Well, point taken. But the 5D2's spread is pretty cramped as it stands. Which makes me think that I'd be fine with 9 points if they spread was wider (and they were cross type).
But come on Neuro, you're just encouraging Canon to be evvvvvvil.
Honestly, I wasn't being sarcastic! Consider...from one point of view, the outer AF points of the 5DII essentially suck, so the 5DII has a very
limited AF point spread, i.e. no spread at all, functionally just one AF point smack in the middle. From that point of view, just dropping the existing 7D's AF sensor, sized for APS-C, into the 5DII is a huge improvement!
Regardless for Focus points, why dont we see coverage on the 4 intersection points of the rule of thirds grid? Looking at the focus maps posted by John, I see coverahe near the grid intersections but non on the intersection points. Heck to make it simple, there should be at least 5 points in every camera, center and 4 grid intersections to start off...
There are limitations on how far from the center the AF points can be spread, especially in the vertical dimension. Basically, three reasons:
- Limitations on the size of the secondary mirror. Light for AF passes through the semi-transparent part of the main mirror (most is reflected up to the viewfinder), then is reflected off the secondary mirror down to the AF sensor. There is limited space behind the main mirror, based on the necessary geometry (i.e. the main mirror has to be at a 45° angle to the incoming light, and the secondary mirror has to be behind the main mirror and at an angle of 90° to the main mirror, so it's length is limited by the distance between the main mirror and the image sensor).
- Distortion. With many lenses, the edges of the frame are subject to distortion (barrel/pincushion), and that reduces the accuracy of phase detect AF.
- Vingetting. The AF system needs a certain amount of light to work. Almost all lenses vignette to some degree, meaning there might not be enough light at the edges of the frame. For example, the EF 17-40mm f/4L has >2 stops of vignetting wide open at the wide end - that means at the edges of the frame, the AF sensors would not heve enough light to operate.
Still, but the 7D and the 1D IV have points that are almost right over the intersections, and the 1DsIII and 1D X come quite close. Down toward the bottom of this article on EOS AF systems
, there's a set of mouse-over comparisons that show the relative AF point spread of the recent xD bodies.
It's worth noting that none of these limitations apply to contrast detect AF, so using LiveView you can autofocus right out to the edge of the frame.