Good time to review autofocus tolerance specs

Started Feb 15, 2008 | Discussions thread
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Good time to review autofocus tolerance specs

There have been enough recent postings about testing autofocus accuracy, being dissatisfied with the results and sending in the camera for calibration. While this is all, of course, fine if you wish to go through the trouble and get a satisfactory response from Canon, I think it may be time to level-set expectations of what Canon is really promising when it sells you a camera. You may not like the spec and want to calibrate more tightly than this manufacturing tolerance spec. However, it's still important to realize that Canon is not necessarily selling you the Brooklyn Bridge by handing you a camera that's not dead-on perfect.

I think the best article for reviewing this is Doug Kerr's excellent article on depth of focus linked here:

http://doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf

For those finding this article a bit too technical, the executive summary is:

1. Your camera is designed to focus on a plane parallel to the sensor.

2. Any point in the subject plane will be focused onto the sensor as a point if focus is perfect but as a small circle if the focus is off. This "blur circle" is also called the circle of confusion.

3. The larger the circle of confusion diameter, the more the subject is out of focus. For diameters below a certain value, we tolerate the misfocus. Above a certain value, we start to perceive things as unacceptably sharp.

4. Based roughly on an 8" x 10" print viewed at a normal distance, this maximum acceptable circle of confusion diameter is about 0.035mm for full-frame and 0.020mm for APS-C. This is about 0.1% of the horizontal dimension in each case. Individuals may have different personal definitions of when misfocus is noticed, but this is a defined number in order to measure DoF and AF accuracy against.

5. Depth of field is defined by the nearest and further planes in front of and in back of the subject plane that project onto the sensor with circle of confusion diameters equal to the maximum tolerable value. Similar depth of focus is defined as how far forward or backward you can move the sensor from the position where the subject is projected onto it with perfect focus such that the subject's circle of confusion reaches the maximum tolerable value.

6. For the standard-precision, f/5.6 autofocus sensors, Canon defines acceptable performance as resulting in the subject having a circle of confusion within the maximum tolerable value of 0.035mm for FF and 0.020mm for APS-C. That is, if you focus on a minute dot but it images as a 0.020mm disc rather than a precise point (ignoring diffraction for the moment) on your 40D's sensor, that's at the limit of acceptable performance. As mentioned earlier, this is about 0.1% of the horizontal dimension.

7. For the high-precision f/2.8 sensors, accuracy is about 3x better. That is, that pinpoint dot on the subject will project to a disc no larger than 1/3 of the max. CoC diameter. That is, no more than 0.035/3 = 0.012mm for FF or 0.020/3 = 0.007mm for APS-C. Again, this is about 1/3 of 0.1% of the horizontal dimension.

That's the gist of it. The exact numbers of 0.035mm and 0.020mm may be wrong since these are extracted from older Canon material and they may have changed their numbers without notice. The specific claim that the high-precision sensor is 3x more accurate may also be wrong, as hard data on this dates back to the 20D implementation. That may also be an obsolete spec. However, whatever the actual numbers, the principles described here still apply.

David

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