ISO Explained

Remember years ago when you purchased film and it was marked with an ASA number?  Different people would tell you that ASA 200 is the best, someone else would tell you that ASA 100 was the best, and someone else ASA 400.  Who was right?  It depends.  ASA was a measurement used with film to let the user/buyer know how sensitive to light the film was.  Each increase, 25 to 50, 50 to 100, 100 to 200, 200 to 400, 400 to 800, 800 to 1600, 1600 to 3200 increased the sensitivity to light by one stop.  A “one stop” difference, means the value would double, or half, depending on which way you were going.  ASA 100 film is half as sensitive to light as ASA 200, or one stop less sensitive.  ASA 200 is twice as sensitive than ASA 100, or one stop more sensitive.  ASA is the measurement used for film, and ISO is the measurement used for digital cameras.  ISO values correlate to ASA values directly.  If a film camera using ASA 100 film requires a shutter speed of 1/60th and a aperture of F8 for a proper exposure, then a digital camera set at ISO 100 will require a shutter speed of 1/60th and a aperture of F8.  ASA equates to ISO. 

The first question people ask, is if ISO 200 is more sensitive than ISO 100, why not leave the camera set to ISO 200 all the time, or better yet the highest ISO the camera can be set for?  Good question.  The mechanics of photography is nothing more than the compromise of variables to achieve the desired composition in the active environment.  ISO is a variable.  Generalizing usually is fraught with peril, so if you do generalize it’s best to qualify the generalization.  In the explanation below explaining why you would choose one ISO over the other, assume we’re using the same camera.  Different digital sensors react to different degrees to a change in ISO, so one digital camera set at ISO 800 could very well perform better in a certain area as another digital camera set to ISO 800.  You must review the models side by side, or against a base reference, to be able to compare their ISO performance against each other.

Lower ISO settings generally mean:

  1. Less noise
  2. Cleaner image
  3. Requires more light and/or a larger aperture and/or a longer shutter speed.

Higher ISO settings generally mean:

  1. More noise
  2. Less detail in the image
  3. Requires less light and/or requires a smaller aperture and/or allows a faster shutter speed.

Noise in digital camera produced images can be directly equated to grain on film images.  The more noise, the less clean the image, the less detail in the image. The less noise, the more clean the image appears, and the more detail in the image.  Ideally you want the cleanest most detailed image possible.  Depending on available light, compromises in ISO settings are required.  During outdoor photography in bright sunlight ISO 100 would be possible and would produce clean detailed images for a given aperture and shutter speed.  During outdoor photography on a cloudy day, ISO might be required, and produce somewhat less clean and detailed images.

ISO, shutter speed, aperture, focal length,  and focal distance are the five main variables a photographer most often balances for optimum results.  In this weeks column I briefly explained ISO.  In future columns I’ll briefly explain each of the other four variables.  Once we have a working knowledge of each variable, then future columns will discuss balancing these variables for the best results in your desired composition.

Homework assignment.  Take your digital compact camera out of the automatic ISO mode, and take a series of pictures of the same subject at the same time, but varying the ISO from the smallest value to the largest value.  Do this on several subjects.  Once completed, view the pictures on your computer monitor where the picture fills half the screen, where it fills all the screen, where it would fill two screens, then four screens.  Keep zooming in at different levels until you become very familiar with each ISO setting for your camera, and how big you must view the picture until you notice the noise (grain), then bigger until the noise becomes objectionable and you notice a loss of detail.  Each camera will be different, compact cameras will produce much more noise and much less detail at a given ISO value than will a DSLR.  Learn your camera.  This information will help you select the most useful variables for your composition.