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本月的Chuck Westfall TechTips裡面就有人這樣問過了
答案如下
1. There are several technical reasons why it's difficult to position focusing points near the edge of the frame on a 24mm x 36mm format camera. It's basically a "Catch-22" situation:
a) The phase-detection AF method that's been in use on most analog and digital SLRs since 1985 relies on a detection module positioned in the base of the camera's mirror chamber. Incoming light passes through the main reflex mirror to an auxiliary or sub-mirror, which reflects it down to the AF detection module. It is physically impossible to increase the width or height of the sub-mirror beyond the limits of today's technology without losing the light from the edges. However, within the limits of focus point positioning, phase detection AF is fast and accurate, and high-quality predictive AF (i.e., focus tracking) is possible. |
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發表於 2008-3-11 19:46
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