Predictive Focus Tracking System
Autofocus is a function that automatically focuses the camera lens on a subject when the shutter release button is at the half-pressed position. By significantly shortening the procedure normally required for focusing, an autofocus function allows camera users to capture clear images at an opportune time with the desired image composition.
The mechanisms of autofocus functions found on cameras can be roughly classified into two categories.
Most single-lens reflex cameras use an autofocus method called the "phase detection system." Using a separator lens in autofocus module, this system produces two images from the image information of the subject captured through the lens. It then measures the distance between those two images using a line sensor, and detects defocus amount. If the focal plane is in front of the camera lens relative to the image surface (front focus), the distance between the two images will be narrower than the optimum value. If the focal point is behind the subject (rear focus), the distance between the two images will be wider than the optimum value (Fig. 1). In other words, if the distance between the two images is narrower than the corresponding distance in an in-focus condition, the autofocus system determines that the focal point is in front of the subject. If the distance is wider than the reference value, the system determines that the focal point is behind the subject. Then, the autofocus system computes instantaneously how much the lens position should be moved and in which direction, and moves the lens accordingly. This focusing operation is extremely fast.
The autofocus of compact cameras, on the other hand, uses a mechanism called the "contrast detection system." Based on the principle that "in focus = highest contrast," this system analyzes the image information of the subject obtained by an image sensor. Then, by moving the lens, this system seeks the lens position where the image contrast is highest (Fig. 2). Since the system does not know before moving the lens whether the focal point is in front of or behind the subject, it calculates the contrast value while moving the lens, and determines the in-focus point based on the locus of changes. Consequently, this method takes a longer time to achieve focusing than the phase detection system, but one of its advantages is that the image sensor can be used for the autofocus function.
Autofocus comes in most handy when trying to focus on a moving subject while following its movement through the viewfinder. If the subject is stationary, it is easy to manually focus on it by viewing the image through the viewfinder or on the LCD monitor. However, focusing accurately on a moving subject and pressing the shutter at the desired time requires a learned skill.
An operation mode called "Continuous-servo AF (AF-C)" maintains continuous focus on a moving subject, but this is not sufficient for taking a sharply focused picture. This is because there is a short time lag between when the shutter is pressed and when the picture is actually taken, which is referred to as the release time lag. To solve this problem, the "predictive focus tracking system" uses special algorithms to forecast the position of the subject at the moment the image is captured based on measurement of the subject's movement, and moves the lens accordingly. Simply put, the predictive focus tracking system detects the subject’s speed of motion and adjusts the focus by taking the release time lag into consideration. Nikon's AF function performs accurate autofocusing operation under any situation by controlling the predictive focus tracking system based on the locus of the subject’s motion. Nikon used extensive data obtained by photographing a large number of moving subjects for the development of the predictive focus tracking system.
To realize high-precision AF for high-speed continuous shooting of a fast-moving subject, it is also important to improve the processing speed of the AF cycle, which consists of a series of internal operations comprised of focus detection, computation, and lens driving. For this, Nikon has adopted a proprietary technology called "Overlap Servo." In a conventional system, the AF cycle of "focus detection to computation to lens driving" is repeated in sequence until focus is attained. In Nikon’s Overlap Servo, on the other hand, the focus detection to prepare for the next shot is performed while the lens driving operation is in process, thus shortening the AF cycle time (Fig. 3). As a result, it can focus on a fast-moving subject quickly and accurately.
The predictive tracking system is very effective for taking pictures of a subject moving at constant speed toward the camera. It is capable of maintaining accurate focus even on a high-speed subject such as an F1 racing car traveling in excess of 300km/h.
However, simple predictive tracking AF does not provide maximum focusing performance for a subject that abruptly changes direction moving at high speed or a subject with low contrast moving randomly. To focus accurately on such a subject, the AF system must accumulate data of the locus of the subject’s movement using multiple focus areas so that appropriate judgment can be made.
The "Multi-CAM3500FX Autofocus Module" built into the Nikon D3 camera has as many as 51 focus areas. This system boasts an advanced AF function. For example, in one setting it selects the most appropriate AF area by taking the photographer’s intention into consideration. There is also a mode which combines adjacent focus areas into groups and automatically selects the focus area from the focus area group in which the subject is positioned.
Moreover, the 15 areas at the center can recognize both vertical and horizontal movement patterns regardless of the type of lens used. Nikon’s superb AF technology has achieved a wider defocus detection range so that the distance to the subject is measured accurately no matter the position of the focal point. In defocus detection, how accurately the focal deviation can be detected in an out-of-focus condition is critical. If the detection range is narrow, after moving the lens, the system must detect the amount of deviation before focusing, thus requiring a longer focusing time.
Nikon’s AF technology continues to advance, now with a simple function that allows users to easily and effectively focus on moving subjects.
Renewed March 2008