How big focus spot

[GyRob]

i read in here that lots just use the centre focus point as its quicker so the midle red sensor - but just how big an area does that spot cover for focus i.e is it the size of the square ?
thanks
Rob.

[Biko]

Not done a scientific test but in experience I would say it can't be far off the size of the square - photographing nature such as birds it focuses from one small branch to another as soon as you move outside the square.

I use different settings depending what I am tracking or shooting or where the subject is to be focussed on so .

[robertwgross]

It has seemed to me that if you take the red focus square, and if you doubled the thickness of the red lines, then that would approximate the actual focus spot. In other words, it is just a little larger than it seems from the square.

---Bob Gross---

[davidwegs]

It is a vertical and horizontal pattern on the 20d and 1Ds2.

I believe it is like a cross in its configuration with the ends of the cross extending out of the red square by about half again as much in each direction.

That's what I hear. :)

[Tom W]

OK, here's the image of the 10D's focus points, which are probably very similar to those on the 300D & 350D Rebels:

http://home.comcast.net/~trwilk3/Images/Focus_Point_Size.jpg

Note that the red shape indicates the sensor, while the black squares are what the focus indicators of the 10D look like. This doesn't speak for the 20D which uses a newer focus system. As well, the 45 focus points on the 1-series are probably a bit smaller, otherwise they'd overlap.

Anyway, it can be seen that if your high-contrast boundary is "seen" anywhere on the area depicted by the red, it can and will be used by the camera to obtain AF.

The original image is Courtesy of Chuck Westfall - I colored it in for clarity.

[GyRob]

WOW that show's it well ,hope the 20d is the same, with a long lens if it was the size of the square theres not a lot to play with.