Basic Learning IV

Focussing

In BasicLearning I, we had explained how light enters the camera through the lens. However, the angle of light entering the lens depends on a number of factors, like the subject size, subject distance, intensity of light etc. Since light can enter the lens at different angles, it is necessary that the quality of light exiting the lens and registering on the sensor be properly adjusted. This process of adjusting is known as focussing.

To achieve focus manually, in most D-SLRs and mirrorless cameras, there is a focussing ring on the lens. This will move the lens elements backward or forward so that the light reaching the sensor is correctly oriented. This is much more easily achieved in the case of a fixed focal length lens like, say, a 50mm lens. In the case of zoom lenses and specially telephoto zooms, the movement required is considerably more. The illustration below indicates how focussing takes place.

The closer the subject, the further is the point at which light converges on the sensor. Hence, the focussing ring will bring the lens closer to the subject. The reverse will happen for a distant subject.

In the case of SLR cameras, a mirror projects the image on to the ground glass which can be used to see whether the image is in perfect focus or not. In most cases, the subject is in the centre of the image, and therefore a split image screen on the glass at the centre is also used to check whether focussing is achieved. In the case of rangefinder cameras, the distance is measured by a coupled parallax unit on top of the camera, allowing the focussing to be set with accuracy.

With the advent of autofocus technology, the guessing went out of focussing, and camera manufacturers used proprietary technology by which the image was focussed automatically by sliding the elements forward or backward according to the need.

An autofocus camera uses a motor and a sensor to automatically lock focus. The motor may be installed in the lens, as in Canon EF lenses, or in the camera body, as was the case with Minolta’s Maxxum range.

Today’s cameras offer a variety of choices in terms of focussing points.

This can vary from 3 to as many as 24. Again, a focussing sensor can be a cross sensor which means it will take care of both horizontal and vertical sensitivities. Depending on the image to be shot, the focussing points that you want can be selected and others switched off. In the case of a moving image, the point can be kept in the active mode whereby it will track the moving subject and keep it in focus.

For the image to come out right, it is obviously necessary to control the amount of light that hits the sensor. If we let in too much light, the image will be washed out, and if we let in too little light, the image will look too dark. Most autofocus systems need an aperture of at least f/ 5.6 to work reliably. In order to get the right amount of light in, as well as gain an acceptable image, modern cameras link the focussing points to the exposure system, thereby resulting in a near- perfect picture.

Although autofocus was patented by Leica, the first really visible system came from Minolta (7000) and Canon ( EOS 650).

Autofocus systems can be grouped into active and passive. Active AF systems use ultrasonic sound waves and infrared light to measure distance and achieve correct focus.

Passive autofocus is achieved by phase detection or contrast measurement. D-SLRs use the first method and mirrorless compact cameras the second.

H. S. Billimoria