In the last two posts I talked about Shutter Speed and F stops for both light control and creative expression. The third part of the exposure triad is ISO or the sensitivity of the chip in the camera that records the image. Right out of the box you may have noticed that the camera was set on Auto ISO which means the camera picks the sensitivity as it perceives the light falling on the scene you are about to photograph. In fact for many cameras any one of the program modes (those little icons on the dial like the running man or the head) will go into this mode without any changes from you. Otherwise you will need to go into the menu to change the ISO.
So just what is this and why should we care if the camera can take care of it? ISO (or for some of us who remember the days of yore of film ASA) was the rating of film as to how sensitive it was in different lighting setups. Slow film was ISO rated at 100 or 64. This was called daylight because it was used for outside daylight shots like landscapes. Plenty of sunshine on bright days which means that film got more than enough light for an exposure. This film would also be called fine grain because of very smooth tones it would produce. The next ISO we would grab is 200 rated film. This was sometimes called the all-purpose film because it would do well in both bright light and shadow. Still a pretty smooth grain film but since it just a little more sensitive allowed a one stop increase in f stop or a faster shutter speed. After that came the fast films of ISO 400 or greater where the higher the number the more it could be used in lower light situations like dark churches or sports. These films would allow even hand held shooting at good shutter speeds to stop action. There was a trade-off for these faster films. It was called grain because in the film were larger silver particles to capture the lower light levels. It would look like sand or grain in the mid tones. Some photographers like this look that this gave to their images for that smoky or harder look.
When the digital revolution took hold many of the old film terms were transferred over like ISO and with it the same problems. In the place of grain we got noise with higher speeds which looked just the same. However the same principle of using higher speeds in low light still applied too. The reason being is that for most cameras auto ISO is going to be a set range of something like 100 to 800 ISO. Which would be fine unless you are shooting a basketball game and need shutter speeds of 1/500 sec or higher to catch the players in midair. Or a night game of high school football where you trying to get all the action on the field.
For most cameras you might have to go to the letter settings or Priority modes of the cameras to set the ISO up to numbers like ISO 1600 or even the H1 or H2 settings for the highest speed in low light. These numbers will allow better shutter speeds but as I said more noise but if the action is what you are going for then it is good trade-off.
One piece of advice, the camera always remembers the last setting you used. So if you are shooting your favorite night sport with high ISOs be sure to reset it back down before going out to do some day shots. It will be obvious after the first couple of images done in full sun that come back all washed out.
With the balance of these three settings, shutter speed, f stop, and ISO you can shoot in almost any lighting setup from full sun to even night time.