FOCUSING MECHANISM

Manual Focus (MF) - A manual focus lens such as the old Canon FD lens on the right, is completely under the control of the user and focus is achieved by turning a focusing ring which is found within the lens barrel. The focusing ring moves particular lens elements accordingly to adjust the focus. Some cameras may also employ a motor-driven system to the focusing mechanism of the lens that can be controlled by pressing focusing buttons or switches. With this mode, the user has to rely on the viewfinder to determine whether the focus is on the intended subject.

In some cameras a focusing screen with precisely arranged microprisms is placed in the viewfinder to help the user make a better assessment on the focus. If the subject is in proper focus the image in the center of the focusing screen will align making the subject very clear. Other recent alternatives to help in manual focusing is the inclusion of a focus confirmation circuit within the lens that provides the user with an audio or visual indication (or both) whenever proper focus is achieved within the specified spot. Some modern cameras also provide a graph indicating 'focus peaking' which indicates the focus status within a selected area of the screen.

Auto Focus (AF) - A lens with an autofocus capability uses an electric motor to drive the focusing mechanism.

Some of the older camera systems used a system called 'screwdrive' where the motor is inside the camera body and is linked to the lens through a shaft in the lens mount. This system makes the lenses cheaper and lighter but is now becoming obsolete because it is generally noisy, imprecise, and tends to be too slow when used with large lenses.

Most AF lenses today have internal motors which could be any of the following types:
DC Motor - a simple small DC motor which tends to be cheap and quieter than the screwdrive, but has low torque and a bit imprecise.
Ultrasonic/Piezo Motor - uses piezo technology making it silent, but is not fast, has low torque, and tends to break easily.
Ring Type Ultrasonic Motor - also uses piezo technology but the motor is a large ring around the lens barrel. This makes it quiet, quicker, more torque, more precise, and tends to be more durable.
DC Stepper Motor (Pulse Linear Motor) - uses pulses to control the rotation of the motor. It is fast, silent, and very smooth which is ideal for video, but has low torque making it useful only for small lenses. It also requires the camera to be powered up even if you want to focus manually.

The focusing process is controlled by the camera using various methods depending on the manufacturer's design. By itself, the focusing system of the lens is not capable of acquiring proper focus, it only receives and implements whatever the camera body instructs it to do. Autofocus lenses usually comes with a switch to disable the Autofocus allowing the user to manually calibrate the focusing whenever there are situations when the autofocus fails or becomes unreliable, such as low contrast subjects or insufficient lighting.

Full Time Manual Focus (FTM) - Whenever autofocus is engaged in a lens, the focusing mechanism becomes mechanically connected to the focusing motor. At this point, depending on the manufacturer, the manual focusing ring is either disengaged or is still linked and is just left to turn along with the autofocus activity. Leaving it linked is quite dangerous if the user instinctively turns the focusing ring. Doing this would force the gears that connects to the motor to turn and possibly break. Lenses with Full Time Manual Focus system uses a special gear system that allows the manual focusing ring to remain useable during autofocus. With this system the autofocus link is temporarily disengaged when the focusing ring is turned and re-engages when the autofocus motor turns.