Full-time manual focus is an EOS system feature that has existed since the earliest film EOS cameras. The idea is that once the camera has focused on a subject you might want to slightly move the focus yourself.
What is full-time manual focus?
The idea of full-time manual focus comes from the early days of EOS cameras – film EOS – when cameras had extremely limited numbers of AF points. You could focus with the central AF point in ONE SHOT AF, and then just slightly move the focus for the actual subject, by turning the lens focus ring. Full-time manual focus can be used in many situations but is especially helpful for close-up and macro work.
The challenge being that the lens needs to be focused by the camera, and then be available to be focused manually without grinding the gears or forcing the focus motor. Canon invented USM focus motors for fast and silent AF. Right from the beginning of EOS cameras, lenses with USM motors were built for full-time manual focus.
Not all lenses had USM focus motors, some early models used a simpler less advanced arc from drive (AFD) motor. With these lenses full-time MF wasn’t possible, requiring you had to move the lens switch to MF to be able to manually focus the lens. Other lenses with DC motors, usually with no motor description in the model name are also unable to do full-time MF.
How to use full-time MF
Full-time MF is simple to use, just focus in ONE SHOT AF mode, and once AF has completed keep your finger on the shutter button and turn the focus ring on the lens.
You cannot use AI SERVO AF with full-time MF as the camera is constantly adjusting focus as the subject moves. If you are using AI focus – please stop!
Special lenses with electronic manual focus
Like all good rules, the proof that they are rules is by the existence of exceptions to the rule. Canon has exceptions, lenses with USM motors that allow you to choose if full-time manual focus is possible or not. EF 50mm f/1.0 USM, EF 85mm f/1.2L USM, EF 85mm f/1.2L II USM, EF 200mm f/1.8L USM, EF 300mm f/2.8L USM, EF 400mm f/2.8L USM, EF 400mm f/2.8L II USM, EF 500mm f/4L USM, EF 600mm f/4L USM, EF 1200mm f/5.6L USM and EF 28-90mm f/2.8-4L USM have USM motors and electronic manual focus.
These lenses have electronic manual focus rings, that use a turn of the focus ring to tell the USM motor to drive the lens; effectively a fly-by-wire focus system. Other USM lenses you turn the focus ring and it is directly connected to the focusing mechanism in the lens. More recently the STM motor equipped lenses are also categorised as electronic manual focus lenses. The EF 50mm f/1.8 STM lens is equipped with electronic manual focus, so it may change operation from one camera to another model.
For such lenses there was a custom function introduced to some cameras to allow the photographer to determine if full-time manual focus was available. It was enabled by default, and appears to be enabled on an EOS 80D, even though there’s no custom function to disable it. The screenshot above is from an EOS 5DS R showing full-time MF enabled. However EOS 250D disables full-time MF by default and has a setting to enable it. You will need to check your camera for the setting and to discover the default.
EOS R mirrorless cameras
Full-time MF is not enabled by default on the EOS R mirrorless cameras, but a custom function exists to enable it. All the RF lenses so far are “fly-by-wire” electronic manual focus lenses.
Interesting. I have a Canon EOS R and tend to use manual focus all of the time.