ISO sensitivity is very simply the 3rd point in the exposure triangle of Speed, Aperture and Sensor Sensitivity. Doubling ISO enables a doubling in shutter speed or a decrease in f number of 1 stop. Providing that a camera has a good enough sensor ISO becomes a useful extra tool, however, it clearly has limits as noise becomes an issue.
In the Images I have created so far, especially in Projects 4 and 5 it was essential to vary the ISO to enable freezing of the action and these images clearly illustrate the possibilities that varying ISO can bring.
There is a massive caveat here. Varying shutter speed and aperture change the amount of light arriving at the sensor, varying ISO changes what the camera does with that light. Herein lies the issue, different cameras react dramatically to different ISO ratings, different cameras also support dramatically different ISO capabilities.
My primary cameras are:
Canon IXUS 950 IS: ISO 80-1,600 (Pub and pocket)
Canon G11: ISO 80-3,200 (walk about or where I want to be discreet and quiet)
Canon 40D: ISO 100-3,200 (underwater and vacation)
Canon 5D2: ISO 100-25,600 (main camera)
In each case the cameras support a pretty broad range of ISO, in fact before getting to the 5D2 the 3 cameras only vary by 1 stop of sensitivity. Most photography takes place between 100 and 800 ISO so all cameras should be fine in this range, however the devil really is in the detail. Image quality varies massively, depending upon the camera used.
To illustrate this I have set up a simple test shooting an indoor scene in my office with each camera shooting at ISO 800, leaving all else on automatic - no flash, cameras mounted on a tripod. I have tried to keep the framing constant and made a crop of a small square area at the center of the images to enable a comparison of the noise. Each image was saved as a JPG in Photoshop with a quality of 5 to reduce the file size. No other image processing has been made.
First of all here is the Initial shot from the 5D2 to show the framing
1. IXUS 950IS - 8MP Point and Shoot
2. G11 - 10MP Bridge Camera
3. 40D - 10MP Crop Sensor SLR (17-55mm f/2.8)
4. 5D2 - 21MP Full Frame SLR (24-105mm f/4)
There are very clear differences between the cameras, first of all the more accurate exposure from the SLR's and much better shadow versus highlight handling. However, the key issue is the noise. The IXUS does very badly in this respect as should be expected from a very basic camera. The G11 is somewhat better, however, the noise is still very visible. The big improvement comes with the two SLR's. The 40D and 5D2 display very little noise at ISO 800.
What this exercise shows is that different cameras perform very differently - ISO latitude varies from camera to camera. The key issue though, is aesthetic, what is tolerable will depend upon the image and the viewer.
With my first DSLR, the Canon EOS 20D, any ISO setting at 400 or above was unacceptably noisy, with the 40D ISO400 works, with the 5D2 the limit is now ISO800 or even ISO1600.
The latest DSLR's now enable ISO to be used in more or less the same way as any other exposure parameter without too much concern for image quality.