I have learnt that fly-by-wire focusing is an electrical manual focusing system. But how does this focusing method work?
- How do you manually focus with fly-by-wire?
- What is the scope of this focusing method?
- Which cameras have this feature?
by Linus Kleen
Function: Normally the focus ring is mechanically connected to the inside of the lens so that when you turn the focus ring some of the lenses inside the lens move directly. With focus-by-wire the movement of the focus ring is measured electronically, and then a motor moves the lenses inside.
Drawbacks: As focussing is now only possible when the lens has power, it is impossible to focus, when the camera is switched off. And for macro work special extension tubes are needed, which have electrical coupling to allow for power and communication between the lens and camera. Also with macro reverse adaptors (and unpowered extension tubes) the only way to focus is by changing the distance between camera and subject.
Lenses which support focus-by-wire:
Focus-by-wire is named after fly-by-wire, a system used in airliners where the flight controls are no longer mechanically connected to the control surfaces (flaps) but instead are connected to sensors which in turn activate servos to move the wings.
Focus by wire works in the same way, the focus ring has sensors which in turn activate the autofocus motors in the lens.
Using a focus-by-wire system is exactly the same as using manual focus, turning the ring back and forth in order to achieve good focus. There may be a slight lag between moving the ring and the lens responding.
Focus-by-wire systems are usually used when there isn't space inside the lens for a mechanical focus linking. For this reason all non-interchangeable lens cameras use focus by wire.
For interchangeable lens cameras, it's a feature of the lens not camera body* with smaller lenses such as those used by mirrorless cameras frequently using focus-by-wire. In DSLRs focus-by-wire is usually reserved for lenses with complicated focus arrangements such as the Canon 85 f/1.2L
The advantages of focus-by-wire, other than compactness, are that it allows the manufacturer to easily control the relationship between focus ring and lens movement so long "throws" are possible.
The disadvantages are that some people don't like the feel and latency of an electronic system, it requires power to focus the lens manually, and it can cause problems when using lenses with different systems (see below).
*the camera body has to supply power to the lens to enable focus-by-wire, and in some cases it must send certain signals also. When using lenses on foreign bodies via an adaptor the adaptor must be able to send these signals.