expect ISO 100 and ISO 200 and so-on to be the same from one camera model to another, irrespective of sensor size, so they design the hardware to make sure that that's the case. And they know that photographers will expect f/8 and f/11 and so-on to be the same from one lens to another too, so they design their lenses to make sure that that's the case. Cameras and lenses are built so that photographers can rely on exposure settings being the same from one format to another.
It's amazing really. It's almost as if the people who design cameras have been to university to learn about it, rather than just endlessly regurgitating half-digested pap from the internet.
Now lenses of the same focal length have different fields of view on different formats, and some of the side-effects of this have led to an obsession among some people with a thing that they call "equivalence". It's a cult that has grown from a faulty understanding of how cameras work that was posted by just one blogger to begin with, but has spread. And this causes a problem. If we leave the incorrect assumptions posted by equivalentists unchallenged then unwary newcomers to photography might fall for it and the cult will keep growing. But when we do challenge the equivalentists, this reinforces their delusion that they are taking part in a two-way debate that they have something meaningful to contribute to -- rather than being merely factually wrong. So unfortunately, threads like this tend to go on and on.
All I can ask you to do is just accept the simple answer that you were given right at the start: you use the same exposure settings on APS-C and full frame, and on medium format and on large format view cameras too. A lens with an f/2 maximum aperture is a lens with an f/2 maximum aperture no matter which format you put it on, and the exposure settings will be the same. The lens really does not magically gather different amounts of light depending on the format you use it on.
Just trust that the people who designed and made your cameras and lenses knew more about what they were doing than random guys on the internet do.