Can someone exain this puzzle to me.
As Neuro has stated above, given cropping of two equally dense sensors of different sizes ( FF & APS ) the larger retains it's benefit of better high ISO performance even when cropped to APS. Presumably this is because there was more light originally on the larger sensor.
When you put say a 135L onto a crop sensor camera, the sensor is only recording about half the area of the image circle from that lens, and so half the light. Due to the difference in circle of confusion that f2 less now produces results in dof terms to (something like) an f3.2 lens.
So why is it that the exposure for that f2 lens remains the same as on a FF camera when the whole of the image circle is being recorded ( or much of it). And it does, even using a separate light meter f2 on the APS camera gives exactly the same exposure via the histogram as a FF camera. Yet the APS is not recording the same amount of light !
How can this be when a FF retains it's high iso performance e en when cropped.