Originally Posted by neuroanatomist
Let me clarify this.
This statement is true in some sense but false in another. It is true in the sense that , if you use, eg, iso100 with both sensors and the same f number (ie, set your camera to f/1.4 for both cameras), the shutter speed will be the same.
However, it isn't the "iso number" that we care about, but rather how much noise we will get in our pictures. In the above scenario, the ff camera will have less photon noise. Alternatively, the ff camera could have used a higher shutter speed and gotten the *same* amount of photon noise as the crop camera. In this sense, f/1.4 exposes more quickly on larger sensors. Put another way, if you put the 85 f/1.2 on a crop camera and put the 135 f/2 on your full frame camera and set the camera isos so that photon noise is the same for both cameras, you'll get the same shutter speed (not exactly- 1.4 * 1.6 is not exactly 2, but close enough). The iso setting will be higher on the ff camera, but the photon noise will be the same.
Thus I think it is correct to think of cropping as changing the "effective f number" the same way it changes "effective focal length" (ie, multiply f number by crop factor to get "effective f number"), because cropping changes both DOF and exposure speed in exactly the same way as multiplying the f number by the crop factor.
John knows all this (after all, it's been hashed over dozens of times on this forum), so it is not my intention to correct him, but I think his statement is easy to misinterpret.
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