Before the rush

Before the rush
by evan-pak

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From the Fujifilm X-Pro 1 manual:

When [¤] AREA is selected for [ ] AF MODE in the shooting menu and S is selected for focus mode, the electronic viewfinder and LCD monitor offer a choice of 49 focus points and the optical viewfinder a choice of 25, allowing photographs to be composed with the main subject positioned almost anywhere in the frame.

What is the reason for this limitation in the optical finder? In either case, the main sensor is used to for contrast-detect focus, so there doesn't seem to be an inherent restriction. Is it just because the parallax of the optical finder inherently introduces uncertainty, and so the optical mode forces larger focus areas to reduce the risk?

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Having dug through a bunch of online reviews and forum comment chains on the X Pro-1 AF (the best of which is X Pro-1 Tips and Tricks), I can only come to the conclusion that since the OVF is not actually showing you exactly what the lens sees as in an SLR-type design, it is impossible to be as entirely accurate as when using the EVF.

I can't find any specifics, but if the OVF is akin to Leica rangefinder OVF's, then there is a parallax discrepancy between what you see through the viewfinder and what the sensor (and, therefor, the AF points) see. I'd assume that the 25 optical focus points are much larger in area than the 49 electronic focus points in order to account for margin of error.

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Didn't I say this in the question? :) – mattdm Apr 27 '12 at 15:33
@mattdm: Yes...I was just trying to add some confirmation for you. :P – jrista Apr 27 '12 at 15:44

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