Purely technical AF question

FidelCastrovich · Nov 28, 2008

Quoting from the 50D user guide :

"With lenses whose maximum aperture is larger than f5.6 : ...all AF points cross type AF ...is possible;
With lenses whose maximum is LARGER than f2.8 : With the center AF point, high precision cross type AF sensitive to both vertical and horizontal lines is possible. The center AF point is...twice as sensitive as the other points".

How come it doesn't say anything about the diagonal sensitivity that i've read about on the Web?
And why does it say "larger" and not "equal or larger"?
Now i have to buy a fast prime to utilize the center point's additional sensitivity?

Garbz · Nov 28, 2008

1. Diagonal = cross. By putting a cross type sensor on the camera becomes sensitive to lines of any orientation regardless of the orientation of the AF sensor itself.

2. Maybe they mean larger and not larger and equal to, maybe the manual was written by marketing people who have no idea. I'll guess the latter since all sites mention that the sensitivity doubles when larger OR EQUAL TO f/2.8

3. Not necessarily a prime. The reason f/2.8 is the standard chosen is that many L series zooms are f/2.8 over their entire range. Basically the centre point's extended precision requires far more light to work so large apertures are a must.

Note I do not own nor have I used this camera. My answers to question 2 and 3 are based on interpreting what I have just read after a few google searches, but I think it all should be right

FidelCastrovich · Nov 28, 2008

Okay, i guess that settles it. I also thought that it was just a mistake by whoever wrote the manual.
The diagonal being the same as cross - that was news to me. I thought cross is vertical and horizontal, and diagonal meant a third plane.

So what makes the center point special if they're all cross type from f5.6?
It's just more sensitive? Not all cross types are born equal, so to speak?

Edit : Primes, because my zooms are f2.8, but i thought that wasn't enough. Now that we've established that it is, primes are not necessary.

Garbz · Nov 28, 2008

hmmm You may be right, it is different. The cross type sensor in the centre from what I gathered for further quick research seems to be two in one. A standard cross type sensor, and a diagonal cross sensor on top of each other. It's the diagonal one that's sensitive to f/2.8 and above.

Also not all cross type sensors are equal. They come in a range of different sizes and quite a few cameras have larger sensors in the centre than the outside. It's well known (or at least should be) that the centre point is by far the best to use.

Not sure entirely what this means for usability. I have never had an AF problem that wasn't a result of user error or complete lack of contrast / brightness. But it definitely seems that you need a f/2.8 lens to take advantage of this.

epp_b · Nov 28, 2008

Autofocus cameras can usually can't confirm focus well at apertures smaller than f/5.6 (remember, when you set a small aperture, the camera only stops down for the split second that it opens the shutter). f/5.6 is usually a standard for acceptable autofocus.

When you have a faster maximum aperture of (f/2.8 or larger is considered "fast") the lens is able to let through more light, allowing the AF sensors to "see" more. When they see more, they can be more accurate and precise, which is more important at wide apertures with narrow depths of field.