Digital Photography

Remember when you used to buy film, and it came in different “speeds” — ISO 200, 400, 800, and so on? Well, those numbers indicate the film’s sensitivity to light; the rating system was devised by the International Standards Organization, thus the ISO abbreviation. The higher the number, the “faster” the film, meaning less light is needed to expose the picture. So a high ISO film lets you use a smaller aperture (higher f-stop setting) or faster shutter speed than if you use a low ISO film. (If that last sentence sounded like gibberish, check out my previous posts, which provide the fundamentals of exposure.)

On a digital camera, the ISO setting determines the light sensitivity of the image sensor. And just as with film, using a higher ISO setting allows you to capture an image with less light. Most cameras enable you to choose from a range of ISO settings, typically from about 200 to 1600, although some newer, higher-end cameras offer much wider ISO ranges, starting as low as ISO 50 and going as high as ISO 6400 or even greater.

Unfortunately, increasing ISO brings a negative side effect: Your picture can take on a grainy or speckled appearance. The example here (a portion of a flower petal) gives you a close-up look. This example shows the digital version of the problem, but you get a similar result with film. In the digital world, the defect is called noise; in the film arena, it’s known as grain.

If you’re an experienced film photographer, you probably know that different types of film produce different levels of grain at a given ISO. Similarly, digital cameras also vary in their noise performance. You may not see any noise at ISOs up to 1600 on some cameras, while others produce visible noise at ISOs as low as 800. It’s important to run a test on your camera so you know just where the level of noise becomes objectionable. Shoot the same subject at each of your camera’s ISO settings and inspect the results. Then, when you’re shooting in dim lighting, you can decide whether you’re better off raising the ISO to get the shot or using a slower shutter speed or lower f-stop setting. Remember that slow shutter speeds can result in motion blur, and your f-stop setting affects depth of field, so you have to balance all three concerns when choosing your exposure approach.

Also check your manual to find out what exposure modes allow you to control the ISO setting. On some cameras, you get no say-so when you shoot in the Full Auto mode or the automatic scene modes (Portrait, Landscape, Sports, and so on). The camera automatically adjusts ISO as needed, always favoring a bright-enough exposure over any noise considerations, and you have to use an advanced exposure mode, such as aperture-priority or shutter-priority mode, to regain the ISO reins. But other cameras let you specify ISO in any exposure mode.

Offering the best of both worlds, some cameras give you an Auto ISO setting but enable you to set limits on the automatic adjustment. Here’s how this feature works: You specify your preferred ISO, but you give the camera permission to adjust ISO if it can’t exposure the picture at your chosen f-stop and shutter speed. In most cases, you also can tell the camera not to go above a certain ISO setting and to intervene only if the shutter speed drops below a certain number.

One final ISO comment: Many cameras offer a built-in noise-removal filters. These filters attempt to soften the appearance of noise by applying a slight blur to your photo as it’s being processed and written to your memory card. They may, in fact, help with noise, but they create new problems: First, you lose a little image sharpness due to the blur, and the camera needs more time to process your photo, hampering your ability to fire off a rapid series of shots. So I leave these in-camera filters off and then use my photo editor’s blur tools to try to soften noise if necessary. Doing the repair in a photo editor lets you limit the blur to the worst parts of the image rather than losing sharpness throughout the whole photo.