ISO in Photography: How Sensitivity Affects Your Images

What is ISO in photography?

ISO is one of the three pillars of exposure, alongside aperture and shutter speed. It measures how sensitive the camera's sensor is to light. The name comes from the International Organization for Standardization, which defines the scale. In practice, your camera's ISO setting tells the image processor how much to amplify the signal from the sensor.

When you raise the ISO, the camera brightens the image by boosting the recorded signal. This is similar to turning up the volume on a quiet recording. The exposure triangle, explained in our exposure triangle overview, shows how ISO works with the other controls to determine the final brightness of your photo.

How ISO increases brightness

Each full stop of ISO doubles the brightness. If you move from ISO 200 to ISO 400, the sensor's signal is amplified twice as much, producing a noticeably brighter image. This is why photographers raise ISO when shooting indoors, at night, or in other low-light situations where they cannot slow the shutter speed or open the aperture further.

Modern cameras offer a wide range of ISO values, from a low base setting to high values that can capture scenes in near darkness. The trade-off is that the amplification also amplifies the electronic noise that is always present. That noise appears as speckles, often called grain, and it reduces the fine detail in your photograph.

The trade-off: noise and image quality

As you increase ISO, the signal is amplified, but so is the random interference from the sensor's electronics. The result is noise, which shows up as colored specks or a gritty texture in shadow areas and smooth tones. Higher ISOs produce more noise, and the effect becomes more visible when you zoom in or make large prints.

Noise is not always bad. Some photographers intentionally use it for a film-like character. However, for most purposes, a clean image is preferable. The key is to find the highest ISO you are comfortable with, while still getting the exposure you need. You can also use noise reduction software during editing, but this may soften fine details.

Sensor size and ISO performance

Larger sensors generally handle high ISO better than smaller ones. The physical size of the sensor allows for larger light-capturing pixels, which tend to produce less noise when the signal is amplified. A full-frame camera will often look cleaner at ISO 6400 than a compact camera at the same setting.

This does not mean you cannot shoot in low light with a small sensor. Modern processing is impressive, and the differences are smaller than they used to be. Still, if you frequently shoot at night or in dark interiors, you may want to consider a camera with a larger sensor. For a breakdown of how sensor size affects image quality, see our sensor size guide.

When to use low ISO and when to use high ISO

Use the lowest ISO possible, typically 100 or 200, when you have plenty of light and you can use a fast shutter speed or a narrow aperture. This is common in daylight landscapes, studio portraits, and any scene where you want maximum detail and no noise.

Raise the ISO when the light is insufficient and you cannot keep the shutter speed fast enough to avoid blur. This happens during indoor events, concerts, astrophotography, or any handheld shot at dusk. The right ISO is a balance: choose the lowest value that still gives you a sharp, correctly exposed image.

Using Auto ISO and setting limits

Auto ISO lets the camera select the ISO for you based on the light and your other settings. It is a useful aid in changing conditions because it reacts to the scene. You can usually set a maximum ISO and a minimum shutter speed, so the camera will not use a value that produces too much noise or a shutter speed that is too slow.

For moving subjects, you might set the minimum shutter speed to something like 1/250 sec. For static scenes, you can allow a slower speed and keep the ISO lower. Many photographers use Auto ISO as a starting point and then review the results to make manual adjustments when needed.

ISO for video and stills

The same principles apply to video, but there is an additional consideration: you cannot use a slow shutter speed as easily because it would create motion blur. Video often requires a shutter speed close to twice the frame rate, so you may need to raise the ISO in low light. Many cinema cameras have dual native ISO, which provides a second clean gain stage at a higher value.

Whether you shoot stills or video, the guidelines are similar. Start at the base ISO, increase only as needed, and watch the noise in the shadows. For cameras that excel in low light, take a look at our best full-frame mirrorless cameras.

What to pick for your use

Questions

What is the base ISO on a camera?

Base ISO is the lowest native sensitivity setting on a camera. It gives the cleanest image with the least noise and the most dynamic range. On most cameras, the base ISO is 100 or 200. Use it whenever you have enough light to expose properly without raising the ISO.

Does higher ISO make photos brighter?

Yes. Increasing ISO amplifies the sensor's signal, which makes the image brighter. Each full stop doubles the brightness. However, it also amplifies noise, so the image quality may decrease. The goal is to use the lowest ISO that gives you the exposure you need.

Why is my photo noisy at low ISO?

Noise can appear at low ISO if you underexpose the image and then brighten it in editing. The sensor's read noise becomes more visible when you push the exposure. To avoid this, try to expose correctly in the camera or use a slightly higher ISO to get a brighter capture.

What ISO should I use indoors without a flash?

There is no single number that works for every situation. Start with an ISO between 800 and 1600, then check your shutter speed and aperture. If the image is still dark or the shutter speed is too slow, raise the ISO until you get a well-exposed, sharp photo.

Is it better to raise ISO or use a slower shutter speed?

It depends on your subject. If you are photographing something moving, a slow shutter will blur it, so raising ISO is better. If your subject is still, you can use a slower shutter and keep the ISO lower for less noise. Use a tripod when you slow the shutter to avoid camera shake.