Quote:
Originally Posted by mfreg
What has more effect on the aquasition (sp?) of focus, the lens or the camera body itself.
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On the acquisition of focus? That's an entirely different question from "speed", which is what the title of the posting said.
The body is in charge of determining what to focus on, tracking the target (if applicable), measuring the misfocus, and (if applicable) projecting what the misfocus will be at the time that the shutter will be open. The 1-series and the 7D have a dedicated processor just for handling AF target selection and tracking.
The only effects that the lens have on this part of the process are:
- in dim light, a slow lens might not provide a bright enough image for the AF sensors where a faster lens might;
- on most Canon DSLRs, at least one AF point (the center) can measure the misfocus with 3 times the precision when the lens is f/2.8 or faster; this gives a more accurate focus.
All of the above is optical/electronic and occurs at computer speeds. You won't notice a difference in
speed from the body, but you will notice better target acquisition and tracking on a 1-series body than on a Digital Rebel.
The actual process of adjusting the focus is entirely done by the lens. The speed at which the lens elements are shifted to achieve focus depends on the type of AF motor—for Canon brand lenses there are four possibilities: AFD, micromotor, USM micromotor, or ring USM—and to a lesser extent on the mechanical process of shifting the focus elements inside the lens.
One other item: if the light is dim and the AF sensors aren't getting enough light to make out an image, AF assist lighting comes into play. With the popup flash, this takes a noticeable amount of time. With hotshoe-mounted external flash units (or an ST-E2), the IR beam assist is much quicker.