How Canon's AI Servo AF Works
John Galt
The following document is information I received from Canon regarding
the
details of how Canon's AI Servo AF systems work. This is Canon
copyrighted material that's not to be reposted without Chuck Westfall's
permission. Also, please note the following from Mr. Westfall:
You have my permission to release the contents of my document as long as
it is credited to me, with the reminder that it was written about the
original EOS-1, not the EOS-1V. Let me re-emphasize that although
certain details have changed over time, the basic principles of Canon's
AI Servo AF system
as they are described in this document remain the same with current
models. Specifically, in a continuous AI Servo AF sequence, the first
exposure is always handled as 'release priority,' while subsequent
exposures in that sequence are handled with 'focus priority' according
to Canon's
definition of that term. In other words, with current EOS cameras, the
issue is not IF
the shutter will be released in a continuous sequence, but WHEN.
Best Regards,
Chuck Westfall
Manager/Technical Information Dept.
Camera Division/Canon U.S.A., Inc.
AI Servo AF with Focus Prediction,
As Used in the Canon EOS 1
1. System Concept
A. Functional Definition
AI Servo AF is an autofocusing system designed exclusively for Canon EOS
cameras. It uses a form of Artificial Intelligence (AI) to determine the
speed and direction of moving subjects, then focuses the camera lens to
a predicted position (Focus Prediction Function) in order to increase
the probability of obtaining a sharp photograph.
B. Control Mechanism
AI Servo AF in the EOS 1 is controlled by a software program that is
built into the camera's exclusive AF Central Processing Unit (CPU). Like
other software programs, the AI Servo AF program follows a set of
"rules." These rules govern the behavior of the EOS 1 when confronted by
a wide variety of (1) subject matter, (2) shooting conditions, (3)
camera settings, and (4) body/booster/lens combinations. Although AI
Servo AF is designed to be simple to use, it must, by necessity, operate
according to the user's selection of these parameters.
2. Subject Recognition
A. AF System Type
The performance of any automatic focusing system depends primarily on
its ability to recognize the subject to be photographed. The EOS 1 uses
a CT-TTL-SIR ("Cross-Type, Through-The-Lens, Secondary Image
Registration") phase detection mechanism for this purpose.
Cross-Type refers to the shape of the focusing area. Unlike other
focusing sensors that detect focusing information in only one dimension,
the EOS 1's Cross-Type sensor can read focusing information in both
horizontal and vertical sections of the viewfinder image. (However, the
vertical component of the EOS 1's AF sensor is only available for use
with Canon EF Lenses that have maximum apertures of f2.8 or greater.)
This feature enhances the camera's ability to recognize difficult AF
subjects, by increasing the area of the image to be evaluated and by
enabling horizontal subject contrast to be detected.
Secondary Image Registration means that the autofocusing system uses a
"secondary image" which is created by an auxiliary optical system that
diverts part of the light beam entering the lens to a point inside the
camera that is geometrically equivalent to the film plane. This system
enables the viewfinder image to remain uniformly bright, while at the
same time, passing sufficient information to the camera's AF detection
system.
Phase Detection means that the AF system obtains its information by
dividing the incoming light into pairs of images that can be compared to
each other. This sort of AF system is classified as a "passive" type,
because it has no moving parts and emits no energy toward the subject.
Instead, it analyzes the image presented to it.
B. Readable Subjects
1. Light Level
An essential requirement for a phase detection AF system is an adequate
volume of light. This area of system performance is measured according
to the Exposure Value (EV) system, an internationally recognized
standard that relates the amount of light in any given environment to a
standard film speed, or sensitivity rating. As a matter of convention,
ISO 100 film speed is normally used as a benchmark for the purpose of
calibrating EV numbers. The EOS 1 AF system has been tested and is
guaranteed to operate successfully down to a light level of EV -1
(bright area luminance). This means that the exposure level for ISO 100
film under such a condition would require camera settings equivalent to
8 seconds at f2.0, a very dark light level indeed. The high end
sensitivity limit for the EOS 1's AF system is EV 18. The correct
exposure for ISO 100 film at this light level would require a camera
setting equivalent to approximately 1/800 second at f16. In reality,
this level of brightness is rarely, if ever encountered.
2. Subject Position
A second requirement for a phase detection AF system is that the subject
must be positioned within the analyzed area of the picture. In the case
of the EOS 1, this position is located in the center of the picture
area. The camera's focusing screen shows this area by means of a set of
engraved brackets, called the AF frame. Subjects that are outside of the
AF frame cannot be evaluated.
3. Subject Contrast
A third requirement for phase detection AF is a sufficient level of
subject contrast. Although as mentioned above, the EOS 1's Cross-Type AF
Sensor is better than conventional horizontal AF systems in terms of
contrast detection capability, it still requires some subject contrast
in order to operate. The required contrast level varies somewhat
depending on the overall light level, with slightly more contrast
required as light levels drop.
Additionally, the contrast level available to the AF system varies
according to the lens type and lighting conditions. For example,
pointing the camera towards a bright light source such as the sun or a
spotlight can create flare. This flare can reduce contrast to the point
of making the subject unreadable for the purposes of AF detection. Flare
control in Canon EF lenses is developed to a very high degree, but is
impossible to eliminate for every condition.
4. Camera Shake and Subject Motion
When either the camera, the subject, or both are moving, these movements
add variables to the process of correct AF detection. The EOS 1 AF
system has been designed to take these conditions into account.
a. Up to a point, camera movement is factored out of the AF calculation
process by the software's ability to recognize that the subject is
relatively stationary and that only the camera is moving. However, if
the camera movement is severe enough, the EOS 1's ability to recognize
the subject may be impaired. For example, if the photographer were
attempting to shoot a near subject from an unstable position such as a
moving sports car, the camera movement may become too excessive.
b. When the subject is moving, it presents a situation that must be
handled differently. In such a case, the software system must evaluate
if the subject movement is steady or steadily accelerating or
decelerating, or if the subject movement is erratic, or if there is any
other unusual characteristic. The method of autofocus control can and
does vary according to these and other factors. In all cases, the EOS 1
uses a form of "sequence control" to handle the possibilities.
3. Sequence Control in the EOS 1 AI Servo AF System
The EOS 1 AI Servo AF system performs best with subjects that pass at
least the minimum requirements for light level, subject position,
subject contrast, camera shake, and subject motion, as outlined above.
However, the process of photographing a moving subject goes far beyond
simple AF detection. Once the photographer presses the shutter release,
a complex chain of events takes place. First, the lens is driven (if
necessary) to the correct focusing position. Next, the camera's reflex
mirror begins to rise. Then, the lens diaphragm is actuated to a
predetermined setting, the shutter opens and closes, and finally the
system is reset for the next exposure. This process is called Sequence
Control.
Sequence Control requires the full integration of all EOS 1 systems,
including AF, AE, viewfinder and LCD panel display, film transport, lens
drive, shutter/mirror charging, self-checking, and others. To a certain
extent, all motor-driven 35mm SLR cameras share the same sequence
control characteristics. However, the way in which the AI Servo AF
function is overlaid on the mechanical functions of the camera is unique
to the EOS 1. This uniqueness is most clearly seen in the responses of
the camera to various shooting conditions. In the following description,
we will outline the possibilities:
A. Single Frame Shooting
Single frame shooting, or the first frame of a continuous sequence, is
handled the same way no matter what the conditions are: i.e., regardless
of whether the EOS 1 body is used alone or with a booster, whether the
subject is readable or not, with any EF lens type, etc.
In this case, focusing begins when Switch-1 (SW-1) is turned ON. This
condition occurs most normally when the shutter release button is
partially depressed. Focusing can also be activated by the EOS 1's AE
lock button when Custom Function 4 (CF4) is ON, by partially depressing
the switch of Remote Switch 60T3, or by using the Wireless Controller
LC-2 Set. For the purposes of discussion, we will simply refer to the
focusing being either ON or OFF.
The photograph is taken when Switch-2 (SW-2) is ON. This occurs when the
shutter release button is fully depressed, or through use of the EOS 1
selftimer, the selftimer/intervalometer function of Command Back E1, or
various remote control devices.
At this instant, lens drive immediately stops, the mirror is released,
the lens diaphragm is stopped down as required, and the exposure begins.
Because the photographer (rather than the autofocusing system) controls
the timing of the shutter release, Canon calls this shooting method
release priority. The release priority method enables the photographer
to shoot when desired, but it does not guarantee sharp focus. In fact,
the first frame of a continuous sequence shot in AI Servo AF is likely
to be out of focus, especially with a subject moving either toward or
away from the EOS 1.
B. Second and Consecutive Frames in Continuous Sequence
In continuous sequence AI Servo AF shooting, shutter release timing for
the second and consecutive frames in the sequence is controlled by the
EOS 1's autofocusing system. Canon calls this focus priority. However,
it is extremely important to note that focus priority shutter release
timing does not guarantee sharp focus. It simply means that the EOS 1
camera, rather than the photographer, controls the shutter release
timing for these exposures. Here's how it works:
Condition 1. Readable Stationary Subject: In this case, the focus is
set once and remains the same for all consecutive exposures. For this
reason, high framing rates (frames per second shooting speeds) are
usually achieved. Focus Prediction is not required, and is therefore not
used.
Condition 2. Totally Unreadable Subject: A good example of this type
of subject is clear sky or any blank, smooth surface. As long as this
condition remains unchanged, the EOS 1 will continue shooting at the
highest possible framing rate. If you try to autofocus this type of
subject before shooting, the lens will go through one search cycle
(current lens position to minimum focus, then back to infinity), then
lock at infinity and display a rapidly blinking focusing indicator in
the viewfinder display. You can shoot as many exposures as you wish
under this condition. However, the viewfinder display is extinguished
during the shooting sequence.
Condition 3. Low Contrast Subject: Many different subjects present
this condition, especially human subjects when the focusing frame is
positioned on smooth skin or non-patterned clothing. If the contrast
level remains too low to read, the camera reverts to condition 2 as
outlined above. If, on the other hand, the contrast level becomes
readable, then the EOS 1 selects the most appropriate response from the
following options:
Condition 4. Adequate Subject with Steady Movement: This is the ideal
subject type for the Focus Prediction Function of the EOS 1's AI Servo
AF mode. When the subject speed reaches a certain level, the EOS 1
identifies it as a moving subject and begins to use its Focus Prediction
capability. In this scenario, the 3 most recent AF measurements are fed
through a proprietary algorithm (formula) to determine subject
direction, speed, and acceleration or deceleration rate. This
information, added to the AF time lag and the release time lag
information, results in a calculated focusing position which is input
from the camera body to the lens microprocessor through the contacts in
the EOS electronic lens mount. Then, the lens is driven to the
calculated focusing position and the exposure is made. The scene is
re-evaluated before the next exposure, and if the subject movement
continues consistently in the same direction, the lens will be driven to
the newly calculated position and the next exposure will be made. This
process is repeated as long as necessary, subject to available battery
power and film capacity.
Sometimes, during the evaluation period between frames, the situation
may change suddenly. This can happen if another subject moves in front
of the original subject temporarily, or if the photographer is unable to
keep the AF frame centered on the original subject. In such a case, the
EOS 1 continues to shoot at the most recent "good" focusing position.
However, if the situation remains uncorrected for more than 0.5 second,
the camera focuses on the new subject if possible, or determines it to
be unreadable and then continues to shoot at the same focusing distance.
The EOS 1 viewfinder indicates that the subject has become unreadable by
means of the flashing AF indicator below the picture area, as described
in Condition 2.
There may be times when the photographer simply wishes to suspend
focusing but continue shooting at a fixed focusing position (sometimes
called AF Lock). The EOS 1 allows for this by the use of Custom Function
4, which makes AF operation independent from shutter release. In the
case described, the photographer keeps on shooting by maintaining finger
pressure on the shutter release, but suspends focusing at will by
releasing thumb pressure from the AE lock button.
Condition 5. Adequate Subject with Excessive Speed: There are
actually two conditions of this type. The first condition occurs with a
subject moving toward the camera, so close or so fast that the required
focusing movement cannot be executed within a preset time limit. For USM
lenses, the time limit has been set at 200 milliseconds (1/5 second).
For non-USM lenses, the time limit is 250 milliseconds (1/4 second). If
the subject requires more focusing time than the preset limit, the
exposure is taken anyway at a focusing position that is as accurate as
possible. However, this type of exposure will be out of focus towards
the infinity direction.
The second condition occurs with subjects that are simply moving too
fast for the AF system to track. In this case, the subject is for all
intents and purposes unreadable, so the camera reverts to Condition 2.
This situation can occur when the camera position remains stationary and
the subject is moving at a high rate of speed, such as a race car or a
downhill skier.
Condition 6. Adequate Subject with Irregular Movement: This is the
most difficult type of subject for the AI Servo AF system to deal with.
A very good example is the stop-and-start, forward-and-backward movement
typically seen with a runway-type fashion model. As long as the model is
walking towards or away from the camera, everything is fine. But if the
model starts moving irregularly, as in certain dance steps, focusing
accuracy may suffer.
If the subject simply changes direction, so that predictable movement
in one direction is simply shifted into predictable movement in another
direction, the Focus Prediction Function can usually adjust to the
change with little or no loss of accuracy. However, if the subject
movement becomes so irregular that it is no longer predictable, then the
EOS 1's Focus Prediction Function is cancelled. If the subject movement
remains unpredictable, then the lens is continually focused to the
subject's most recently detected position. In other words, the focusing
position will change if the subject distance changes, but the AI Servo
AF system will not execute a false prediction.
4. Conclusion
The EOS 1 AI Servo AF system was designed to function as the finest
central-area autofocusing system in the world for moving subjects. When
tested to its maximum performance limits, we believe that it clearly
outperforms other central-area systems, regardless of manufacturer. In
spite of this, Canon is constantly working to improve the performance of
EOS cameras, including even better autofocusing systems in future
models.
Canon U.S.A., Inc.
Camera Technical Dept.
Supplementary Technical Information
© 1992 All Rights Reserved
“AI Servo AF With the EOS 1”
Publication #C3-11/06/92
John Galt
Obakesan
interesting
In article <3956D8A1...@nospam.net>, John Galt <nos...@nospam.net>
wrote:
> Cross-Type refers to the shape of the focusing area. Unlike other
>focusing sensors that detect focusing information in only one dimension,
>the EOS 1's Cross-Type sensor can read focusing information in both
>horizontal and vertical sections of the viewfinder image. (However, the
>vertical component of the EOS 1's AF sensor is only available for use
>with Canon EF Lenses that have maximum apertures of f2.8 or greater.)
hmmm worth knowing ... counts out most of my AF lenses ...
>© 1992 All Rights Reserved
wonder how much (little) its changed since then?
--
See Ya
(when the bandwidth gets better ;-)
Chris Eastwood Please remove undies for reply
Photographer, Stunt Programmer WWW -> www.powerup.com.au/~cjeastwd/
Motorcyclist and dingbat
Stephen M. Dunn
$In article <3956D8A1...@nospam.net>, John Galt <nos...@nospam.net>
$wrote:
$> Cross-Type refers to the shape of the focusing area. Unlike other
$>focusing sensors that detect focusing information in only one dimension,
$>the EOS 1's Cross-Type sensor can read focusing information in both
$>horizontal and vertical sections of the viewfinder image. (However, the
$>vertical component of the EOS 1's AF sensor is only available for use
$>with Canon EF Lenses that have maximum apertures of f2.8 or greater.)
$
$hmmm worth knowing ... counts out most of my AF lenses ...
If you have an EOS-1 (or others that require faster lenses for
their cross-type sensors), yes.
The low-to-middle bodies don't have this restriction; their
cross-type sensors work with lenses as slow as f/5.6. It's only
the high-end bodies that require faster lenses for their
cross-type sensors. From memory, that's:
EOS-1 and EOS-1N: f/2.8
EOS-3 and EOS-1V: f/4 for the central sensor, f/2.8 for the others
The obvious question is why the high-end bodies have their
cross-type sensors crippled in this way, whereas the lower bodies
don't. AFAIK, the cross-type sensors on the high-end bodies are
much more sensitive than those on the lower bodies, so if you
_do_ have a fast lens, they'll grab focus more quickly and more
reliably. And let's face it - if you have a pro body, chances
are you have pro lenses, most of which are fast enough to permit
the cross-type sensors to operate.
--
Stephen M. Dunn <ste...@bokonon.ussinc.com>
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