MintMar Offline Upload & Sell: Off
|
John Caldwell wrote:
I have never understood the basis of restricting AF regions to the center areas of the frame. Is this a matter that most shooters would not use peripheral AF points, or is there some design tradeoff that makes peripheral AF points difficult to execute?
This is not a bad image (Canon's own) to explain:
It's greatly simplified, but here you can see how the multiple images of the same subject are transferred to the AF sensors. The images from horizontal and vertical couples of sensors (also called baselines) are then read out and the correct focus movement is established.
Now, that image does not show the aperture of the lens, so you must imagine the aperture (wide-open is also sort of "stopped down ideal" ), and since the aperture reduces the circle where the autofocusing images can travel, the aperture obscures the worldview for the AF sensors. Therefore their grouping to the center.
This also explains why for most of the time, f/8 AF worked only on certain cameras and in center only: the worldview was so obscured for the AF baselines, that only the center AF sensor worked. This was also helped by a parabollic submirror of the first 45pt AF cameras like EOS-3 and 1D til Mk4.
With Canon, there are also high-precision baselines, which can focus fast lenses better, but they require quite unobstructed worldview. Therefore, some of the better sensor stopped working with lenses slower than f/2.8. Also, since they were wider, they had to be placed near center:
This is the 5D3 AF sensor. You can see how wide are the baselines of the five high precision X-shaped sensors in the middle of the viewfinder:
Since Canon quite expands the AF area with recent models, you can see various classes of lenses which limit the AF sensor and useful AF points according to how they obstruct the worldview by their maximum wide open aperture.
|