> Is there an understandable explanation for this? > If I were to play dumb, I could argue that AF speed > should depend on working aperture, or *available* > light, but it is determined by max aperture, > whatever the working aperture is. Why is that?
Well, a f/2.8 lens wide open lets through more light than a f/5.6 wide open, right? You don't AF while pressing the DOF preview button (I hope ;-). So it is wide open that you - and the AF sensors in the body - look through the lens. Meaning a f/2.8 provides the body more light for the AF sensors than a f/5.6 given the same illumination on the scene... The sensitivity of the body does not change because of the minimum f/stop of the lens attached but the f/stop does impose how much light is reaching the body... Without going to do the math for you (EV to f/stop relation considering fixed focal length and fixed shuttertime), it's logical that a f/2.8 allows you to AF with lower available light than a f/5.6. When dimming the light, you'll reach the AF limit of the body sooner with the f/5.6. Cheers, Stefan * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
