Wednesday, June 29, 2011

Elements of Exposure - Part II - ISO

In Part I of my Elements of Exposure series posted on June 16, 2011, I introduced you to the exposure triangle.  The first component of the exposure triangle is ISO.  Here I discuss what ISO is for and the advantages and disadvantages of different ISO settings.

Exposure Controls
Photography is always about compromise when it comes to adjusting our exposure controls to get a proper exposure. Let’s look at each adjustment individually and what the advantages and disadvantages are of changing each of these controls.

Originally from the film world, ISO is an acronym for International Standards Organization; this organization set the standards of film sensitivity so that it was the same from one manufacture to another. So a roll of ISO 100 film from Kodak, Ilford, Fuji, etc. would have the exact same sensitivity. In the film days our ISO was set for the entire roll of film loaded in our camera and the only way to adjust it would be to replace it with a different roll of film with a different ISO rating. Digital has changed that by allowing the photographer to change the ISO at any time. The problem with increasing ISO settings is that higher ISOs introduce noise into our images that reduces the overall quality of the image. Lower ISOs while delivering less noise and higher quality images may prevent us from capturing an image in the way we want to because there may not be enough light captured to give us a proper exposure. What is noise? In film and in digital photography, noise appears as graininess due to the capture of a different exposure or color in grains or pixels as opposed to the surrounding grains or pixels making it standout. As we increase our ISO it increases the occurrences of noise. As photographers we must find a balance between quality and noise that conveys the scene as we desire. As a general rule, we should always aim to use the lowest possible ISO if we want the highest possible quality. As a standard, this is generally around ISO 100. While there are films and digital sensors with lower ISO settings the quality of the images is not significantly better to warrant going much lower than this standard. The main reason for going lower than the standard would be because you desire to use a slower shutter speed or wider aperture than the standard allows. And, the obvious reason for increasing your ISO above this standard would be that it does not let in enough light to get a proper exposure at the shutter and aperture setting you desire to use. If we relate ISO to stops than a 100 ISO setting is half as sensitive as a 200 ISO setting or we could say a 200 ISO setting is twice as sensitive as a 100 ISO setting. If we move from 100 to 200 ISO we have doubled the amount of light being recorded or increased exposure by one stop. Or, if we move from 100 to 400 ISO we have increase exposure by 2 stops, and so on. Conversely, if we move from 400 ISO to 100 ISO we have decreased the exposure by 2 stops. In photography, stops are the common language of describing the intensity of exposure adjustments.

In my next post I will discuss the second exposure control, aperture.  I want to hear from you, please leave your comments, questions or topic suggestion for future blogs in the comments section.

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