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p.1 #4 · p.1 #4 · Taping pins vs. native f/8 autofocus capability | |
AvianScott wrote:
Has anyone used both methods? Is native f/8 autofocus a big improvement over taping pins?
Generally, f/8 native AF is superior because it performs within the specifications rather than just happens to work if the lighting and contrast and wind direction are all just right.
The f/8 limitation has never been as much about a lack of light at f/8 (although it doesn't help to reduce the light and hence contrast available to the AF system) as it is about the angle of approach of the image light to the AF system. The smaller the aperture, the smaller the angle of approach and the harder it is to distinguish between the light coming from nearer the top (or left) of the lens and the light coming from nearer the bottom (or right) of the lens. At wider aperture it is easier to make this distinction even in poor light and so the AF can operate better.
The phase detection AF splits the image that it sees and concentrates on the parts of the light from the intended focus target coming from different directions to figure out not only whether or not the subject is in focus, but also which direction the focus needs to be changed. There is a bundle of lenses and filters in the AF assembly at the bottom of the mirror box to do this.
Another factor that restricts the ability of the AF to see the target from multiple directions at the same time is that the image has to bounce of the sub-mirror that is behind the main mirror. That sub-mirror is too small to allow the entire scene to reach the AF module and that is why until very recently the cross-type AF sensors were all at or near the centre of the image, whether or not they worked at f/8. Getting them to work closer to the edges is a great improvement but it hasn't yet made it at f/8 as well.
In nearly all cases, Canon used firmware to disable AF when the perceived aperture was smaller than the minimum allowed, in order that whatever AF you got was sure to work within specs. Taping the pins allows you to get into that not so perfect zone that is not fully within specs but might work OK in good conditions, slowly or less precisely in poorer conditions, and not at all in bad conditions. Nikon did not do this. It's specs were generally f/5.6 or bigger until very recently but you never had to tape pins to go beyond the specified AF limits.
- Alan
(Post #10,000 ! Yay ! Now I should probably go out and get a life )
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