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One thing that I've noticed with Canon DSLR camera is they will disable or limit autofocus when the lens is below certain aperture values, which are specified in my answer to What are the aperture limits for autofocus on Canon DSLR cameras? Note that Nikon cameras are also typically limited to f/5.6 except for models featuring the latest revision of the Multi-CAM 3500 AF system as in the D4 and D800.

As far as I am aware, Olympus and Pentax do not have these limitations, even if autofocus may be slow or unreliable beyond f/5.6. (Sony/Minolta requires at least f/6.3, with one exception for a 500mm f/8 autofocus mirror lens, and the Alpha 850 and 900 have a center AF point with high precision at f/2.8.) For example, on Pentax, 9 of the 11 points are cross-type and remain so at all apertures. The Pentax AF system has been reported to work at f/8 and beyond, though extremely slowly. Olympus Four Thirds cameras will continue to attempt to focus below f/5.6 as well. Why do Canon and Nikon choose to limit AF functionality instead of attempting to focus with reduced performance beyond the above cutoffs?

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Just out of curiosity, is there a lens/body combination that you are attempting? – dpollitt Oct 8 '11 at 3:49
I don't have a Canon DSLR, but this is something that I don't find with Nikon, Olympus, or Pentax (the system I use). I also recognize that high-precision points truly require the specified apertures, but the fact that a certain minimum aperture is needed for AF to work at all on Canon raises this question. – DragonLord Oct 8 '11 at 4:01
As a fellow Pentax shooter, I can tell you that at narrow apertures, the camera hunts like crazy. You're better off, sometimes, just going manual. – John Cavan♦ Oct 8 '11 at 22:36
Post edited to reflect the fact that Nikon has these restrictions as well--but not Olympus or Pentax. – DragonLord Mar 31 '12 at 19:26

2 Answers

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The auto focus systems are not capable of using the highest precision cross type focusing points if you do not use a large aperture lens. They aren't artificially limiting the precision, they are simply working within the constraints of the maximum aperture.

Canon puts these limits in to ensure reliable AF. If you don't believe that theory, add a piece of tape between your teleconverter and lens, on the top three left pins. This will trick your body into thinking that you don't have a teleconverter attached, and your lens is simply a 300/4.0 or whatever you have.

Furthermore, only the Canon 1-series cameras are capable of AF with a f/8 lens, albeit at the center point only. Usually this setup comes from combining a long lens with a extender.

This really comes down to a issue of user experience. Sure, they could include ISO 204800 or 409600, but the quality would not make anyone happy. They are trying to keep the majority of users satisfied with what the camera can do, and most consumers are satisfied if AF doesn't hunt endlessly.

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From what I've read, the tape trick results in unreliable AF. The camera may cope in brighter conditions with high contrast, but will hunt and fail to acquire focus in low light or low contrast, depending on the camera body. – DragonLord Oct 8 '11 at 14:48
I agree 100%, its a workaround, not a complete solution. – dpollitt Oct 8 '11 at 14:51
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In any case, while every phase detection AF system has a hard limit (typically around f/11 AFAIK), the fact that Canon has these limitations that are set above these hard limits means that this answer most directly answers the question. – DragonLord Oct 8 '11 at 19:11
up vote 13 down vote

Aperture limitations are a consequence of the design of phase detect autofocus systems and are not specific to Canon.

Phase detect autofocus in SLRs works by directing light to the AF sensor using a second mirror behind the semi-silvered main mirror. The AF sensor uses a pair of lenses to focus light from the subject onto a pair of 1-dimensional sensors, effectively lines of pixels. By measuring the offset between contrast patterns measured on each sensor the distance to the subject can be calculated. See this crude diagram (not to scale!)

The size and separation between the pair of AF sensors determines the accuracy of the AF measurement. However there is a trade off - the further apart they are, the wider the aperture must be in order to direct light to reach both sensors. In the diagram I have included an aperture stop that represents, say f/4.0 (the lens is f/2.8 wide open). With the lens stopped down to f/4.0 incoming light would be blocked from reaching the AF sensors.

Modern AF units have multiple AF points, each composed of a pair sensors at different separations and orientations. The centre point often has an extra wide separation sensor pair and thus only works at maximum accuracy at f/2.8. Often AF points have pairs of 1D pixels at 90 degrees in order to detect either horizontal or vertical detail. One pixel row will have greater separation so the AF point only works as a "cross type" sensor when using a large aperture lens, otherwise either the vertical or horizontal sensor is disabled, leaving a regular single-orientation AF point.

The sensors may work a bit past their specified aperture, as the pattern of light might overlap the sensor just enough to get a match and calculate the distance, which is why taping the pins of a teleconverter sometimes works. But in general it's not designed to do this so to prevent inaccurate or intermittent performance, Canon disable AF when the reported aperture is too small.

Canon has optimised their AF units to be very accurate with wide aperture lenses (which Canon offers at even long focal lengths, for example the 400mm f/2.8 or 600mm f/4.0), rather than to be more flexible continuing to work but with less speed and accuracy at small apertures.

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To help understand this, I tried looking through the viewfinder of a manual focus film SLR with split prism focusing aid. I noticed that that opposite parts of each side of the split prism blackens as the lens stops down in depth-of-field preview, and that focusing with the split prism is no longer possible by f/16-22 - even with a bright flashlight aimed at the center of the lens. It seems this would explain why phase detection doesn't work past a certain aperture. Though it may still function to an extent beyond f/8, it certainly won't work at f/16. – DragonLord Oct 8 '11 at 18:52
Sweet answer! I like the conclusion on the tradeoff. Do you want high precision at larger apertures?! Then give up AF at smaller apertures! – dpollitt Oct 8 '11 at 21:55
... which is why allowing both requires additional AF sensors, one for faster lenses and one for slower lenses. This is the approach Canon takes in its AF systems. Nikon, Olympus, and Pentax don't take this approach, which may explain why the Pentax DA* 55mm lens shows variable accuracy on different camera bodies, as noted in the conclusion of dpreview's review of this lens. – DragonLord Oct 8 '11 at 22:39

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