All these letters describe the same thing, a form of image stabilisation that allows sharp, hand-held photographs, to be taken at lower than normal shutter speeds. The different names are used by Canon, Nikon, Sigma and Tamron to describe very similar technologies, each designed carefully to avoid the patents of the other companies.
My first encounter with Image Stabilisation (IS) was at a Nature Photography Fair in 1995 where Canon had a demonstration 75-300mm IS zoom lens on show, the first IS lens available. When I fitted it onto my camera, and half-pressed the shutter release, there was a loud whirring noise and the image in the viewfinder wobbled around queasily before settling. Panning the lens caused the image to stay still initially then eventually catch up after panning had stopped. From this unpromising start, I felt that the technology could have some benefits and soon bought one of the lenses.
Like many 75-300mm lenses, the performance was good from 75-200 but fell away towards the 300mm end. At full aperture (large) at 300mm there was some softness which disappeared once the lens was stopped down to f/8 - f/11. Image stabilisation allowed me to hand-hold the lens at the optimum aperture and get sharper photographs. The sharpness gain was not due to reducing camera shake, just by allowing the lens to be hand-held reliably at the best aperture. This was before the days of digital photography when you could change ISO values for individual pictures.
As the technology improved, shutter speed gains rose from 2 stops to 3 stops, and eventually to 4 stops, as is the case for most recent IS lenses. This does not mean that the full 4 stops benefit over the normal 1/focal length minimum shutter speed is guaranteed. It depends on how steady your hands are, how windy it is, whether you are on a moving platform, and so on. However, if you take several pictures, even at very low shutter speeds, the chances with IS are that at least some of them will be sharp.
How does this affect landscape photographers? Often they use sturdy tripods for every picture and some would say they have no need for IS lenses. In fact, the early IS lenses actually could reduce the sharpness of photographs taken on tripods if the IS was left switched on. This was because a feedback loop could develop in the absence of any movement causing the IS group to hunt around. Most modern IS lenses automatically sense tripod use and switch off the stabilisation.
A number of current lenses, such as the 70-200mm models, have available both IS and non-IS versions, and the difference in cost often is considerable. If you use a tripod all the time, you could save money and get the non-IS version, but often I take pictures, especially early and late in the day, that capture a fleeting moment which would be gone before my tripod was erected. I depend on IS for these pictures and would not be without it now.
Both photographs this month are hand-held at slow shutter speeds with IS lenses. The photograph of moving water was shot balancing on a slippery rock. I dropped the shutter speed as low as it would go (1/10th second), to gain the feeling of movement in the water. The second image shows water (from a watering can) bouncing off a Shasta daisy. It was taken at 1/30th of a second at 277mm focal length, equivalent to 443 mm on full frame. I tried a range of shutter speeds from 1/8th to 1/250th but this one was the best, combining the movement of the water and the tracks of the splashes without the flower head moving about during the exposure.
Read this article, and many more, in High Definition, inside Issue 28 of Landscape Photography Magazine.