Light is both a photographer’s best friend and worst enemy. Consider what happened to me recently, for instance. I visited a wild animal park with the hope of taking some pictures of wolves, cougars, and bears. Unfortunately, it had rained all morning, and the sky was dark and overcast when I arrived at noon.
As I prepared to take my first photo, I found that the camera was so starved for light that it wanted to set the exposure at about 1/10 second–far too slow to get a sharp image of active animals, especially since I was hand-holding the camera.
So what did I do? I increased the ISO.
ABCs of ISO
As you might recall from the days of film cameras, film comes in a variety of speeds. Film speed is measured using a numbering system called ISO (sometimes also referred to by its geezer name, ASA). Lower ISO numbers are known as “slow” films, which are less sensitive to light, while higher ISO numbers are more sensitive to light. It’s pretty typical to shoot with ISO 100 or 200 in normal daylight, and use 400 film for lower light photography. Super-fast films like ISO 800 and even ISO 1600 are available for photography in near darkness.
Here’s the secret code that governs film speed: doubling the ISO number of the film doubles its sensitivity to light. So ISO 200 film needs half the light to take the same picture as ISO 100 film. ISO 400 film needs a quarter of the light that ISO 100 needs. In other words, if your camera would try to capture a particular low-light scene with a shutter speed of 1/15 second at ISO 100, you could capture it at 1/60 second at ISO 400. That’s an incredibly powerful capability that means the difference between getting a blurry mess and a sharp photo.
What ISO Means for Digital
Big deal, right? This is the world of digital now, not film. We don’t buy memory cards with ISO speeds marked on them the way we used to pick film boxes for their ISO numbers. So what does all this have to do with us?
As it happens, you can control the sensitivity of your camera’s light sensor by changing the camera’s ISO setting. By default, most digital cameras have an ISO somewhere around 100. If you need more film speed for low light conditions, getting more sensitivity out of your camera is as simple as selecting a higher ISO from the camera’s menu. Most digital cameras offer a range of ISO values, such as 100, 200, and 400. Some cameras go much higher–all the way to 3200, in fact. Just dial in the ISO you need. By increasing the ISO, I was able to get pictures like this wolf at the animal park–but at the default ISO, everything would have just been a blurry, slow-shutter mess.
The Dark Side of Film Speed
That sounds great, right? So I know what you’re thinking: If higher ISO film is more light sensitive, why not always use a high ISO number?
Unfortunately, light sensitivity brings its own baggage. High speed film was notoriously grainy. As you increased film’s sensitivity to light, the light-sensitive grains of silver halide in the film emulsion got bigger and more noticeable. In the final print, that manifested itself as grain–irregular, pixel-like elements in the photo that took away the photo’s smooth, high-resolution look. Ironically, in the world of digital, we have almost exactly the same effect when choosing ISO values. Increasing the sensitivity of the camera’s light sensor introduces digital noise into the picture. Similar to film grain, noise is random pixels of color.
You may not notice noise from a distance, but if you zoom in on a digital photo on the computer screen or look closely at a print, it’s hard to miss. All digital images have some amount of noise, but it gets worse as the ISO goes up. Check out the small detail of a high-ISO photo, for instance. The noise–appearing in the form of colored speckles–is quite apparent.
So what should you do about ISO? I suggest shooting at the lowest possible ISO all the time. In ordinary shooting conditions, stick with the camera’s lowest ISO level, since that’ll give you the least digital noise. But when you notice that the camera is recommending a really slow shutter speed, crank up the ISO. Just remember to drop it back down to the lower value when you’re done, so you don’t accidentally capture a month’s worth of pictures at ISO 800.
Another thing to keep in mind: most digital cameras don’t allow you to adjust the ISO (or any other setting for that matter) when you’re in Automatic exposure mode. To tweak the ISO, you’ll want to be in Program, Shutter Priority, Aperture Priority, or a Scene mode.
Want to learn more? Check out my recent Digital Focus newsletter on minimizing the effects of digital noise.