OS in-lens stabilisation
A few casual tests revealed that on average the in-lens optical stabilisation works better than the in-body SSS when shooting between 150mm and 250mm with shutter speeds around 1/30-1/60th. Neither of us liked the floating effect of this stabilisation, or the sleep-mode parking off centre, and the sudden return to centered position. But it works just a little better than the Alpha 700 SSS at the long end where it counts most.
The floating OS group parks itself ‘up’ when the camera is held for a normal landscape scene. Viewing through the lens with the camera turned off, or when the OS has gone into sleep mode, the image appears as it would if the entire lens was shifted about 2.5mm vertically (like a 2.5mm rising front) at the 18mm setting. At longer focal lengths, the offset is greater, but normally you would keep the lens with the zoom locked at 18mm when walking around. When you turn the camera on, or touch the shutter button and wake the system up, the image shifts to the ‘centre’ position and stabilises, taking half a second or so. If you release the shutter during this period, the result will be blurred ; unlike in-body SSS, you must wait for the image to settle. This is a key difference between in-lens OS and in-body SSS/SS, and it has practical implications. For the two cricket shots below, in-lens OS was turned off. There was a possibility of the lens parking the OS between shots, as cricket can be a leisurely game, but the timing of a single shot is critical.
I did not want to have this small delay in safe shooting caused by the OS. In-body SSS/SS would have been perfectly usable as it only operates during exposure and no warm-up or settling time is involved. At the speeds used here, faster than 1/1000th of a second, I felt no need to use any form of stabilisation.
On the other hand, you may ask why Sigma decided to leave OS in a lens intended for the Alpha mount. They have omitted it from some earlier designs, which are OS for Nikon and Canon only, plain non-OS for Sony and Pentax. They reasoned that users can make the choice. Perhaps they also did some tests and found that in-lens OS can work a little better under some conditions, and that users may like the stability of view offered especially at the long end. There is no doubt that hand-holding can be a better experience with a stabilised lens when you are working at 375mm full-frame equivalent.
I won’t show shake tests, as I have no equipment to repeat them exactly. I just took a load of pictures at difficult or impossible settings using both systems and compared them. Neither achieved a perfect hit rate, but with a shutter speed of 1/40th at 250mm one in six shots was visually shake-free at 100% with SSS, compared to two out of six for Sigma OS. At shorter focal lengths I can only say that Sigma OS appeared better able to deal with unreasonably long exposures like 1/8th or 1/4. Sony SSS seems to have the most benefit in making marginal shots (like 1/60th at 100mm) critically sharp every time.
You can turn OS on or off, but when it is on, you must turn SSS off on the body. Using both together is a disaster, it only took a few test snaps to show why. You end up with amplified shake, not corrected shake!