Sunday, April 26, 2009

Exposure...Part 3 - ISO

ISO - Sensor Sensitivity to Light
ISO stands for the "International Organisation for Standardization". The ISO was founded in early 1947 and is headquartered in Geneva, Switzerland and is composed of representatives from various of it's 158 members national standards organisations.

One of the standards defined by this organization is photographic film's sensitivity to light. An image sensors 'film speed', is often referred to as its "ISO number."

ISO - and Stops of Exposure

Just like aperture and shutter speed ISO is set up as multiples of 2. ISO 100 allows twice as much light as
 ISO 50. ISO 3200 allows twice as much light as ISO 1600. Most DSLR cameras have a minimum ISO of about ISO 200 though many have an extended range at each end of the scale that can be accessed if needed. The number of stops in the ISO range is typically much less than the number of stops available with aperture and shutter speed. The common ISO stops are:
  • 100
  • 200
  • 400
  • 800

Higher end camera extend the range:
  • 1600
  • 3200
  • 6400

and so on. The Nikon Dx3 is capable of ISO 25,600.
Like aperture and shutter speed most cameras allow adjusting ISO in 1/3 stop increments.

How ISO is used

ISO sensitivity is selected according to the amount of light that is available. On a bright sunny day ISO 100 or 200 is appropriate and the image sensors sensitivity is low. If there is a hard overcast you may want to use ISO 400 to keep your shutter speed reasonable for hand holding the camera and the image sensor becomes a bit more sensitive to light. If you are inside and don't have auxiliary lighting you may need ISO 3200 or more to make a viable exposure.

The way camera manufacturers alter the light sensitivity of the image sensor is by using different pixel output amplifiers. The amplifiers used at ISO 100 don't amplify the signal as much as the ISO 1600 amplifiers do.

In our last installment of the Exposure series we'll show how the 3 legs of the exposure triangle relate. First we need to discuss some of the inherent limitations to ISO, namely image noise.

ISO and Image Noise

Image noise is a random, usually unwanted, variation in brightness or color information in an image.
Image noise is usually most noticeable in those parts of an image with low signal level, such as shadows or underexposed images.

Noise tends to increase as ISO increases. The images produced by most cameras at their highest ISO settings are the images that will usually have the most image noise.

The left image was made at a high ISO setting in very low light and required a shutter time of 10 seconds and the noise is readily apparent. Long shutter times also contribute to image noise. The right image was made with a low ISO setting and a 1/10 shutter speed because auxiliary light was used.

At a later date we may explore some of the technical causes of image noise but suffice for now to think of noise in terms of luminosity noise, or variation in adjacent pixel brightness, and color noise or variation in adjacent pixel color.

There are image editing software available that can be used to diminish image noise in the post processing work flow if in fact the photographer feels it must be reduced to make an acceptable image.

In the next installment we'll start pulling all this exposure stuff together.

See you then,

All images courtesy of Wikimedia Commons.

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