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[quote name='Yakim' timestamp='1331544174' post='16597']

This statement is true only WRT DoF, not WRT light gathering.

[/quote]

Wrong.



Both lenses/sensor combinations will have these two factors EXACTLY the same:



The sensors will capture light from the SAME angle of view.

The lenses will let light through through the SAME size of aperture.



Now explain how there would be a difference in "light gathering" as you put it?



Your statement can only be right (the 2nd part of it) when you take two lenses on the SAME size of sensor.



Let me put it to you in a different way. Suppose a flat surface of size X has "gathered" an amount Y of light over its entire surface, over a certain period of time.

Now we cut that flat surface in half, and those halves into half again.

Will each resulting surface have captured Y amount of light, even though they are 1/4 X size now? Of course not.



Or yet in another way.

A 24mm f2.8 lens has a LOT smaller aperture than a 90mm f2.8 lens. How come, when you put then in front of the same sensor, the sensor will capture the same amount of light over a period of time, when one aperture is so much bigger?

Because one lens (the 24mm one) captures light from a MUCH wider angle of view.

Now if we are to crop that wider view, we are left with less light to capture.



Similar to a smaller sensor... Light gets "cropped". So the sensor needs a longer duration of exposure to capture the same amount of light to form the image, if you use the same focal length / aperture ratio.



This though is obscured from the user, due to the great big equalizer called ISO in the digital era.