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Canon AF System
It is worth understanding how the Canon AF system works if only to avoid various problems that can arise due to poor usage and technique causing AF inaccuracy.
There are a number of useful links to information which are reproduce here:
Autofocus - Wikipedia
mkoehler.de - Canon Autofocus Facts
fredmiranda.com - Canon autofocus information
Canon's latest application for a US patent on their autofocus system Go to www.pat2pdf.org and enter patent # 6,603,929
Canon's pamphlet on getting the most from EOS DSLRs EOS_Digital.pdf (No longer Available for Download)
Distilling this down to the most important points and adding itallic comments for emphasis:
Each AF sensor consists of a pair of short lines of pixels forming an array. One array comprises the outer sensors. Two crossed arrays (one vertical, one horizontal) comprise the center sensor. With lenses or f2.8 or faster, the camera activates a second vertical array in the center.
The arrays are sensitive to linear details that run perpendicular to the orientation of the array. Therefore, the horizontal arrays (identified by the horizontal rectangle marks on the viewscreen) are sensitive to vetical linear details; the vertical arrays (identified by the vertical rectangle marks on the viewscreen) are sensitive to horizontal linear details.
They are blind to linear details that run parallel to the array direction. The center array, being a crossed combination of a vertical and a horizontal array, is sensitive to linear details running both vertically and horizontally. When the second vertical array is activated, it's combined input increases the accuracy by a factor of three.
The pixel arrays are actually three times longer than indicated by the viewfinder markings. This is to cover the fact that the viewscreen has a significant amount of "slop" in its horizontal-plane positioning. Therefore, the sensors actually see details that are somewhat outside the viewfinder markings, and may focus on them instead of details within the sensor markings, if those outside details are more perpendicular to the array than the details inside the markings.
The upshot of this is the problem of trying to AF on relatively small details that do not full cover the AF point such and/or have more than one depth. A user could put an intended subject in the mark, but if there is a strong detail just outside the mark (but within the sensor area), the camera would focus on that strong detail.
Examples are trying to focus on a small bird in a tree and ironically some of the less thoughtfully design AF accuracy tests such as those with a slanting distance scale or using round batteries.
The resultant effect can run from pulling the AF off subtly to causing the AF system to "capture" targets at different depths, hunting from one to the other.
This issue is probably responsible for more reports of soft lenses and AF problems than anything else.
When you half-press the shutter release (or the * button, if
you've used the custom function to move focusing control there), the activated
AF sensor "looks" at the image projected by the lens from two
different directions (each line of pixels in the array looks from the opposite
direction of the other) and identifies the phase difference of the light from
each direction. In one "look," it calculates the distance and
direction the lens must be moved to cancel the phase differences. It then
commands the lens to move the appropriate distance and direction and stops. It
does not "hunt" for a best focus, nor does it take a second look after
the lens has moved (it is an "open loop" system).
If the starting point is so far out of focus that the sensor can't identify a phase difference, the camera racks the lens once forward and once backward to find a detectable difference. If it can't find a detectable difference during that motion, it stops.
Although the camera does not take a "second look" to see if the intended focus has been achieved, the lens does take a "second look" to ensure it has moved the direction and distance commanded by the camera (it is a "closed loop" system). This second look corrects for any slippage or backlash in the lens mechanism, and can often be detected as a small "correction" movement at the end of the longer initial movements.
So this suggests the One-Shot AF mode focus point estimation is an open loop system but the lens position control is a closed loop system with at least a second iteration.
When the camera determines how far and in what direction the lens must move to cancel the phase difference, it does so within a tolerance of "within the depth of focus" of lenses slower than f2.8 (down to f5.6) or "within 1/3 of the depth of focus" of lenses f2.8 and faster. The depth of focus is the range at the sensor plane within which the image of a point will be reproduced as a blur smaller than the manufacturer's designated "circle of confusion" (CoC). Canon's designated circle of confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C format. The CoC is based on maintaining the appearance of sharpness in a 6x9 inch print at about an 10 inch viewing distance (as revealed by the Euro-Canon web site). There is no guarantee that images enlarged any greater than this will appear sharp.
As a result of this tolerance (within the depth of focus or
within 1/3 of the depth of focus), the camera can place the actual plane of
focus at random anywhere within the tolerance range, and not necessarily at the
same place each time.
The depth of focus is the sensor side conjugate of the subject side depth of field, so neglecting pupilary magnifications issues the depth of focus and depth of field at a given distance can be considered to both have the same constraints.
Note that having said this some lenses show more AF than others, this is probably due to mechanical and servo control loop variations in lens design. In general with good light and contrast levels the AF system does seem to be fairly accurate, however it is important to remember that it will normally vary significantly from the ideal plane of sharp focus.
Also the above description does not mention light levels and contrast levels, clearly when some combination of these two factors falls below required levels, probably a signal-to-noise ratio, the AF system will stop work reliably or all together. Considering the system is phase difference driven there is probably a threshold effect. Below the threshold the system does not work at all, just hunting, just above it probably works with reduced accuracy but rapidly improves to its design specification; this is rather speculative but is deduced from a knowledge of other phase derived ranging and direction finding systems.
One Shot: When you set the camera to "One Shot," you set the condition "The subject is definitely not moving." The camera is in a "focus priority" mode. The shutter release is locked until the camera achieves what it thinks is the proper focus. This is best if your subject and the camera will be motionless, because it allows you to focus and change the framing without the camera refocusing automatically.
AI Servo: When you put the camera into AI Servo mode, you have set the condition "The subject is definitely moving." The camera is in a "shutter priority" mode. Therefore, the camera goes into a routine that continually collects data to predict the subject movement and move the lens to intercept the subject at its new position. You can shoot even if out of focus (however, the camera cannot release the shutter if the lens is actually in motion). If you know your subject will be in constant motion, this is the best mode. If the subject is actually not moving, the chance of a misfocused shot increases as the camera goes through its data-collection routine. However, often a handheld camera does move (as the photographer sways naturally) for AI Focus to detect and correct for the sway. AI Servo will use whichever focus point you have activated. However, if you activate all the focus points, you must put the center point on the subject and half-press the shutter release for about half a second for the camera to "acquire" the right subject. After that, while you hold the shutter release, the camera can intelligently "hand off" the subject focus from point to point as the subject "wanders" over the viewscreen.
AI Focus: The camera is normally in One Shot mode and the shutter will lock until it achieves focus. However, if it detects the subject moving (that is, the subject goes out of focus), it will automatically switch into AI Servo mode and try to maintain focus. If you are focusing on something that frequently stays still but could move suddenly (like a toddler) this mode comes in handy. The important point wiht AI Focus is that it does not lock the shutter. However, the camera will usually interpret "focus and recompose" as movement of the subject, and will refocus.
Note that using the AI Servo mode with a static subject effectively provides a iterative AF control, the evidence of this is visible by observing the lens range indication converge on the correct focus distance, particularly when a non-report teleconverter or stacked converters are used.
For testing it is important to eliminate as many variables as possible. So the target used here is designed to be parallel to the camera sensor to eliminate any ambiguity regarding target range and has high contrast in both vertical and horizontal directions.
Tests should be set-up carefully with the camera and target mount level to ensure the required alignment. Ideally a tripod and mirror lockup should be used with either a cable release or timer mode to reduce vibration.
The target, instructions and a calculator are available for download Home Grown Focus Test Chart and Calculator V1.12.
Last Updated 06/08/2008
All Content © 2005-13 Lester Wareham All Rights
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