In the old days, once you had chosen your film and put it in the camera, you were restricted to that sensitivity setting until the film was finished. It was possible to change film part-way through a roll, but so awkward that most never bothered. Then dawned the digital age, and the sensor’s sensitivity could be changed as easily as the aperture or the shutter time. Early digital SLRs had a fairly limited ISO setting range, and once you strayed above ISO 400 or so the noise was very noticeable. More recent cameras have a much larger range – up to 54000 and more – and the noise is quite tolerable at the higher settings. And when there is noticeable noise post-processing software can do a good job of removing it.
Having a wide range of useable ISO, selectable shot-by-shot, extends the possibilities in low-light conditions. Instead of varying aperture and shutter time to get the exposure we can now juggle with sensitivity as well without being afraid of noise. Where previously you would have had to use flash, or a tripod if the subject was static, now you can quite happily boost the ISO and still have a reasonable depth of field (aperture) and exposure time. Indoor photography has never been the same.
Camera manufacturers have long offered equipment with the semi-automatic modes Tv and Av. Tv lets you set the desired shutter time and the camera will choose a matching aperture, while Av lets you set the aperture and have the camera select the shutter. Now there is a new semi-automatic mode TAv, where you can set both the aperture and shutter time and the camera will vary the ISO to match. This is ideal in, for example, street photography, where the light conditions are constantly varying as differently-dressed people pass through your target area but you want to keep the speed and depth of field under control. It is also great indoors, in the theatre or places such as museums where flash is just not an option. My Pentax allows ISO up to 54000, and can give quite useable pictures up to about 30000. Where previously I most often used the Av setting, nowadays my preferred setting is TAv.
Variable sensitivity extends your photographic options – take advantage of it.