Lenses That Focus By Wire

A quick-ish technical post about focus by wire, what it is and when you might not want it in a lens you are thinking of buying.

A camera lens is made up of a number of lenses, and to avoid confusion the individual lenses are called elements.

A lens may have as few as three and as many as fourteen elements. I may have missed a lens with even more elements. Modern lenses tend to have more elements because lens manufacturers are constantly trying to refine how well a lens can render a scene.

Camera sensors get better all the time, and lens manufacturers want to keep up.

That’s the intro, now to the details.

Lenses with a mechanical focus have a focus ring directly coupled to the lens elements. Turn the ring and the elements move backwards or forward to change the focus distance.

Focus by wire uses electronic signals to control focus. The photographer turns the focus ring but that doesn’t move the elements.

Instead, turning the focus ring controls the motor(s) built into the lens. The motor(s) take their instruction from the movement of the focus ring, and the motor changes the focus.

Focus by wire gets its name from fly-by-wire systems used in aircraft.

Fly-by-wire is a flight control system that replaces traditional mechanical linkages (such as cables, pulleys, and pushrods) between the pilot’s controls and the aircraft’s control surfaces (ailerons, rudder, elevator) with an electronic interface.

The pilot doesn’t move the control surfaces on the wings directly because it would be impossibly hard. Instead, the pilot presses a pedal or turns a dial and electronic motors move the control surfaces..

And because the aircraft is so big, any small errors are not that relevant.

In cameras it is different because small errors matter much more. This is true in still photography and in video.

And that has been the source of the criticism of focus by wire – that the systems are laggy and prone to overshoot.

The photographer turns the focus ring quickly, and the system plays catch-up.

When that happens, the photographer feels divorced from the focusing, which is the exact opposite of what he or she wants.

The pressure might be off in a studio or with landscapes, both of which are situations where the photographer has time to turn the focus ring slowly to focus.

But on the street or with any fast paced action, the photographer wants to feel an immediate, consistent response.

High-quality lenses by major manufacturers use powerful and accurate motors and you would never know whether they are mechanical or motorised.

But it’s something to watch out for – particularly with some kit zooms. You can feel it when you turn the focus ring quickly and the system can’t keep up and wheezes and whirrs and the clutch grabs and won’t let you turn the ring as quickly as you want.