Autofocus
From Freepedia
Autofocus (or AF) is a feature of some optical systems that allow them to maintain correct focus on a target.
Contents |
General
Simple AF systems rely on a single focus sensor. Advanced systems consist of an array of focus sensors. The Canon EOS 3 has as many as 45 sensors, each accessible with their unique eye-control, to focus the camera on the photographer's subject. Most modern SLR cameras use data from the AF system to assist in the light measuring.
The speed and accuracy of autofocusing can be very good; normally the performance is better than could be achieved manually. Modern cameras measure several areas of the picture and decide where the subject is. Some cameras are able to decide if the subject is moving towards or away from the camera, what speed it has, at what rate it accelerates, and can follow the subject as it crosses the image.
Different AF systems
There are two different approaches to systems for automatic focusing:
- active systems
- passive systems
Active systems
These systems usually work in two steps:
- distance measuring (separated from the optical system)
- focus adjustment to the optical system
There are several ways to measure distance: sound-waves (some Polaroid cameras), IR light (some compact cameras, early video camcorders). Sound-waves and IR light are emitted from the camera, and by measuring the delay in the reflection of the sound- or light-waves, distance can be calculated.
Focus adjustment is mainly controlled electronically. The data collected from the distance measuring are calculated in an electronic unit which activates a mechanical or electromechanical system to adjust the focus accordingly.
An exception to the two-step approach is the mechanical autofocus provided in some enlargers, which adjust the lens directly.
Passive systems
Passive systems don't rely on any energy (in form of electromagnetic rays or soundwaves) to be sent from the camera to the subject. Instead, these systems utilize the light that is naturally reflected by the subject. Autofocusing can be achieved in two ways:
- phase detection
- contrast measurement
Phase detection is achieved by dividing the incoming light into pairs of images and comparing them. This system is often utilized in SLR cameras. Contrast measurement is achieved by determining when maximum focus is adjusted to achieve the maximum contrast within the sensor field. This is a common method in video-cameras and digital-image cameras, and is measured through the actual optics in use. This way, passive autofocusing may not involve actual distance measuring at all.
Pros and cons of the systems
Active systems will typically not focus through windows, since sound waves and IR light are reflected by the glass. With passive systems this will generally not be a problem, unless the window is stained.
Passive systems may not find focus when the contrast is low, notably on large single-coloured surfaces (walls, blue sky, etc) or in low-light conditions. Passive systems are dependent on a certain degree of natural illumination to the subject, while active systems may focus correctly even in total darkness.
