Using a higher ISO can help freeze the motion in a blurry photo.
To a photographer, the term "ISO" refers to the speed of a particular film or a digital camera's sensitivity to light. With a film camera, you must purchase film of a certain ISO and use those settings for the entire roll. On a digital camera, the ISO setting can be adjusted for each individual photo. Understanding ISO can help you take the best possible picture for a given light situation.
Other Terms for ISO
ISO stands for International Organization for Standardization. Older cameras may use film speed number values marked as ASA (American Standards Association). Older European cameras may list film speed as DIN (Deutsches Institut für Normung) values, which consist of a two-digit number followed by a degree symbol. In all cases, the higher the number, the faster the film.
Working With Light
A low ISO number, such as 50, indicates that the film is slow; in other words, a high amount of light is required to make an exposure. This film speed works best outdoors. Fast films, such as ISO 3200, require far less light and work well in low-light indoor situations or if you don't want to use a flash. The ISO numbers apply in the same way to digital cameras.
Using ISO With Shutter Speed
An important reason to consider ISO is to figure out what shutter speeds are possible with a given light situation. On a 35 mm camera equipped with an average lens, shutter speeds of 1/30 of a second or lower will result in blurry images if the camera is handheld. With a digital camera, you can observe the shutter speed the camera chooses in its automatic mode. If the speed is 1/30, you can increase the ISO by one stop, allowing for a shutter speed of 1/60. You may also find that you need the fastest possible shutter speed to freeze motion in an action scene. Using high-speed films or fast settings gives you the opportunity to do so.
Grain
The advantage of working with a low ISO is that the film grain or digital noise is less noticeable. If the lighting allows a workable shutter speed, you should use the lowest ISO number possible for fine grain. A high ISO, on the other hand, produces large, noticeable grain. This may be an advantage to some photographers for creative effect. You should expect an entire roll of high-speed film to produce grainy results, whereas you can select this mode on a digital camera just for images that you shoot in low light.
Contrast
Contrast is another consideration when adjusting your ISO. Slow film or a low ISO setting produces a wide range of values. This is favorable when you are trying to capture a great amount of detail in the highlights and shadows of a scene, especially one that is lit very well. High-speed film or a high ISO setting increases the contrast of a scene. This may be favorable for some images but problematic in others.
Tags: digital camera, film speed, entire roll, freeze motion, given light, given light situation