10mp 10fps 1.3 crop

Started Feb 17, 2007 | Discussions thread
T3 Forum Pro • Posts: 21,546
Re: 27pt AF is an upgrade? NT

thx1138 wrote:

Dave Chrismer wrote:

Yes, one of the problems with the 45 pt is they are so small they
lack the sensitivity of the larger point used in the 5D. Also how
many use it in 45 pt mode? I rather a newer 27 pt system, with at
least 9 + 4 x-type points. This would be placed in a rule of thirds
grid, but with the centre point surrounded by 4 x-type points.
Unlike the current system the off centre points would be x-type @
f/4, not f/2.8 and the centre point would be x-type @ f/5.6.

A larger AF point doesn't necessarily mean it's more sensitive, and conversely a smaller AF point doesn't necessarily mean it's less sensitive. A larger AF point is simply larger, eg, its contrast detection strip is longer. But that doesn't mean it's more sensitive. It simply means that it covers a larger area. You can have a shorter contrast detection strip that is just as sensitive as a longer contrast detection strip. Saying that a larger AF strip is more sensitive than a shorter AF strip is like saying that a 12 inch ruler is more accurate for measuring a one-inch object than a 6 inch ruler is. Both rulers are equally accurate (or sensitive).

When you have a more broadly spaced AF array, either the focus points are farther apart, or the focus points extend farther beyond their marked AF position. Neither cases necessarily produces higher sensitivity. In fact, if an AF stip extends too far beyond its marked focus position, that can cause percieved errors in focus because areas outside the market focus position (but within the true area of the focus sensor) are snapping into focus. So in light of that (no pun intended), one can argue that is you want greater AF precision, you would want a smaller AF point (eg, a smaller contrast detection strip) to get a more precise point of focus, just like a small spot meter offers a more precise, point-point measure of light within any given scene.

A cross-type sensor puts a contrast detection strip along the vertical and horizontal axis. This allows the camera to "see" contrast lines along two axises, rather than just one.

Anyways, if you want to read more about how AF in SLR cameras works, read here:

http://www.howstuffworks.com/autofocus3.htm

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