Getting to grips with...ISO

ISO sensitivity

Once you've mastered the aperture and shutter speed controls of your camera, the next key element to achieving great pictures is to understand the benefits and pitfalls of controlling the light sensitivity of your camera's sensor.

Let's start by looking at the benefits. By being able to increase the sensitivity of your camera's sensor to light, less available light is needed to correctly expose the shot. This allows you much more freedom in the way you shot, allowing you to use faster shutter speeds or smaller apertures when they'd normally not be enough light to expose the shot correctly. For instance, rather than being stuck with a slow shutter speed when shooting indoors, you can crank-up the sensitivity of the sensor, allowing you to use a faster shutter speed and still expose the shot correctly - without having to alter the aperture.

They'll also be times though when you want a low sensitivity - long shutter speeds for blurring movement aren't suited to high sensitivities, while it's a similar story for shallow depth-of-field shots on a bright day. With your aperture wide open, you'll want a low sensitivity selected to avoid overexposing the shot. All this isn't a problem because we can control the sensitivity of our camera's sensor to light for each specific shooting situation, choosing whether we need it to be less or more sensitive.   

Your camera's sensitivity is referred to as ISO - a throwback to the days of film, but rather than referring to how sensitive the film is to light, it's a measure of how amplified the electrical signals are that have been converted from light hitting the sensor.

ISO 100 is a typical baseline rating for a camera, with an increase to ISO 200 increasing the sensitivity by one stop. This will allow you to either use a shutter speed one stop faster (1/125th of a second at ISO 100 would be shortened to 1/250th of a second at ISO 200 for example), or an aperture one stop smaller - so rather than shooting at f/5.6 at ISO 100, you can shoot at f/8 at ISO 200 and maintain the same exposure as before. Each time you double the ISO, the sensitivity of the sensor increases by one stop, so at ISO 3200, you can shoot with a shutter speed five times faster or an aperture five stops smaller than you could at ISO 100. This means that 1/30th of a second at ISO 100 would be shortened to 1/1000th of a second at ISO 3200, or rather than shooting with an aperture of f/2.8 at ISO 100, you'll be able to shoot at f/16 at ISO 3200.

ISO sensitivities are an industry standard measurement across all manufacturers, so ISO 100 on a Nikon DSLR will be the same as ISO 100 on a Canon DSLR, or a Sony compact for that matter. What does vary is the ISO range from camera to camera - a typical ISO range of a modern DSLR is 100-6400, but some high-end models offer expanded ISO ranges up to 102,400! Naturally, the broader the ISO range, the more flexibility you have when shooting in low light. There are downsides however - turn over the page to find out more...