IF (internal focus) is a focusing system using a lens group between front lens and iris.
RF (rear focus) is a system using the rear lens group to focus.
SH (straight helicoid) is a rather rare system of front focus, where the front lens extends straight forward without rotation.
FF (front focus) means the front lens is used to focus, which results in a rotating front element and - speaking about Canon EF lenses - rotating filter thread and lens hood mount (can't use flower shaped hoods, exception: EF 2.8-4/28-80 features FF and a freely rotatable filter thread).
IF, RF and SH are all focusing systems supporting a fixed filter thread position (supporting flower shaped lens hoods and polarizers).
FLE (floating lens elements) can actually be a combination of the above (commonly the first three, FF is a mechanism found in rather simply designed entry level lenses), but its main purpose is a special optical correction for narrow focus.
Parfocality is the classic zoom advantage, enabling you to zoom in, focus, zoom out and shoot. Most cheaper zooms change their focal point on differing focal lengths and are therefore varifocal.
SIC (Super Inner Cam) is a mechanism to prevent or minimize a change of focus while zooming (regards IF and RF systems), which - if not addressed - will necessarily result in a varifocal zoom. Still SIC being a feature will not automatically mean a truly parfocal lens. It also allows for very compact lens designs.
E-M (Electronic-Manual) is a manual focusing method/system available with some (mostly) older lenses, that used the built-in AF motor for accelerated manual focus operation. You could often choose between 3 different speeds of focusing, where the number of available speed options is given within the (...): 180°, 320° and 640° ring turn to focus from minimum distance to infinity.
PF (Power Focus) is an electronically supported focus mode for movie shooting.
AFD (Arc Form Drive) is a discontinued fast, but noisy AF drive.
MM (Micro Motor) is the standard entry level solution. Light-weight and compact and little less noisy then AFD at times, but neither that fast and precise as well.
STM (Stepper Motor) is the latest development of a near silent AF drive supporting full-time manual focus and high precision continuous focus for movie shooting.
Ring USM (Ultrasonic Motor) is an electro-magnetic drive, silent, reasonably fast and precise and offers FTM (full-time manual focus without switching between AF/MF).
Micro USM is a silent drive based on a micro motor, until now always offering FTM as well. Not that silent, fast and precise like Ring USM.
Nano USM ... piezo drive supporting FTM
HSM (Hyper Sonic Motor) is the Sigma version of Ring USM (see above).
DF (Dual Focus) is a MF/AF switch mechanism similar to Tokina's Focus Clutch, utilizing the focus ring by pushing it forwards or backwards to enable or disable AF, but combined with an additional switch giving choices between AF with rotating or non-rotating focus ring, MF using focus ring or with focus locked (no AF nor MF).
MO (Manual Override?) is a focus mode to switch the AF off for a predefined period, before it automatically is switched on. The length of the delay must be programmed using the USB Dock. That said, it's a feature of selected Global Series lenses, first introduced with the 150-600 DG OS HSM | S.
FC (Focus Clutch) is a MF/AF switch mechanism originally invented by Tokina utilizing the focus ring by pushing it forwards or backwards to enable or disable the AF (very similar to Sigma's Dual Focus).
HLD (High/Low torque modulated Drive) ... was introduced in 2017 and offers Full-time Manual focus override.
PZD (Piezo Drive) is one of two (actually the newer one) Tamron specific ultrasonic lens drives. This one is based on a standing wave.
USD (Ultrasonic Silent Drive) is the original Tamron version of Ring USM (see Canon specifics above). It's based on the traveling wave principle.
FC (Focus Clutch) is a Tokina specific MF/AF switch mechanism utilizing the focus ring by pushing it forwards or backwards to enable or disable AF.
Type of AF motor used: There are no official statements by Tokina about this feature, nor for a long time did they have a name for it. Therefor I will just refer to the noisy DC motor type as MmDrv and to the silent drive type as SilDrv.
SD-M silent drive supported by GMR (Giant Magneto-Resistance) sensors was introduced with the 16-28mm FX. The SD-M is still based on DC motor technology. It's not clear, whether SD-M and the older SilDrv modules are identical or not. According to tests a la Photozone SD-M seems still not comparable to USM or HSM when speaking of noise.
VCM-S (Vibration Control Mechanism - Supersonic) was introduced with the 70-200mm F4.0 SD FX (still to be released?). It's a combination of vibration control and supersonic drive. As usual with Tokina, there isn't the slightest bit of information available other than that.
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