September 2011

The Mechanisms behind Fast, Comfortable Autofocus

Photo 1: Layout of the EOS-1D Mark IV AF sensor

Photo 1: Layout of the EOS-1D Mark IV AF sensor

Autofocus (AF) is an essential function in modern photography. This month’s technical report discusses the mechanisms behind two AF methods employed in digital SLRs, phase detection and contrast detection.

1. Phase detection

Figure 1: Basic principle behind phase-detection AF

Figure 1: Basic principle behind phase-detection AF

The phase detection AF method works by dividing the incoming subject image that passes through the photographic lens into two images and then detecting the difference in the focus point position between the two images. The original image is split into two images with two secondary microlens arrays in the AF sensor unit. Two line sensors measure each focus point.

When the plane of focus is in front of the subject (i.e., closer to the camera), the subject is said to be in front focus. In this case, the two images are closer together on the sensor than when in the correct focus position.

Conversely, when the plane of focus is in behind of the subject (i.e., farther away from the camera), the subject is said to be in back focus. In this case, the two images are farther apart on the sensor than when in the correct focus position.

Simply put, the autofocus unit calculates the direction to move the lens based on a reference distance between the two images when in perfect focus: i.e., if the distance is narrower than the reference, the subject is in front focus; if the distance is wider than the reference, the subject is in back focus. The autofocus unit calculates the amount to move the lens based on the amount of difference measured at the line sensors. After calculating these two parameters, it drives the photographic lens to the correct position (Figure 1).

Because the camera can immediately figure out the direction and amount to move the lens, phase-detection AF can focus very quickly.

Different focus accuracies at f/2.8 and f/5.6

As explained above, the basic principle behind the phase-detection AF mainly used in digital SLRs is that the AF unit detects and calculates the phase difference between the subject images formed on the left and right AF sensors and then outputs the amount the image is out of focus.

But given the same amount of lens movement to achieve focus, using f/2.8 light rays results in a larger change in the formed image’s position on the sensor than if f/5.6 light rays were used. Consequently, more accurate focus detection is possible using f/2.8 light rays (figures 2 and 3).

Figure 2: Comparison of principles for f/2.8 and f/5.6 light rays

Figure 2: Comparison of principles for f/2.8 and f/5.6 light rays

Figure 3: Principle behind cross-type AF sensors

Figure 3: Principle behind cross-type AF sensors

Multipoint AF and cross-type AF points (Figure 4) (EOS-1D Mark IV)
(1) Lens maximum aperture of f/2.8 (or greater)

In this case, 39 of the 45 AF points can be manually selected to function as high-precision cross-type AF points (simultaneously horizontal-line and vertical-line sensitive). The other six AF points function as horizontal-line sensitive points.

With automatic selection, the cross-type AF points are reduced from 39 to 16. The remaining 26 AF points function as horizontal-line sensitive points.

(2) Lens maximum aperture of f/4

The center AF point functions as a high-precision cross-type AF point. The remaining 44 AF points function as horizontal-line sensitive points.

(3) Lens maximum aperture of f/5.6 or f/8

With an f/5.6 lens, all AF points function as horizontal-line sensitive points. With an f/8 lens, the center AF point functions as a horizontal-line sensitive point. AF cannot function with lenses with an f-number higher than f/8.

Figure 4: Layout of AF points on the EOS-1D Mark IV AF

Figure 4: Layout of AF points on the EOS-1D Mark IV AF

2.Contrast detection

Contrast-detection AF is the method commonly employed on most video cameras, TV cameras, and compact digital cameras. Because the contrast is highest when the image is in correct focus, the camera analyzes the contrast information from the image on the imaging sensor and moves the lens to the position that gives the maximum contrast value.

The drawback with contrast-detection AF is that the lens must traverse the full focusing range because the peak contrast location cannot be found without moving the lens and because there is no way of knowing initially in which direction the peak contrast lies. Consequently, contrast detection requires more time to reach focus than phase detection (Figure 5).

The benefit of this method, however, is that digital cameras, by necessity, have an imaging sensor for taking photos and, therefore, no additional AF sensor unit is necessary. Contrast-detection AF is also used on SLRs for autofocusing during Live View shooting and movie recording.

Figure 5: Basic principle behind contrast-detection AF

Figure 5: Basic principle behind contrast-detection AF

Aiming to respond in the deciding instant and to broaden the creative possibilities of photography

AF Technology

Canon’s AF technology has continued to evolve in the pursuit of faster, more comfortable operation. We asked Mr. Tomokazu Yoshida from the Camera Development Center at Image Communication Products Operations to give us a detailed explanation of Canon’s development aims and evolution history, the mechanisms behind the functions, and future directions in the area of AF technology.

*1 Cross-type AF Points

Cross-type AF points exhibit excellent capture performance regardless of the subject’s pattern because they have a horizontal-line detection sensor and a vertical-line detection sensor arranged in a cross pattern. And cross-type AF points sensitive to f/2.8 light rays can focus with even higher precision. Of the 45 AF points on the EOS-1D Mark IV, 39 are cross-type AF points sensitive to f/2.8 and f/5.6 light rays (see Figure 4 for the layout). Of the 15 AF points on the EOS 5D Mark II, the center AF point is a cross-type AF point sensitive to f/2.8 and f/5.6 light rays. All 19 AF points on the EOS 7D are cross-type AF points sensitive to f/5.6 light rays and the center AF point is a dual cross-type point sensitive also to f/2.8 light rays. All nine AF points on the EOS 60D are cross-type AF points sensitive to f/5.6 light rays and the center AF point is a dual cross-type point sensitive to f/2.8 light rays. And of the nine AF points on the EOS Kiss X5, the center AF point is a cross-type point sensitive to f/2.8 light rays.

EOS 5D Mark II

EOS 5D Mark II

EOS 7D

EOS 7D

EOS 60D

EOS 60D

EOS Kiss X5

EOS Kiss X5

*2 AF Sensor

The AF sensor is the heart of AF and determines the AF’s ability to detect subjects and focus precisely. On the EOS-1D Mark IV, the AF sensor reliably detects subjects and focuses with high precision even in low-light / low-contrast situations that were difficult for previous AF sensors.

AF sensor on the EOS-1D Mark IV

AF sensor on the EOS-1D Mark IV

*3 AI Servo AF (AI Servo AF II)

This is an AF function that predicts where the subject will be in the next instant. It does this by finding the correlation between speed and distance of motion based on past focusing data and then calculating a correction amount to offset the focusing error. The AI Servo AF II, with a newly developed algorithm, closely checks any focusing results that seem suspicious based on data obtained from actual shooting conditions encountered by professionals. Because the function begins predictive control from the first focusing results immediately after the subject begins moving, it can track movement instantly.

*4 AF Customization

The easiest settings for ordinary shooting conditions are chosen as default settings. High-end models, such as the EOS-1D Mark IV and the 1Ds Mark III, have several custom functions to handle special shooting intentions or unusual shooting conditions. Custom functions such as AI Servo tracking sensitivity, AI Servo AF tracking method, AF expansion with selected point, and selectable AF points give flexible control over the AF function.

Canon’s AF technology: aiming for faster and more comfortable shooting through multipoint AF, cross-type AF points, and faster continuous shooting speeds

Our AF development has proceeded over the years based on the motto “faster and more comfortable” while always being aware of the need to focus instantly on the desired subject. The very first AF systems used only one center AF point to focus, but over time we have developed multipoint AF, increasing the number of AF points from three points to five, seven, nine, 19, and even 45 points. Our pro models now have as many as 45 AF points, and we are continuing to add more AF points to our entry-level models.

Another feature of our products is cross-type AF points (*1). Cross-type AF points consist of a horizontal-line detection sensor and a vertical-line detection sensor arranged in a cross for a given AF point on the AF sensor (*2). Because of their shape, cross-type AF points are able to focus on many more kinds of subjects. In the future, we want to employ cross-type AF points at as many AF points as possible.

One more characteristic function we can mention is our Predictive AI Servo AF (*3). One of the biggest needs of SLR users was AF tracking of randomly moving subjects. We put all our effort into developing this function. Naturally, what photographers were looking for was not just AF but also fast frame rates. Today, our EOS-1D Mark IV can shoot at continuous speeds as fast as 10 fps.

Enabling photographers to clearly observe and shoot moving subjects through the viewfinder has been both the aim and history of our development of multipoint AF, cross-type AF points, and high continuous shooting speeds.

f/2.8 light rays for greater precision, f/5.6 light rays for a wider detection range

Our digital SLRs use two types of light rays, at f/2.8 and at f/5.6, to autofocus. As described above, using f/2.8 light rays allows for more accurate focus detection. But this does not mean that we can make do with f/2.8 light rays alone.

Using f/2.8 light rays is advantageous for two reasons. Because the aperture is larger and optically brighter, it is possible to detect the focus in darker conditions. And because the base line used in the triangulation principle is longer, more accurate focus measurements are possible. The disadvantage is that it may take some time to search and find the focus when the subject is well out of focus because the detection range at f/2.8 is narrow.

The advantage of f/5.6 light rays is that autofocus is implementable with almost all lenses. Furthermore, because the distance range the sensor can detect is wider, it is easier to lock on the subject, whether at point-blank range or off in the distance.

Using these two types of light rays together greatly increases the AF capture performance. For example, even when a bright f/2.8 lens is attached, f/5.6 light rays can first be used for detection if the defocus range is prohibitively large and then f/2.8 light rays can be used to fine-tune the focus.

Given that Canon supplies all kinds of EF and EF-S lenses with different maximum apertures and focal lengths for different applications, one of the most attractive aspects of the EOS SLRs is the AF implementation that selectively uses these two types of light rays.

Enabling more comfortable AF by tailoring the AF configuration for each camera model

Users of high-end SLR models tend to use more kinds of composition in their photography. To give more compositional freedom, the AF sensor layouts on high-end models include cross-type AF points all the way to the edge. And on the EOS-1D Mark IV, 39 of the 45 AF points are vertical-line sensitive at f/2.8. We considered this advantageous because high-end SLR users tend to use large aperture lenses.

On the entry-level EOS Kiss X5**, the center point included an f/2.8 horizontal-line sensitive point along with a cross-type point that is both horizontal-line and vertical-line sensitive at f/5.6. This was believed to be an optimum configuration in consideration of the camera and lens combinations EOS Kiss users would likely use (Kiss users are more likely to use f/5.6 lenses) and their style of shooting.

**EOS Kiss X5 [Japan], EOS Rebel T3i [Americas], EOS 600D [Europe, Asia, Oceania]

We use enormous amounts of historical data to provide fast, comfortable focusing on subjects that move irregularly

Predictive AI Servo AF is a useful function for shooting moving subjects. This function looks at multiple records of past focusing data to predict where the subject will be in the next instant. This is necessary because, with phase-detection AF, there is a time lag between when the shutter is released and when the exposure is made because the main mirror and sub-mirror must be raised. It is possible that the subject moves in the instant the photo is taken when it is impossible to gauge the focus distance. Because the depth of field on digital SLRs is shallower than on compact digitals, the amount the subject may move during this time lag cannot be ignored. The problem then is how to provide fast, comfortable focusing while predicting the subject’s movement during the time lag given all the types of possible shooting conditions, all the types of possible subjects, and all the types of motion. This is the hardest part of predicting motion, but Canon has an algorithm that optimizes focusing on many kinds of subjects using data that we have accumulated over many years. This is both one of the features of EOS SLRs and one of our assets.

Another aspect to remember is that the objective and style of shooting varies between camera classes. On the EOS-1D Mark IV and other high-end cameras favored by professional photographers, settings can be customized to suit the user’s preferences (*4). On the other hand, most EOS Kiss series users want to be able to use the camera straight out of the box without bothering with detailed settings. Therefore, these cameras are set for general users. Similarly, different predictive functions are provided for each class.

Problems with definitely improving performance and developing the dream AF technology

I feel that user needs are quickly becoming more sophisticated especially with respect to predictive AF. The increasing resolution of image sensors has made it possible to image data at larger sizes than ever before. Because of this, we have to provide AF functionality that can deliver images that are perfectly in focus even when viewed at the actual pixel size. But considering the influence of temperature conditions, it is extremely difficult technically to focus perfectly 100 percent of the time. Nevertheless, we want to get as close to 100 percent as possible. The issue we must tackle next is developing AF that accurately focuses in keeping with the user’s intentions regardless of how difficult the shot is.

We are also conscious of improving AF performance for movie recording. This is not easy because the demands for motion in still images and movies are diametrically opposed: where users want crisp focus in still images, they want fluid focus in movies. Still, it is a challenge for us to determine how to improve shooting functions for both applications.

Even as innovations like 45-point Area AF and cross-type AF points have led to more compositional freedom and broadened the possibilities of photography, we are still devoted to developing in the near future AF technology that will focus where you want it just by thought alone without any physical operations. This “dream AF” will astound you all.