ISO is one of the three pillars of photography exposure — alongside aperture and shutter speed — and understanding it will help you take control of your camera in any lighting situation. Get it right and you'll capture clean, detailed images even in challenging conditions. Get it wrong and you'll end up with grainy, noisy shots that are hard to recover in editing.
What is ISO?
ISO refers to your camera sensor's sensitivity to light. A low ISO number (like ISO 100) means the sensor is less sensitive — ideal for bright conditions. A high ISO number (like ISO 3200 or above) means the sensor is more sensitive — useful in low light, but at the cost of increased digital noise (grain) in your images.
Low ISO (100 – 400)
Use a low ISO whenever you have plenty of light — outdoors on a sunny day, in a well-lit studio, or when using flash. Low ISO produces the cleanest, most detailed images with the least noise. It's the default starting point for most shooting situations.
High ISO (800 and Above)
Raise your ISO when light is limited and you can't use a slower shutter speed or wider aperture — for example, shooting indoors without flash, at a concert, or in low-light street photography. Modern cameras handle high ISO remarkably well, with many full-frame mirrorless cameras producing usable images at ISO 6400 or beyond.
ISO and Noise
Every time you double your ISO (e.g. from ISO 400 to ISO 800), you double the sensor's sensitivity but also increase the amount of digital noise in your image. The key is to use the lowest ISO that still gives you a correctly exposed image with an acceptable shutter speed — this is called your "base ISO" approach.
Auto ISO
Most modern cameras offer an Auto ISO mode, which automatically adjusts sensitivity based on the available light. This is a great tool for fast-moving situations like events or street photography, where you want to keep your aperture and shutter speed fixed while the camera handles exposure. You can usually set a maximum ISO limit to keep noise under control.
ISO and Exposure
ISO works together with aperture and shutter speed to control exposure. Doubling your ISO (e.g. from ISO 400 to ISO 800) doubles the sensor's sensitivity — equivalent to opening your aperture by one stop or halving your shutter speed in terms of exposure.
Keep Learning
Now that you understand ISO, check out our Simple Guide to Shutter Speed and Simple Guide to Aperture to complete your understanding of the exposure triangle. Ready to put it all into practice? Browse Ted's range of mirrorless cameras online or visit your nearest Ted's store.