Autofocus : Sony

Autofocus

The name DYNAX has long been synonymous with advanced auto focus (AF) systems. In this section, we look at the AF technologies used in the DYNAX-series of single-lens reflex cameras. Though these technologies were introduced in the DYNAX more than two decades ago, updated versions continue to be employed today by most manufacturers of SLR AF cameras.

TTL Phase-Detection System

Of the two AF systems used in the industry during the early days of autofocus development--the TTL (through the lens) phase detection and contrast systems--TTL phase detection was considered superior in terms of AF speed and accuracy.

DYNAX engineers greatly improved TTL phase detection and designed an innovative AF sensor to further increase its speed and accuracy. Upon incorporating the new system into the camera body, Minolta was able to launch the world's first true and easy-to-use AF SLR camera--the 7000.

Subsequently, competing camera makers adopted the same TTL phase-detection system for themselves and today the technology can be found in most SLR cameras.

Focus Priority AF

With focus priority AF selected on the 7000 and subsequent models, the user holds the shutter release all the way down and the camera fires as soon as the correct focus is achieved. This solved the problem of missing critical shots due to the time it takes the user to make sure the focus is correct and then to release the shutter button, by which time a moving subject could be out of focus.

Using this function, a mother, for instance, can take well-focused shots of her children even as they play around or ride on tricycles.

AF Illuminator

To improve AF performance in low lighting conditions, the 7000 incorporated an AF illuminator. It works by momentarily beaming a randomly distributed striped pattern onto the subject, briefly illuminating it and helping the AF to identify and measure the subject accurately. A more powerful version has been incorporated into external flash units.

Moving-Subject Detection Technology

Moving-Subject Detection technology debuted in the DYNAX 7000i. It continuously analyzes the AF data after focus has been achieved and informs the AF control if the subject is stationary or moving--and, if moving, at what speed and in what direction. Predictive focus control calculates where the subject will be when the shutter is released. This ensures that even a fast moving subject remains sharply in focus--even when shutter-lag time adds between several tens to hundreds of milliseconds to the shooting process.

It works by employing a sensor to detect the moving subject and to continually measure the subject's direction and speed. This enables it to both predict where the subject will be in the time it takes for the shutter to be released and to compensate for the subject's movement during that time.

When introduced in the DYNAX 7000i it greatly extended the camera's range in shooting fast-moving subjects. In fact, it was developed with the goal of being able to capture a full-frame shot of the world's fastest runner with the camera held vertically. For increased convenience, Automatic focus-mode selection, which chooses between single-shot AF and continuous AF modes, was also included. Single-shot AF mode is used to lock in the focus when the subject is stationary; continuous AF mode is used to keep a moving subject in focus.

Multi-Point and Auto-Tracking AF

The purpose of this technology was to enable AF to "Focus wherever the subject is in the frame." DYNAX engineers expanded the AF coverage area by increasing the number of sensors from just one targeting the center of the composition, to three. Yet simply increasing the number of sensors was not enough; also required was a new technology that would "know which area of the viewfinder the photographer wanted to use to focus on a subject." DYNAX was the first to incorporate this technology and was able to further increase the number of sensors to seven then eleven. As the number of sensors multiplied, additional technology able to track a moving subject became necessary. So DYNAX engineers developed Auto-tracking AF technology that continually kept track of a moving subject as it moved into range of one AF sensor from another, and then decided which sensor was best suited to focus on the subject.

Eye-Start AF System

Ideally, an AF system should work like the photographer's eye and focus in on the subject the moment he looks through the viewfinder. This is the guiding concept behind the Eye-Start AF system. When the photographer looks through the viewfinder, an eyepiece sensor adjacent to it signals the Eye-Start System to activate the AF, so that it is ready to focus immediately on whatever subject the photographer will target. To work exactly as intended, however, the system has to be finely calibrated. Should it activate too early, the AF could focus on the wrong subject; if it is slow to activate, then the technology fails in its goal. The Eye-Start AF system, introduced first in the DYNAX 7xi, was developed after exhaustive and repetitive testing of users' actual manner of shooting.

Availability of a Reflex Telephoto Lens

This compact, reflex ultra-telephoto lens is unique to the DYNAX-system series. In designing it, DYNAX engineers faced an additional major technology challenge: how to provide sufficient luminous flux for the AF system to work effectively? The answer came through achieving a tighter focusing of the light from the primary mirror, which enabled the use of a smaller sub-mirror. This enabled the AF to obtain the necessary light to work efficiently. In addition, the engineers also overcame the problem of color aberration that is inherent in ultra-telephoto design. No other camera manufacturer has been able to produce a reflex ultra-telephoto lens that can work with an AF system.

DYNAX Technology Autofocus