What is ISO?

ISO, (International Standards Organization) is the rate at which the camera’s sensor responds to light. In the days before digital cameras, ISO was called ASA. And in order to change the film’s speed, you needed to change the film. The advent of the digital age has allowed us to change the speed of the camera’s response to light with the mere flip of a button.

ISO is expressed in numbers:
50, 100, 200, 400, 800, 1600, 3200, 6400 and beyond, depending on your camera.

An ISO speed of 100 responds to light twice as quickly as an ISO speed of 50, an ISO of 200 responds twice as quickly as a speed of 100 and 4 times as fast as a speed of 50.
Why do we need so many choices in speeds? Basically different lighting conditions require different exposure settings. ISO is part of the exposure equation which includes Aperture and Shutter Speed. All three are needed to get to the perfect exposure for the situation. We use ISO to help us in the event that we require or can’t change the aperture or shutter speed to reach the desired exposure.

Suppose you find yourself at a concert or an indoor recital or play, your camera warns you that the shutter speed and/or aperture settings will not allow you to get the correct exposure needed? Flash might not be an option, as in this type of situation, flash may not be allowed. Changing the shutter speed might result in a blurred shot of the subject if it is in motion on the stage. Changing the aperture might result in a shallow Depth of Field, where the subject might be in focus but other parts of the scene are not in focus. This leaves us the option of changing the ISO, which will increase the sensor’s ability to translate the light into an acceptable image. In this situation, I might elect to use ISO 400 or 800, this would allow more light onto the camera’s sensor without excessive noise destroying the image.

Higher ISOs would be used in low light situations, such as late afternoon, early evenings. The highest ISOs would be used for night-time photography, where the camera’s sensor requires the highest sensitivity in order to properly expose the scene.

There are trade-offs, though, to using higher ISOs also equals noise. Noise is roughly equivalent to “grain” in film images. Some noise is acceptable in an image, however, using higher ISOs usually leads to less than satisfactory results and attempting to lighten an image taken at a very high ISO will usually result in the image being totally unusable.
Despite these drawbacks, high ISOs can also save us in low light situations and give us the shot we want.