40D centre AF point

[Astaroth]

Looking at the above camera with keen interest but when reading the specification at http://www.canon-europe.com/Images/13_459036.pdf it states:

A central AF point is capable of accurate focusing up to aperture f/2.8, with eight outer points working up to f/5.6.

now is it me or shouldnt the central AF point have a higher (numerically speaking) f-stop capability than the others? at 2.8 it wouldnt work with most lenses or am i missing something?

[solidamber]

The 20D and 30D already had a "more accurate" sensor at the center focus point which is only enabled when you mount an f/2.8 of better lens. But the "high accuracy" sensor at the center point was only "vertical line sensitive" in the 20/30D.

But with the 40D, both lines in the "X" now have a "normal" and a "high accuracy" sensor. So this is something of an improvement. I also think it makes sense to arrange them as an "X" instead of a "+" because they'll be more likely to find a useful target that way.


Sourced from

http://forums.dpreview.com/forums/re...ssage=24506624

[Astaroth]

I assume therefore that it is 5.6 for normal focusing and the 2.8 for the more accurate?

[docsmith2k1]

It is better to think of the central sensor as more sensitive rather than accurate. With an f 2.8 lens more light is available to hit the sensor, and it can distinguish between less "contrasty" areas and so improve focus capability. (focus is dependent on the sensors ability to determine differences in contrast)

[Astaroth]

More sensitive to me however would mean it can focus with a small appeture therefore would be able to use a f4 with a 2x extender where as the f5.6 of the non-centre AF points would exclude this.

[docsmith2k1]

More sensitive in that it can distinguish between shades of grey that are closer together rather than less sensitive which require a more obvious difference in tonality.