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 p.6 #7 · Canon autofocus information | |
RDKirk wrote:
2. Canon's latest application for a US patent on their autofocus system
Go to www.pat2pdf.org and enter patent # 6,603,929
Wow! That patent is a great source of information! The focusing mechanism described there is revolutionary!
However, I am left wondering which cameras actually use circuitry described here. I made a Google search on that patent number and Canon 20D - and I got exactly one match, which was this very thread!
Fortunately there is a simple method of finding out whether 20D uses the focusing mechanism described in the patent. If the autofocus sensors are visible beneath the main mirror (visible in the sensor cleaning position), then 20D does not use the new system. I don't have a 20D myself, so I cannot check it. 10D has clearly several holes there, for the AF sensors. Also, if the shutter does not move when focusing, 20D cannot use this method.
The autofocus system described in that patent is revolutionary. It uses the current Canon (D)SLR focusing first. That method is based on the phase difference and blind (from the camera point of view) movement of the focusing group to the proper position. However, after that, the second method is engaged.
The second method is based on contrast, i.e. moving the focusing mechanism to the position where the best possible contrast is produced. The information for this focusing method is gotten from the image sensor itself, though the half mirror. The shutter, however, needs to be opened for the focusing operation, but the mirror stays in the position. There is, no longer, a smaller mirror beneath the main mirror, but the AF sensor is built inside the pentaprism housing.
Because the aufocus is based on the data that the sensor provides and because the lens is driven with a special "precision mode" (depending on the lens type), there will be no autofocus errors ever with this method. No back- or frontfocus, no matter how imprecisely the lens or the camera has been adjusted.
Provided, of course, that there is enough light and contrast for focusing in the first place and that the AF point is pointed to where the photographer wants it. But when the focusing succeeds, it is not possible to gain any better focusing by any means (=manual focusing is about dead after that). After all, the lens is stepped one step at a time and the best contrast (as seen in the final image itself) is found out.
One important corollary of this method may be that third party lenses either stop working altogether or at least they cannot be used in the precision mode. The patent describes a different lens focus mechanism movement which is used with the contrast focusing mode.
The patent describes three different lens mechanisms, each of which has a specific precision mode, depending on the implementation technology (two types of stepper motors and USM). Because that "new" mode is not used with the current focusing system, there is no way it can have been found out through any reverse engineering by Sigma, Tamron or Tokina. This might mean, in the worst case, that all third party lenses need to be rechipped.
My gut feeling is that none of the current Canon cameras use this new autofocus mechanism. In that case, maybe the 1D Mark II n is the first one? And it will be a big success, if that is the case!
Think about it - always perfect focus with AF!
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