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Re: Educate Me | Autofocus

For those interested in how phase type autofocus conceptually works, I suggest reading this: doug.kerr.home.att.net/pumpkin/Split_Prism.pdf

Once that is understood take a look at this picture:

This is the AF sensor chip of the 400D (and 30D)

Schematically the line sensors of the AF chip, their orientation and their sensitivity are shown here:

The orientation and sensitivity of the corresponding AF points shown here:

In summary:
9 AF points:
6 Vertical F5.6
2 Horizontal F5.6
1 vertical (zigzag) F5.6 and 1 horizontal F5.6 or F2.8 combined to yield a "+" type sensor

OK, let's look at the 450D's AF sensor chip:

which compared to the 400 AF chip has a very similar general AF line sensor layout (plus a couple of color sensitive areas). Hence the 400D and 450D have the same number of and the AF point configuration.

In order to get a F2.8 capable cross AF point, the (baseline)distance between the the two parts of the sensor pair needs to be about the double of a corresponding F5.6 point. To avoid having to increase the size of the AF sensor (read: keep cost low) the 2 pairs used for a F2.8 capable cross AF point, are instead rotated by 45 deg. which among other things yield a 1.4 x longer baseline. This design has until now been deployed in the 40D, the 50D and the upcoming 7D will use it as well, but has not yet been ported to the xxxD series.

The details for the 1D mk lll AF can be studied in the associated white papers (google for "Canon white paper")

Hope that helped

- hans -

Sep 10, 2009 at 05:57 PM

  Previous versions of hpjfromdk's message #7514448 « Educate Me | Autofocus »