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Autofocus
In the old'n days (some might call them the good ole days!), you focused a camera using a dial on the lens. Now if you use a DSLR camera, you can still do this - usually by first flicking a little switch on the lens from 'AF' to 'MF'. And there are times when you might want to do this ... we'll come to them later. But first a little background information.
What is focus?
When you point a camera at something, millions of light rays from any point on that object enter the camera through the lens. For that object to appear sharp (i.e. not blurred), those light rays must be made to converge to a point on the sensor. When this happens, the camera is focused on that subject. If they do not all converge at the same point, then they end up forming a blurred circle called a 'circle of confusion' and the camera is not focussed on the subject.
The tip of the arrow is in focus on the sensor as all the light rays converge to a single point on the sensor.
In this image, the rays of light from the arrow are brought into focus in front of the sensor, so at the sensor itself the tip of the arrow would appear as a blurred disc.
In a camera lens, there are usually quite a few separate glass elements that make up the compound lens. Moving some of this allows you to change the distance in front of the lens that appears in focus. On cheaper lenses, it is often the front element of the lens that moves to achieve focus, although some lenses achieve it by moving internal glass elements.
Autofocus
Autofocus is simply a system whereby the camera is able to decide when a subject is in focus, and can control a motor to move the focusing glass elements instead of the photographer having to do it.
Autofocus first appeared on a commercial camera in the late 1970s, although it only started to gain popularity in the early 80s when Pentax closely followed by Nikon brought out autofocus SLR cameras. In the early Minolta and Nikon cameras, the autofocus sensors and the motor were part of the camera body, however Canon decided to develop a completely new system for their autofocus cameras (called EOS) which put the motor into the lens itself. Nikon's recent consumer models have also done away with the focus motor in the camera body.
The early AF systems used an active autofocus system whereby it directed something at the subject (an ultrasonic soundwave or infrared beams) to determine the distance to the subject.
Passive Autofocus
Modern digital cameras use passive autofocus where nothing is directed towards the subject (although some cameras to have autofocus assist lights for low light conditions). These work in one of two ways:
Contrast based systems
These are used on nearly all compact / point-and-shoot cameras. It looks at the image coming straight from the sensor - specifically the contrast of the image around the location where focus is being achieved. The contrast in the image is greatest when the subject is in focus so by moving the focusing lens elements the camera is able to see when the contrast is at a maximum.
This system is usually quite slow as it needs to move the focus beyond the point of correct focus until the contrast starts to diminish - only then does it know that it has passed the point of maximum contrast and so has to return back to the focus point.
The advantage is that this system is cheap to implement as it requires no special AF sensors in the camera - it is using the data straight from the image sensor and runs it through a software algorithm on the image processing chip which in turn controls the lens motors.
As more and more DSLR cameras start to include live view, contrast based AF is starting to appear as an alternative AF method on DSLRs.
Phase based systems
The incoming light is divided into a pair of images and these are compared. Two optical prisms capture the rays of light from the opposite sides of the lens and divert it to the autofocus sensor. By analysing the phase difference of the two images, it is possible to work out whether the subject is nearer or further away than the distance the lens is current focused on. The phase difference also gives a fairly accurate indication of how far the lens focus needs to be moved. This is why a DSLR which uses phase-AF is able to focus so much faster than a compact camera that used contrast-AF.
Auto Focus Sensors
On a camera that uses a contrast based autofocus system (such as most compact cameras) there are no dedicated AF sensors. The camera is able to focus on any point in the image as it is using data from the main image sensor.
However on a DSLR which uses a phase based autofocus system, specific AF sensors are built into the camera (they usually sit in the base of the camera with light reflected down to them by a secondary mirror sitting behind the main reflex mirror).
Generally the more upmarket the camera, the more autofocus points will be available. For example Nikon's D40, D40x and D60 only have 3 autofocus points, whereas their professional D3 camera has 51 autofocus points. A camera can only use the information from one AF sensor to focus - which one is used can either be manually selected or left to the camera to decide.
AF Point Selection
Manual selection involves the photographer selecting one of the focus points and the camera simply uses the data from this sensor to lock focus on the subject.
Automatic selection may vary depending on the camera. If the camera only contains a small number of sensors, it will look at the data from all AF sensors as soon as the shutter button is half pressed. It then selects the one which is over the nearest point in the scene - this becomes the active AF point for that shot.
Cameras with many focus points often use sophisticated algorithms to decide on the active point. For example, it may give priority to points higher up in the frame even though they may not necessarily be the nearest point to the camera.
Linear / Cross-type AF Sensors
Most AF sensors are one dimensional (i.e. a strip with a height of only a few pixels and a width of 20-50 pixels). This makes focusing on something with strong vertical lines very easy, but can cause problems for a subject with horizontal lines. Therefore most DSLRs incorporate at lease one cross-type sensor which has a horizontal and a vertical row of pixels.
In consumer models, usually only the centre focus point is cross-type, however on the semi-pro and pro models, many more of the sensors are usually cross-type sensors.
Speed of Autofocus
The speed at which a camera is able to focus not only depends on the type of autofocus system used, but also on the speed of the focusing motors in the lens. On a DSLR camera, the sensor is usually able to determine the focus distance almost instantaneously, however it then needs to move the lens to that position using the motors in the lens or the camera)
The speed at which the motor can move the lens to the correct focus position and the motors ability to stop immediately when focus is achieved governs the overall focusing speed.
Most lens manufacturers offer two different types of motors in their lens - basic mechanical motors and more advanced motor based on ultrasonic vibrations - the latter being much faster and considerably quieter while focusing. Different manufacturers call these faster motors by different names (which if part of a particular lens will be included in the lens description).
- Canon - Ultrasonic Motor (USM)
- Nikon - Silent Wave Motor (AF-S)
- Minolta, Sony - Super Sonic Motor (SSM)
- Pentax - Supersonic Drive Motor (SDM)
- Sigma - Hyper Sonic Motor (HSM)
- Olympus - Supersonic Wave Drive (SWD)
- Panasonic - Extra Silent Motor (XSM)
Sometimes the Camera needs Help
The lower the light - the more difficult it can be for a camera to accurately lock focus on the subject. Obviously the more light you can provide to the AF sensor, the better it will be able to focus. That is why many cameras have AF sensors that work better when larger aperture lenses. For example, many of Canon's EOS range of DSLRs have a centre AF sensor that offers additional precision when focusing with f/2.8 or faster lenses.
In very low light, even a large aperture lens may not be able to focus accurately - therefore many cameras are equipped with an autofocus assist system. Sometimes this will use the flash gun to fire pulses of light which the camera can then use to lock onto focus. This is not ideal, as if you are taking pictures of people, the pulses of light from the flash can be off-putting. Other systems may use a steady light source shone onto the subject, or even a grid of lines to give the camera something visible to lock onto when focusing on smooth even coloured surfaces. Look in the user manual of your camera to see what system (if any) your camera has.
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