Re: Musings on EOS autofocus sensor development
BiscottiGelato
wrote:
Thanks for the explanation and diagrams. Very informative and learned a lot on how these AF diagrams are supposed to be read.
In actual fact the mapping of the line sensor to AF points is a little more complicated than detailed in my diagram. For example the uppermost parts of the vertical line sensor pair 4 maps onto AF point 4 and not onto AF point 2 as one would assume, etc.. This "reversing" is caused by the optics of the SIR systems, but since the AF sensor is pretty symetric in its layout, IMO it is mostly relevant Canons HW & SW engineers as it does not change the basic understanding of how the phase type line sensors map to an AF point.
In terms of sensors then, it seems the center point of 40D/50D/7D are all pretty much the same. In terms of processing tho, I agree that there is more advanced processing on the 7D (wavelength based, selective read-out of sensor for spot AF, more processing power for faster AF in general, etc).
Even though this is partly speculative in my behalf (i.e. not officially published by Canon marketing) I believe the 40D thus 50D AF sensor chips to contain areas used to compensate for CA offsets (wavelength offsets). In total 4 sensor pairs are used with one sensor in the pair more sensitive to "normal" wavelengthes and the other sensitive to red and near IR. These sensor areas were taken out on the 7D AF chip to make room for the additonal AF points. Instead of having only 4 of these areas/zones covering the AF chip, the 7D uses the 63 zones of the AE sensor, which results in an resolution and hence CA compensation at AF point level.

So in my book the CA/wavelength compensation is not new, it was just dramatically improved now allowing it to cater AF wise for very localized illumination (small colored edges etc..).
One interesting aspect of the "new" 7D AF system is however also how much of its improved responsiveness and capabilities originate in the fact that it has a
dedicated
AF processor - something the 5DmkII lacks for instance
In the end what counts is not the amount of claimed new features, but rather that they work as advertised in real life..
~ hans ~