Digital Photography - ASA-ISO Sensitivity

The sensitivity rating of film or a digital light sensitive device is called the ISO (International Standards Organization), ASA (American Standards Association), and decades ago DIN (Deutsche Industrie Normen) a German standard found mostly in Europe now replaced with ISO).

Common ISO ratings begin with values of 25, 50, 100, 200, 400, 800, 1600, 3200, etc. Notice that each value is exceeded by doubling the sensitive value; this is commonly referred to as "Stops.

The lower ratings (25 - 200) will produce images with very fine grain and much lower sensitivity to light. The higher ratings (400 - 3200 will have much higher sensitivity to light and produce images with visible grain patterns.

In normal photography Grain Patterns is not an important consideration; the patterns will become visible after the image is enlarged to 8 x 10 inch and larger. Where Grain Patterns may become important is when making fine portraits, macro images, and low light photography.

Depending on the Artistic intent of the image, grain can become an asset to the image, or a detraction. The choice is yours when planning the photograph.

What is grain in a photograph?

In film photography grain is the chemical elements that become light sensitive and record the light patterns. Changes in the chemical make-up will change the sensitivity of these elements to become more sensitive to lower light levels, this can become a visible part of the overall image.

Development of exposed film can be "Pushed" beyond original specifications to extract details in shadows under low lighting conditions. "Pushed" processing enhances the grain in the finished image.

Commonly "Pushed processing will effectively increase the ISO 1 - 2 stops.

In digital photography we have photo sensitive devices that are electronic in nature and convert light into electrons.

We can add, or subtract voltage to the photo sensitive element to cause it to become more or less sensitive to light.

When we make the photosensitive element more sensitive to low light we introduce electronic noise into the image, this becomes "Grain" in the photograph.

We can use software to Post Process the images and reduce the effective grain patterns.