The lens never stops down during auto focus. The sensors have a minimum light sensitivity, usually expressed in EV. The maximum aperture is the widest lens opening. So a faster lens can focus in lower light. Second, if the maximum aperture is f/8 rather than f/4, the greater DOF makes it harder to detect focus accurately. The camera lens minimum and maximum focus points are further apart, while a lower F stop (wider aperture) lens has a shorter distance between min and max DOF (depth of field).
The f-stop is the (focal length) / (lens diameter), so a wider lens opening, maximum lens diameter (or maximum aperture), the smaller the f-stop value. EV is a measure of the light, standardized to ISO 100 speed. The auto focus range for a digital rebel is: EV 1-18 @ ISO 100 with f/1.4 lens EV 1 is the minimum light for the sensor to work. So with an f/5.6 lens you need 4 stops more light than an f/1.4 lens in order for the auto-focus to work under low light. The Canon 1DmII spec is: EV 0-18 (at 20°C & ISO 100) and the 5D is: EV -0.5 to 18 (at 20°C, ISO 100) So the 1DmII can focus in the same light with a 1 stop slower lens. With plenty of light, the minimum aperture the auto focus can work at might relate to the maximum DOF the sensor auto-correlation function can work with to reliably auto-focus. Even with manual focus, for example, the 85/1.2 lens can be more precisely focused than the 28-135 @ 85mm (f/5.x). Or take an f/4 lens and manually stop it down to f/8 and see how easy you can focus it compared to f/4. The camera has the same problem. Independent of the aperture setting used to take the picture with. Wayne At 05:21 PM 3/7/2006, you wrote: >I need a refresher. Could someone ploease explain, or point me to an >explanation, of why it's the *maximum* aperture and not the working aperture >of a lens that decides AF speed and accuracy with different bodies, and how >those bodies differ? For instance, the 1 series is said to be able to "focus >to f/8" (making an f/4 lens on a 2xTC feasible) while other bodies need f/5.6 >or so? - this is what I want to understand better. * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
