ISO SENSITIVITY
The ISO setting on your camera is the third part of the exposure triangle. Here’s how it works…
“Each time you increase the ISO, you get a small decrease in image quality”
Digital cameras, of course, do not use film – but the same ISO scale that was used for film is used to control your EOS camera’s sensitivity to light. As the ISO is increased, your sensor becomes more sensitive to light – and this helps you get that fast-enough shutter speed you may need in low light when hand-holding.
This makes ISO a powerful tool, helping you to get sharp shots in a variety of lighting conditions. You’ll find ISO is a simple set of numbers. The base sensitivity of Canon EOS cameras is ISO100. But this can be increased by pressing the ISO button and then rotating the correct dial (on some cameras, ISO is changed through the red menu). The scale is simple, in that doubling the ISO number doubles the sensitivity of the sensor – for instance, increasing the ISO from 100 to 200 means you can use a shutter speed that’s twice as fast, such as 1/250 sec increasing to 1/500 sec.
Each doubling of the ISO increases the sensitivity by a full exposure stop, with the full-stop ISO scale progressing 100, 200, 400, 800, 1600 and so on. The top ISO setting varies depending on the age and cost of your Canon EOS camera, from ISO3200 to ISO102,400 (see right).
The top ISO settings on many models are hidden, and must be expanded in the ISO menu. There’s a good reason for this – each time you increase the ISO, you also get a small decrease in image quality, as boosting the picture signal also amplifies impurities known as noise. This noise shows up as grain and colour mottling in the image – and this gets progressively more noticeable the higher the ISO is set. This is why Canon suggests that the highest ISO speeds produce so much image noise that they should be used with caution!
When to increase ISO
However, pumping up the ISO can sometimes improve your photos, because this lets you use a faster shutter speed for your shots – eliminating camera shake as well as capturing sharp shots of moving subjects. Remember, a grainy picture is better than a blurry unusable one.
Also, a higher ISO can enable you to use a narrower aperture when you’re shooting handheld, increasing depth of field for scenic shots with more depth of sharpness.
Although higher ISO settings are invaluable in low light, they are not essential for all low-light situations. In fact, if you can keep your camera steady, they are often best avoided. If you are using a solid tripod, the slowest ISO setting (ISO100) is usually the best option, because you can then use a longer shutter speed to make up for the lack of light. Similarly, if you are using flash, high-iso settings are not needed (although increasing the ISO increases the effective range of your flash).