Originally Posted by Q.G.
We do!
There is less light, reflected off the piano, reaching the camera than at 150 m, or 10 m.
But don't forget that the image of that piano will be proportionally smaller.
As mentioned before: both image size and light intensity follow the same geometry. So though there's less light, that light has to fill in a smaller spot on film. The intensity per area unit will be the same at 300 m as it would be at 150 m, or 10 m.
So the setting to use is the same too.
If at 300 m we use a tele lens and we fill the image with the piano, a lesser quantity of light fills the entire image, so we have a different exposure.

If I place on the piano a grey card and read it with a spot reflected meter, which only reads light reflected from the card (that is, supposing the card "fills" the reading angle of the spot lightmeter), my understand of physics tells me that my spot meter will give me a different exposure than the incident light meter used near the piano.

I am not convinced that my understanding of physics is right, though. Actually I very much doubt it.

If I take a picture of a lit monument at night, the exposure for the monument isn't the same even if I am far from the monument?

I'm really puzzled.

Fabrizio