Comparison of f/6.7 and f/2.8 Light Sources

Comparison of f/6.7 and f/2.8 Light Sources The TTL phase-detection AF system used in most AF SLR cameras works by obtaining the amount of defocusing (i.e., the degree to which the image is not focused) by detecting and calculating the phase difference in the subject image passing through the objective lens, which is captured by the left and right sensors. The focusing position is then obtained from this defocusing amount. If the subject defocusing amounts are the same, as shown at left, this is resolved using the difference between f/6.7 light and f/2.8 light. This is because the defocusing shift on the sensors is greater for the f/2.8 light than for the f/6.7 light, resulting in more accurate light measurement. When the picture is taken, faster f-stop settings also produce more marked defocusing. In the Dynax 7, the use of sensors suited to f/2.8 light in the ' + shape' and 'x shape' dual cross-hair sensors give improved AF accuracy when fast lenses are used.