AI Servo Information by Canon USA.

This document was originally posted to the Usenet newsgroup in June 2000, by John Galt. The page is hosted here by as a public service. The original post included the following remark by Chuck Westfall, Canon USA’s technical manager, who wrote this document.

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.

Note that I’ve reformatted the document for improved clarity over the original Usenet post, but none of the content has been altered in any way. Copyright 1992 Canon USA, 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