Basic Learning V
Autofocus
In Basic Learning IV, we had explained how focussing is achieved by moving lens elements forwards and backwards until the sharpest image is projected on to the sensor. Autofocus uses a miniaturized motor that focusses the lens for you. Companies like Canon, Nikon and Panasonic install this motor in the lens, whereas Olympus and Sony (and Nikon in some models) incorporate it in the camera body. Since the advent of commercially viable autofocus cameras in the mid 80’s by the likes of Minolta and Canon, autofocus technology has advanced considerably. In the early days, the main system to use for autofocussing was the active autofocus system. This system used sound waves or an infra-red signal to estimate the distance to the subject and achieve autofocus. However, both these technologies had their limitations, and in most cases, were effective only within a finite distance. They were also not very effective when the subject was very bright, or when shooting through glass. Conversely, active autofocus systems worked well in the dark, thus making flash photography easier. Active systems are used mainly in compact cameras. The most commonly used system today is the passive autofocus system. Most D-SLRs use this system which determines the distance to the subject by computer analysis and drives the lens back or forth to achieve the best focus. It does this with the help of an AF sensor which is actually a CCD. This CCD measures the difference in intensity (phase detection) between different elements of the subject to achieve focus. Under the contrast detection system, commonly used by mirrorless cameras, light and image contrast is used to lock in to the subject. Thus, if you try to focus on a black wall, contrast measurement will be very difficult, and therefore autofocus will not possible. Passive autofocus systems react to both vertical and horizontal detail, depending on the orientation of their AF sensors. Modern cameras, therefore, have both cross-type and vertical-line sensors to make focussing quicker, more precise and easier. The number of autofocus sensors used by a camera varies widely, and can go up to 45 or more in the case of highend D-SLRs.. Autofocus systems are now linked to exposure systems as well in order to enable the camera to get the best possible picture. Once achieved, autofocus can also be locked. Focus lock works by depressing the shutter button half-way and holding it till you recompose the picture. Although autofocus systems are highly sophisticated, a complicated subject with either poor light or very bright light can fool the autofocus system. In such cases, manual focus can be used to fine-tune focussing and achieve better sharpness. Autofocus can be in ‘One Shot’ mode or ‘Continuous’ mode. The latter is necessary for moving subjects since the ‘one shot’ mode cannot anticipate subject motion. Canon and Nikon have also developed a ‘one shot’ or AI Servo mode where the focussing system predicts where the subject will be in the future and focusses at this distance. This definitely increases the chances of focussing correctly with moving subjects.