Noise is the grainy, speckled texture that appears in photos taken in dim light. It is closely linked to ISO, but the link is often misunderstood. Knowing where noise really comes from makes it much easier to control.
What noise looks like
There are two kinds, and they usually appear together.
- Luminance noise is a fine grain of lighter and darker specks, similar to film grain. In moderate amounts it is not unpleasant.
- Colour noise (chroma noise) shows as random red, green and purple blotches, most visible in dark areas. It looks artificial and is the more distracting of the two.
Noise also hides fine detail. Hair, fabric texture and distant foliage turn soft and smeared as noise increases.
Why higher ISO means more noise
The real cause of noise is a lack of light. Light arrives at the sensor in a slightly irregular way, and the camera electronics add small errors of their own. When plenty of light is collected, these irregularities are tiny compared with the signal and the image looks clean. When very little light is collected, they become a large part of the signal.
Raising ISO does not create that noise; it amplifies a weak signal together with the noise already in it. You normally raise ISO precisely because there is little light, so high ISO and noise go together. The practical conclusion is this: the way to reduce noise is to give the sensor more light, through a wider aperture or a longer shutter speed, whenever the subject allows it.
Noise is worst in the shadows
Dark areas of a photo have received the least light, so they contain the most noise. This has an important consequence. An underexposed photo taken at ISO 800 and brightened later on a computer will usually look noisier than a correctly exposed photo taken at ISO 3200. Brightening in editing is amplification too, and it is applied to shadows that had very little light to begin with.
What else affects noise
- Sensor size. A larger sensor collects more light in total for the same exposure settings. A full-frame camera is typically about one stop cleaner than an APS-C camera, and a phone sensor is several stops noisier.
- Viewing size. Noise that looks severe at 100% on a monitor is often invisible in a small print or on a phone screen.
- Long exposures and heat. Exposures of many seconds can produce bright “hot pixels”, especially when the camera is warm.
Rough expectations by ISO
| ISO | Typical result on a modern APS-C or full-frame camera |
|---|---|
| 100–400 | Clean; noise not visible in normal use |
| 800–1600 | Slight grain in shadows; fine for large prints |
| 3200–6400 | Visible grain, some loss of fine detail; good for screens and medium prints |
| 12800 and above | Strong noise, weaker colour; use when there is no alternative |
How to keep noise down
- Open the aperture first. Going from f/5.6 to f/2.8 allows ISO 1600 instead of ISO 6400.
- Use the slowest shutter speed that still gives a sharp picture, or use a tripod for still subjects.
- Expose correctly in the camera instead of brightening later.
- Shoot RAW. It gives far better noise reduction in editing than a JPEG.
- Apply noise reduction in moderation. Colour noise can be removed almost completely; heavy luminance reduction makes skin and textures look like plastic.
Common mistakes
- Underexposing deliberately to “keep the ISO low”, then brightening the file afterwards.
- Judging noise only at 100% magnification instead of at the size the photo will be used.
- Leaving in-camera high ISO noise reduction on its strongest setting, which smears detail in JPEGs.
- Blaming ISO when the real problem is a small aperture such as f/8 used indoors.
Practice exercise
- Place the camera on a table or tripod in a dim room, aimed at a scene with both dark and bright areas.
- In Aperture Priority at f/5.6, shoot the scene at ISO 100, 800, 3200 and 12800.
- View the files at 100% and compare a shadow area, then compare them fitted to the screen.
- Take one more frame at ISO 800 with exposure compensation at −2, brighten it by two stops in editing and compare it with the ISO 3200 frame.
Next in this series: High ISO vs Low ISO Explained, Best ISO Settings for Beginners and Overexposure vs Underexposure Explained. You can also browse all photography lessons.
