Today's Photo Skill: Controllers of the Light - ISO

Having explored aperture and shutter speed the third modifier affecting perfect exposure is ISO. ISO in the film days referred to the "speed" of the film. The faster the film the more grain in the pictures. Slower film needed more light for perfect exposure and had little graininess. That is great for still subjects and bright scenes. Dark scenes and fast moving subjects (requiring fast shutter speeds) required faster film at the expense of graininess in the images. This concept has been brought into the digital age as a setting on your camera. The higher the ISO setting, the more light you are letting in (at the expense of potential graininess) allowing you to shoot in darker settings, or to get higher shutter speeds for stopping action.

Here's how I use ISO. I decide what aperture is acceptable based on my depth of field needs. Then I select a shutter speed that gets me a perfect exposure. If that shutter is too slow for the subject  - say a child, young restless dog or an athlete in action, then I begin boosting my ISO until I can support the shutter speed I need.

ISO is measured in numbers that (depending on your camera's features) will be in amounts that equal an f-stop of light. A low ISO is 100. After that in full f-stop increments ISOs are 200, 400, 800, 1600, 3200, 6400, 128,000. Each camera is different with regard to picture quality at higher ISOs. For example, my previous camera system (Nikon D2X) had good quality through ISO 800. Above 800 the images were unacceptably grainy. what is acceptable is up to the photographer. With my current camera system (Panasonic Lumix GH3) 3200 is still good and with a little post production tweaks 6400 can be usable.

Please see the 2 sample photos. Look in the blue shirt of the man on the right to really see the grain difference. Their exposure settings are labeled in the caption.

To determine your ISO "comfort zone" take pictures of the same object, keeping the exposure good, at each ISO setting. That means that as you increase the ISO you will have to either speed up the shutter, make the aperture smaller or both. Then look at the resulting images. You will find a place where you like one image and not the next. The last image you like tells you the highest ISO setting you're comfortable shooting. Remember that this varies from camera to camera so you'll want to do this test each time you get a new camera body.

Here's my best advice for when you find yourself shooting a high ISO - nail the exposure. If you are the littlest bit underexposed (dark) on a high ISO shot the image is lost. When you use your software to lighten the image you'll lighten and highlight the graininess and it will be awful. The higher the ISO the less tolerance for an imperfect exposure. Yet the high ISOs on modern cameras expand the range of what we can capture. It's worth the effort to learn to use ISO as part of your toolbox for controlling the light!