In film photography ISO refers to the film’s sensitivity to light and is also called the film speed.  The chemical makeup of the film is altered to either increase or decrease its sensitivity and speed.  The lower the ISO number the slower and less sensitive the film is to light.  Low speed film also results in less grainy photographs.  The higher the ISO number the faster and more sensitive the film is to light.  In simple terms that means in well lit situations, such as outdoors on a sunny day, you need a lower ISO film such as an ISO 100 and in less lit situations and action shots, such as indoors and sporting events, you need a higher ISO number such as 400.

So this may lead you to wonder why there is an ISO setting on your digital camera.  There is no film to chemically alter and you either set it and forget it on auto or manually adjust your shutter speed and aperture to compensate for different lighting conditions and motion.  So how does ISO fit into digital photography?

In a digital camera the film is replaced with the sensor.  So ISO in a digital camera sets the speed and sensitivity to light of the sensor.  Being able to adjust your ISO allows for expanded shooting options in different conditions.  For example, if you leave your ISO on automatic the camera will, in most cases, chose a low ISO.  In daylight conditions this will probably be acceptable.  But in low light conditions you will need to compensate by extending your shutter speed or opening up the aperture.  But a longer shutter speed can lead to camera shake and blurry pictures unless a tripod is available.  So an easy solution is to increase your ISO.  The faster “film” speed and greater sensitivity to light allows for shorter shutter speeds and less chance of blurred pictures from camera shake.

You will want to keep in mind that the higher the ISO the more noise a digital image can have (grain in film photography).  Depending on the end use and final printed size of your image this may be an acceptable compromise to get the shot you want.

I shot the three photographs below in Scottsdale, AZ.  The ISO setting from left to right was 100, 200 and 400.  The aperture was at f/6.3 and the shutter speed was at3.2 sec for all three.  As you can see, increasing the ISO allowed for the more sensitive sensor to take on more light and get a very different picture.

Night Phtograph Shot at ISO100 Night Photograph Shot at ISO 200 Night Photograph Shot at ISO 400

Click on the images to see a larger version.  Then go out and take your camera off ISO auto and experiment.

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