Add glass and mix
I’m almost ready to provide a list of how the three finder modes work, and to add this a reference to the Sony Alpha NEX E-mount system and its LA-EA1 lens adaptor. First you need to know about different lens types.
The basic Alpha lensmount, from 1985 onwards, is in-body motor driven focus. Everyone calls this ‘screw drive’ and who am I to argue with such an inaccurate term? We all understand it, so ‘screw drive’ is what I’ll call these lenses.
The Alpha system jumped in early 2000s to a supersonic motor drive, built in to the lens, controlled electronically and supplied with battery power from the body. Only two Minolta lenses ever had this, the 300mm f/2.8 G SSM and the 70-200mm f/2.8 G SSM, by the time Sony took over in 2006. SSM was introduced to compete with the speed and silent operation of Canon’s USM and Nikon’s new AF-S, and the lens choice reveals why – it made a big difference in sports and action photography with large telephotos and zooms which were difficult to drive from the in-body motor.
The SAL 70-200mm f/2.8 G SSM – inherited from Minolta – offers the best type of focus compatibility with the A580, SSM supersonic motor drive
It was never a matter of the body motor being too weak; sometimes, it was too powerful or too fast. The first generation of screw drive lenses needed a lot of power. Later ‘HS’ high speed variants were easier to drive. Then Minolta made the in-body motors even more powerful to help handle the increased demand for closer focusing, fewer ‘screw’ rotations, and even more complex zooms… upon which some of the HS lenses ended up overdriven, focusing too fast for accuracy or even hitting the end-stop with gear train crash results. So SSM was actually a body independent solution with the motor precisely matched to the focus action required, sometimes slower than older screw-drive but always smoother and finer in adjustment.
Sony overlooked the need to convert all lenses to SSM as soon as possible. Their new Carl Zeiss 16-80mm, their own 16-105mm, even the CZ 85mm replacement were still screw drive. They obviously did not foresee the HD video revolution in DSLRs, despite a long history of both the Sony and Minolta teams being almost prescient in their innovations from the 1980s on. Catch-up demanded a solution and their found this by going back to Canon’s early days and putting ordinary micro motors into lenses – the SAM (Smooth Auto Focus) range of ‘easy choice’ budget optics.
These Chinese made plastic bodied lenses are optical surprises, far better than their feel indicates. Mechanically, the SAM motor is anything but ‘smooth’; it can be damaged if you use the body switch to change from AF and MF and then try to turn the focus ring, it is vital to move the AF/M switch on the lens instead. They make noises ranging from little buzzes to prolonged grinding. But Sony did not mean ‘silent’ or ‘silky’ when they said smooth. They just meant capable of very fine adjustments under full control, like the SSM lenses without the same cost. And that is needed for the A580/560 with contrast detect autofocus in live view mode, and hopefully for real-time AF during video shooting in future cameras.
The same function also enables use of the SSM/SAM lenses on the NEX with LA-AE1 adaptor.
Beyond the Sony range, there are third party lenses which have screw drive, and some which have ultrasonic motors (Sigma HSM, Tamron just beginning to roll out) or micromotors (Tamron, including models rebranded as Sony). While the motors may be similar the protocols within the lenses don’t appear to match. I have been unable to get reliable CDAF with third party lense; it is best to select Phase Detect focus in Focus Check LV mode via Camera Menu 2. This then has to be switched back on the same way if you want to use it with genuine SAM/SSM lenses.