Choosing between a low and a high ISO is a trade. Low ISO gives the best image quality but demands a lot of light. High ISO lets you work in dim conditions but costs some quality. Neither is right or wrong; the correct choice depends on the subject and the light.
What counts as low and high
- Low: ISO 100 to 400
- Medium: ISO 800 to 1600
- High: ISO 3200 and above
These bands shift with the camera. On a recent full-frame body ISO 6400 is quite usable, while on a small-sensor compact ISO 1600 already looks rough.
Side-by-side comparison
| Low ISO (100–400) | High ISO (3200+) | |
|---|---|---|
| Noise | Very little | Visible grain and colour speckle |
| Fine detail | Maximum | Reduced |
| Dynamic range | Widest; highlights and shadows recover well | Narrower; less room for editing |
| Colour | Rich and accurate | Slightly flatter |
| Light needed | A lot, or a tripod | Little |
| Shutter speeds possible | Slow in dim light | Fast even in dim light |
When low ISO is the right choice
- Outdoors in daylight, for almost any subject.
- Landscapes, architecture and product photos taken from a tripod. With the camera fixed, a shutter speed of several seconds is no problem.
- Studio or flash photography, where you control the amount of light.
- Deliberate long exposures, such as smoothing water or recording light trails.
- Photos intended for large prints or heavy editing.
When high ISO is the right choice
- Moving subjects in poor light: indoor sport, children playing, dancing at a wedding.
- Handheld shooting indoors or at night where a tripod is not practical.
- Places where flash is not allowed or would spoil the atmosphere, such as museums, ceremonies and concerts.
- Long telephoto lenses, which need fast shutter speeds to avoid shake.
- The night sky, where stars start to trail after roughly 15 to 20 seconds, so ISO 3200 or more is needed.
The same scene, two approaches
Consider a city street at dusk, photographed at f/4.
- Low ISO: ISO 100 at 1/4 s. The file is clean, but a handheld shot will be blurred and any walking people become streaks. On a tripod this gives a high quality image with a sense of movement.
- High ISO: ISO 3200 at 1/125 s. The file has visible grain, but the shot can be taken handheld and passers-by are sharp.
The total brightness is the same. What changes is whether motion is frozen and whether a tripod is required. The subject decides: a still building favours low ISO, a moving person favours high ISO.
A simple decision rule
- Decide the aperture you want for the picture.
- Decide the slowest shutter speed that will keep the subject sharp.
- Use the lowest ISO that makes those two settings possible.
Following this order means ISO is never higher than it needs to be, and never so low that the photo is blurred.
Common mistakes
- Using high ISO on a tripod for a still subject, which throws away quality for no benefit.
- Staying at ISO 100 indoors and accepting 1/15 s for a portrait. The person will move and the photo will be soft.
- Using high ISO in bright sun. The camera may reach its fastest shutter speed and still overexpose.
- Assuming a noisy photo is a failed photo. Many memorable pictures are grainy; very few are blurred by accident.
Practice exercise
- At dusk or in a dim room, photograph a moving subject such as a person walking or a ceiling fan at ISO 100 in Aperture Priority.
- Repeat at ISO 6400 without changing anything else.
- Compare motion blur in the first frame with noise in the second.
- Find the intermediate ISO where the subject is just sharp. That is the best compromise for that scene.
Next in this series: Best ISO Settings for Beginners, How ISO Affects Image Noise and How to Balance ISO, Aperture and Shutter Speed. You can also browse all photography lessons.
