Idaho Airships, Inc.

Most of us use Canon, so most of us use Canon’s autofocus. There are some things you should know about Canon’s autofocus…

I’m avoiding the technique questions-you’ll need to read your camera’s manual to develop your own Back-Button AF, AF Point selection and AF mode strategies. They vary by camera and application.

Canon’s autofocus was not designed to achieve perfect focus. According to their Patent Application any print larger than 6×9 is not “guaranteed” to be sharp when viewed from a distance of “about 10 inches.” This involves the calculation of something called a “Circle of Confusion” which you might wish to research on your own, and obviously presumes normalized human vision.

In short, Canon’s autofocus is designed to meet a surprisingly modest goal.

Obviously the focus is achieved with the lens at maximum aperture, and if the image is acquired stopped down there will be some Depth of Field advantage in the exposure configuration. This is yet another advantage-in addition to optical performance-to shooting stopped down.

I can’t speak to differences in autofocus processing between various camera models, however, it is reasonable to expect that prosumer cameras do not offer the same autofocus performance as their 1-Series stablemates. You get what you pay for, with all respect to generational improvements to various models.

One advantage in shooting aerials is that you will likely be approximating focus at infinity, where the Depth of Field metrics are compressed and less prone to significant error.

I shoot with the Canon 1DsMIII, and the following comments apply only to that camera.

According to the camera manual (mine is well worn like a family Bible…is yours?), there are 19 high-precision (“cross type”) autofocus sensors among the 45 total sensors. A note of special interest to gear-geeks: The sensors are actually located in the bottom of the Mirror Box and are approximately three times the size of the viewfinder marks!

The high precision sensors are sensitive to both horizontal and vertical lines, while the remainder of the “assist” sensors are sensitive only to horizontal lines. This is very important to keep in mind if you are focusing using a lens that uses few or no high precision sensors. High precision sensors are about twice as sensitive to vertical lines as they are to horizontal, and all other sensors can only respond to horizontal lines. A judicious composition when setting autofocus can exploit that bias. If you have access to high precision sensors, prefer vertical detail with them, while if you are “stuck” with other sensors ensure that you are providing them some horizontal detail.

Please re-read and re-read that last paragraph until you understand it fully. It is critical. It may be worth tattooing on your forearm.

Lenses with maximum apertures of f2.8 allow the camera to use all 19 high precision sensors. In low light or other situations that are hostile to autofocus, that’s a big deal. Lenses with a maximum aperture of f4.0 use only the center focus sensor in its “high precision” mode, and use the other 44 sensors in their “horizontal line” only mode. Lenses with a maximum aperture of f5.6 use all sensors in their “horizontal-only” mode, and lenses with a maximum aperture of f8 use only the center sensor point, and that with horizontal sensitivity only.

Thus, while the 24-105 f4 L does offer image stabilization, it does not harness all of the camera’s high precision autofocus sensors. Your decision to use it instead of a 24-70 f2.8 L for a low-light shoot must be made weighing this and a variety of other factors.

Since we’re talking about image sharpness some mention should be made of a terribly misunderstood feature: Canon’s IS. There…I mentioned it, and will cover it in depth in a subsequent post.

There are a number of other factors that influence image sharpness. Perhaps none will surprise you more than the fact that images are blurred during capture-Canon’s response to moire’ is to blur the image a bit before it reaches the sensor, and then reconstruct it’s virtue during demosaicing. Canon also blurs detail at the point of resolution extinction.

Another example of the perils of digital processing is presented by Norman Koren (whom I have met and dined with-it was a great experience). Norman authored the GamutVision application which I cannot recommend highly enough. On his website, Norman has demonstrated how a poorly constructed output .icc profile can destroy image sharpness.

There are many more hardware and procedural influences on image sharpness, but I trust that the information above will go the majority of the distance in clearing up common misconceptions and perhaps contribute to improved production practices within PAPA.