Full-time manual focus differences

[Tijn]

Hai,

Not having touched/experimented with a very vast array of lenses yet, I'm a bit confused about "full-time manual focus". I know what it is, and what it's for; I understand its use... Still, there seem to be differences in it.

Most or all autofocus-able lenses (at least from Canon) have an auto focus / manual focus switch on the lens itself. 'Primitive' lenses (some cheap kit lenses among others) do not allow manual focussing at all as long as this switch is on "auto focus". This means that the focus ring will not turn while this switch is on "af".

Now as I understand it, for lenses with full-time manual focus, while in "af" mode and after focussing, you're able to adjust the focus by turning the ring.

The confusion that I have is what happens when I turn the focus ring on my 70-200 f/4L IS lens (my only L lens by the way), without autofocussing first. The focus changes perfectly. It's as if the lens is in manual mode; no need to focus first, you can always manually focus even without half-pressing the shutter, even with the camera off, while it's on "af" mode (even though that's useless because when half-pressing the shutter it'll first AF first again anyhow). But when I once "felt" a 135mm f/2L, the focus ring turned but did nothing unless you had autofocussed first.

So there seem to be at least two different "types" of full-time manual focus (perhaps more if counting micro USM seperately, but the two lenses mentioned above both have ring-type USM). One that will always make the focus ring focus, regardless of af/manual mode; and one that will allow manual adjustments only after focussing with AF. What is it that distinguishes these two? Is there a name for it, is it possible to see which lenses have the type of FTM that my lens has, and which have the type that the 135mm has?

[neuroanatomist]

But when I once "felt" a 135mm f/2L, the focus ring turned but did nothing unless you had autofocussed first.

I'm not sure what was going on with the 135L you tried, but my 135L behaves like all of my other FTM lenses - you turn the focus ring, and the focus elements move, regardless of whether an AF was done first or not.  I can say that the focus ring on the 135L turns much more smoothly than some other L lenses.

Are you sure you're not thinking of the 85L?  That lens has electronic manual focus ('focus-by-wire'), meaning turning the MF ring doesn't move gears, but rather activates actuators which drive the AF motor.  That means MF with the 85L only works with the lens connected to the camera and the camera powered on.

This means that the focus ring will not turn while this switch is on "af".

Not usually.  Setting the switch to AF on non-FTM lenses, the focus ring does turn, but doing so may damage the lens. 

...you can always manually focus even without half-pressing the shutter, even with the camera off, while it's on "af" mode (even though that's useless because when half-pressing the shutter it'll first AF first again anyhow).

That's why many people (me included) use back-button AF, where a shutter half-press activates metering only, and you press teh AF-ON button to AF.

[Tijn]

That's why many people (me included) use back-button AF, where a shutter half-press activates metering only, and you press teh AF-ON button to AF.

I've seen that option. Was looking for the opposite, something to stop the AF while in AI servo mode to change the framing without losing focus. I did find that...

I guess that if you use back-button AF instead, you can just stop it servo-focussing by releasing the button? Personally I've not tried it yet, but I did find that back button a bit fiddly to find (for AF stop) for something as important as focussing.

And you're correct, it might have been the 85L. But it was either that or the 135L, it was an 'odd number'. But it was an f/2 prime for sure. The person who had it didn't know much about it either (she had just got her new body for a week and borrowed the lens from a friend or something).

Thanks for yet another explanation.

[gbchriste]

That's why many people (me included) use back-button AF, where a shutter half-press activates metering only, and you press teh AF-ON button to AF.

I've seen that option. Was looking for the opposite, something to stop the AF while in AI servo mode to change the framing without losing focus. I did find that...

I guess that if you use back-button AF instead, you can just stop it servo-focussing by releasing the button? Personally I've not tried it yet, but I did find that back button a bit fiddly to find (for AF stop) for something as important as focussing.

And you're correct, it might have been the 85L. But it was either that or the 135L, it was an 'odd number'. But it was an f/2 prime for sure. The person who had it didn't know much about it either (she had just got her new body for a week and borrowed the lens from a friend or something).

Thanks for yet another explanation.

The beauty of back button focus is that the focus doesn't change with each press of the shutter.  Suppose I'm composing a portrait (typical for me) wheren I'm using one of the outer focus points to focus on the eye, then recomposing to get a better framing of the subject.  Using the shutter button for focus, I would have to do the focus-recompose sequence on every shot.  With back button focus, once focus is set, it isn't changed again until you hit the backbutton focus switch again.  So for a stationary subject, I can set the focus point, and then take multiple exposures with slightly different composition, provided my subject doesn't move out of the depth of field limits.

[neuroanatomist]

I guess that if you use back-button AF instead, you can just stop it servo-focussing by releasing the button? Personally I've not tried it yet, but I did find that back button a bit fiddly to find (for AF stop) for something as important as focussing.

Yes, AF stops when you let go of the AF-ON button.  I agree with gbchriste about the utility of AF-ON for changing composition for different shots without having to wait for AF each time. 

FWIW, the supertele primes have AF-Stop buttons on the lens barrel which, when pressed, pause the AF system - that's useful to sports shooters who want to stop AF when a referee crosses in front of the lens, for example (although some of the customizations on the 7D/1DIV/1D X are reasonably good at keeping a lock on the subject under those circumstances, if configured properly). 

[The_Arsonist]

That's why many people (me included) use back-button AF, where a shutter half-press activates metering only, and you press teh AF-ON button to AF.

I've seen that option. Was looking for the opposite, something to stop the AF while in AI servo mode to change the framing without losing focus. I did find that...

I guess that if you use back-button AF instead, you can just stop it servo-focussing by releasing the button? Personally I've not tried it yet, but I did find that back button a bit fiddly to find (for AF stop) for something as important as focussing.

And you're correct, it might have been the 85L. But it was either that or the 135L, it was an 'odd number'. But it was an f/2 prime for sure. The person who had it didn't know much about it either (she had just got her new body for a week and borrowed the lens from a friend or something).

Thanks for yet another explanation.

I use the AF-ON button to stop my AF, which it sounds like you've tried? I'm used to the shutter button metering and auto focusing, so whenever I want to manual focus, or lock focus for a series of pictures, I hold the AF-ON (made to be AF-OFF with a custom function) until I want to use auto focus again. I will switch the lens to manual focus if I want to keep it locked for more than a few pictures.