Sorry, apparently this forum does not allow posting links from other Fuji forums. Here's the text.
Here's a summary of the differences using collared and non-collared lenses.
"Sure - it's hard to have the cake and eat it too. Clutch-based lenses are different to operate because AF and MF operation as related to the position of the lens' focus ring inherently contradict each other. Contrary to normal non-clutch lenses, the 14mm, 16mm and 23mm lenses feature what's called an "absolute" position encoder to deliver their angular focus ring position to the camera's CPU (which then issues drive commands to the lens's focusing element).
In its MF position,
every angular focus ring position corresponds to exactly one unique lens focus distance, and
the focus ring features hard stops for its near and far focusing limits.
So, a clutch-based lens's absolute position encoder provides a position signal that depends on the ring's angular position.
Normal, non-clutch lenses don't have focus rings with hard stops and feature a different position encoder implementation. Their encoder doesn't provide an absolute position signal whose minimum and maximum values are defined by any hard stops. Rather, their encoder signal is relative to the position where the lens's focus elements happen to be at the time when manual focusing begins. Such a relative encoder simply records any focus ring motion in terms of direction and rotational angle into a number of "ticks" or pulses which the camera's CPU translates into forward or backward motion steps for the lens' focusing motor(s).
The contradiction I mentioned above is due to the fact that clutch-based lenses don't have a relative position encoder in the ring's AF position, and their focus ring is locked in AF position. It's theoretically possible to implement such an encoder, but that would require a second position encoder and a different clutch design (changeover between AF and MF modes would become a lot more complicated and even less intuitive), a different focus motor design and a different signal path design for encoder and focus motor drive signals. All of this would make the lens heavier and yet more expensive.
Nikon did something like this in their DSLR lens line, but none of their designs both using their "screwdriver" and ultrasonic focusing motor designs offer a convincing user experience. Either the focus ring has to move during AF operation in their "screwdriver" designs which bears the risk to damage the in-camera focus motor if you don't keep your fingers out of the way, or you lose the hard stops and the DoF scale as in their USM lens designs.
So, this contradiction could only be cured by changing the lens's internal hardware!
Continues in next post (I exceeded the post editor's max. allowed character count)..."
"Part 2:
So, after all these ramblings, how can we best use Fuiji's clutch-based lenses based on the existing hardware and firmware? Let's take a look at how the camera/lens combo behaves if you have a clutch-based lens attached, and AF+MF plus Focus Check are enabled (my description relates to the X-T1's firmware, and to some aspects of the X-T10 and X-E2. Neither the X-T10 nor the X-E2 feature Dual Display EVF mode. Please correct me if there's a mistake or discrepancy in any of the descriptions below, particularly in reference to the X-T10 and X-E2):
1) Clutch in AF position, MCS rotary switch on camera body on S:
AF-L button function is available w/ full functional scope: On while pressing or toggle-switch operation. Pressing AF-L will start an AF run and lock focus (green AF frame indicates locked state).
Focus Assist button can access focus magnification if Focus Check is On and focus has been locked; neither focus peaking nor split-image assist functions are available - you're limited to basic ground-glass focus magnification (typical for normal AF-S operation)
2 EVF display modes available: Full and normal
Since the focus ring is locked, you cannot use the AF+MF function.
2) Clutch in AF position, MCS rotary switch on camera body on C:
As above, but Focus Assist button won't provide focus magnification.
So, this combination of settings doesn't offer any practical value.
3) Clutch in AF position, MCS rotary switch on camera body on M:
· 3 EVF display modes available: Full, Normal and Dual *
· No AF function available, neither via shutter button half-press nor via AF-L button.
· No focus tweaking as focus ring is locked
So, this combination of settings doesn't offer any practical value.
4) Clutch in MF position, MCS rotary switch on camera body on S:
2 EVF display modes available: Full and Normal *
AF-L button is disabled (this could be changed in firmware, but would probably require a major software re-write because of EVF display modes, see further down)
Lens will perform an AF run on half-pressing the shutter button (AF-L button isn't needed!), and you can manually tweak focus via focus ring while you keep the shutter button half-depressed.
Manual focus tweaking will be done while the lens is stopped down to its manually defined or automatically determined working aperture.
Focus magnification kicks in automatically if you turn the focus ring (remember, AF+MF and Focus Check are enabled).
Focus Assist button is disabled, but isn't needed anyway as Focus Check is enabled.
Focus distance setting bis lost as soon as you let go of the shutter button. Thus, you a) need to proceed directly to triggering the shutter after focusing, b ) you can't revert to normal EVF magnification before taking your shot, and c ) you need to position your AF frame before you start focusing as focusing and reframing is impossible because it would require to go back to an un-magnified EVF display.
5) Clutch in MF position, MCS rotary switch on camera body on S:
As in 6) below
So, this combination of settings doesn't offer any practical value.
6) Clutch in MF position, MCS rotary switch on camera body on M:
3 EVF display modes available: Full, Normal and Dual *
AF-L button is disabled (this could be changed in firmware, but would probably require a major software re-write because of EVF display modes, see further down)
Lens will be focused manually while the lens' aperture is wide open (i.e. minimal DoF)
Focus magnification kicks in automatically if you turn the focus ring (rememberFocus Check is enabled).
You revert to normal un-magnified EVF display by pressing Focus Assist; if you turn the focus ring, magnification will kick in again. This lets you manually focus the lens, and allows you to use focus & recompose.
On half-pressing the shutter button, the lens will stop down to its manually defined or automatically determined working aperture, so you can check DoF.
Focus distance will be retained as long as you don't touch the focus ring. This also holds true if you half-press the shutter button, of if you switch off your camera and power it up again.
Summary:
Use scenario 1) for wide angle lenses at medium to far distances. This scenario doesn't require much thought and can be used when handing thze camera to someone else.
Use scenario 4) for longer focal lengths or for macro shots that don't require focusing/recomposing. Position the AF frame to a desired location before focusing.
Use scenario 6) for all focal lengths in landscape photography, for wide angle photography and for macro photography if you have the time and/or need careful setups.
* Two comments (this could be a firmware bug, or at least an opportunity to improve camera firmware):
There seems to be a software relationship between the function of the AF-L button and the EVF display modes for manual focus mode - they are mutually exclusive.
Another firmware dependent interrelation appears to be between the focus ring position and the focusing modes of the MCS switch: Dual-display EVF mode is only available if both the focus ring and the MCS switch are set to MF; if the MCS switch is on S while the focusing ring is in MF mode, then Dual Display mode isn't available although you're still essentially in MF mode. This is somewhat of a pity because if you previously had selected split-image focusing in the M position of the MCS switch and then change to the S position of the MCS switch, then focus magnification won't display a split-image, but will revert back to standard magnification Thus, split-image isn't available when trying to use the AF+MF function when the MCS switch is in its S position (BTW, this also happens if I use a non-clutch lens)."
