Flake said:
61 focus points and all of them crowded together in the centre of the frame! What's the point in that? They could have used the 19 point of the 7D and had similar results. Is there anyone here prepared to admit that the only place they choose to focus is in the centre of the frame? Why not spread the focus points so they have a broader coverage which makes them a lot more usual in the real world.
torger said:
I'm no expert, but isn't this limitation a pure technical one? That is there is too poor sharpness, or too steep angle of incoming light or something far to the sides so it is really not technically feasible to do? That is full coverage is asking for the impossible?
Right and wrong. There are technical limitations on the spread of the AF points - at best, they can only occupy the middle area of the frame, because of simple geometry and optics. In a nutshell, there are three reasons:
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[*]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, you're below f/5.6 and AF sensors would not heve enough light to operate.
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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.
Regarding using the 19 point AF of the 7D, one of the advantages of a denser array of points is improved AI Servo AF tracking.
Canon states that the 1D X has "
expanded AF coverage area," but they don't say what that's relative to - could be expanded relative to the 1Ds III or the 1D IV.
I have a PS file with the focusing screens of all the xD bodies scaled and superimposed in layers. When they are all scaled relative to the frame size / rule of thirds grid, the 1D IV is the clear winner for AF spread. The 7D comes close, then the 1DsIII, with the 5DII at the bottom, I added the 1D X AF layout to this, and in terms of relative coverage, it slots in between the 1D IV (still the best) and the 1DsIII. I suppose Canon is technically correct - the 1D X AF coverage area is expanded from the 1DsIII, and since the absolute area is larger than the 1D IV (smaller sensor means smaller AF sensor), the 1D X is bigger than both.
The attached images show superimpositions of 1D X with 1DsIII and 1D IV AF points. As you can see, in the first comparison (1D X with 1DsIII) the rows of AF points at the sides extend a full point past where the 1DsIII stops, and the top-bottom extent is the same; the 1DsIII's coverage extends beyond the 1D X at the 'corners' (45°, 135°, 225°, 315°). Comparing the 1D X to the 1D IV, they have the same lateral extent, although the 1D X has a whole vertical row at the edges vs. a single point, but the 1D IV has a full extra row at top and bottom, including something that the 1D X lacks - AF points that sit right on the rule-of-thirds intersections.
The bottom line is that while the 1D X's AF system falls a little short of the 1D IV in terms of relative coverage, it's overall an improved system. Previous bodies had only the center AF point as a high-precsion point with f/2.8 lenses, the 1D X has a row of 5 such points. More importantly, cross-type points are much better than singe-orientation points. For the 1D IV and 1DsIII, you needed an f/2.8 or faster lens to get cross-type points active at all but the center AF point, else they were single-orientation only. The 1D X has 21 central points that are cross-type with f/5.6 lenses, and two 10-point clusters on either side that are cross-type with f/4 or faster.
To clarify the overall number of cross-type points by lens speed and camera body, see the summary table below (just edited to reflect the specs on the Canon USA website).
