The ABCs of ISO

iso comparison
A comparison of grain at different ISO's - by HuttyMcphoo

The three legs of a good exposure are shutter speed, aperture, and ISO. In the old days ISO was called ASA and was dictated by the type of film you had in your camera. The standard for color and clarity in those days was Kodachrome 64 and, for really fine work, you might pick Kodachrome 25. The lower the number on film, the finer the grain and the slower the film, meaning it needed more light to get good results.

In modern digital cameras the theory is similar but the application is quite different. Modern image sensors can change ISO, the sensor’s sensitivity to light, to suit the scene. Most digital sensors are optimized to produce the best results in natural daylight at lower ISOs, but it takes better eyes than mine to see the image quality degrade at ISO 400 and below. And the noise introduced by digital cameras at higher ISOs is not a constant. Ever year sensors get better and noise reduction algorithms in your camera’s computer also improve in every generation of hardware. What’s true today may change in a couple years.

Just like in film, the amount of noise in a digital picture increases at higher ISOs. It starts getting more noticeable at ISOs over 800 and becomes really noticeable at 1200 and above.

As a general rule, use the lowest ISO you can and still get a reasonable shutter speed. Ironically, long shutter speeds can introduce their own brand of image anomalies and sometimes it’s a matter of picking your poison.

When it comes to the three legged stool of exposure, ISO still works like it used to…for now. But expect this one to change sooner than either of the others.