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In-depth answer to a basic digital photography question
You’ve probably read or heard about unacceptable noise in digital photos. What does that mean and how does it occur?
A reader asks: “What is ‘noise’? Why is it bad to have noise in a photograph? How can I avoid it?”
All photos by Mason Resnick
In digital photographs, “noise” is the commonly-used term to describe visual distortion. It looks similar to grain found in film photographs, but can also look like splotches of discoloration when it’s really bad, and can ruin a photograph. Noise tends get worse when you’re shooting in low light.
From a technical point of view, noise is the visual manifestation of a lower signal-to-noise ratio, which is measured in decibels. While the amount of noise that you may consider acceptable may be different from what the next guy might accept, but most professional photographers want to see photos with at least a 30dB signal-to-noise ratio.
How much noise is too much noise?
While there are absolute numbers that are used to define levels of noise, the amount of noise that would be considered unacceptable to one photographer might be fine for someone else. A snapshooter, for instance, may not care about noisy images as much as an enthusiast or professional photographer. It also depends on usage. Higher noise might be tolerated in, for example, a nighttime sports action shot, but that same amount of noise would render an architecture photo, where detail and color fidelity are critical, unusable.
The size of the prints you make also factor into the “how much noise is too much” equation. Bigger prints will show noise artifacts more than small ones. Noise that might be huge and distracting in an 11x14-inch print might not look so bad on a 4x6-inch print, and might be almost invisible on a computer monitor.
So, “acceptable” noise is a very subjective concept!
What causes digital noise?
ISO: Higher ISO, which you may need when shooting in low light, is the main culprit in causing more noise. Think of ISO as the gain knob on an amplifier for an electric guitar. The more gain, the louder guitar’s sound, but it also becomes distorted compared to the clean sound of the guitar without gain. In a photograph, the higher the ISO, the more distortion (noise) shows in the image.
Sensor size: When it comes to noise, sensor size matters. Cameras with smaller sensors, such as cell phones and compact cameras, have thumbnail-sized sensors, and on these cameras noise can reach unacceptable levels even at ISO 400. By the time you reach ISO 800 or higher, the picture may end up looking like an impressionist painting and lose sharpness, detail and color fidelity. Cameras with larger sensors, such as DSLRs and Mirrorless Interchangeable-Lens Compact cameras, produce lower grain at higher ISOs. The larger the sensor, the better the grain at comparable speeds.
Pixel density: A sensor with 14 million pixels (megapixesl) will produce more digital noise than an equal-sized sensor with 10 megapixesl. That’s because, in order to squeeze those extra 4 million pixels, the actual pixel size has to shrink, which means each pixel will let in less light (think of smaller apertures in lenses letting in less like than larger apertures). To compensate, the “gain” is turned up, and this causes distortion. Conversely, a larger sensor with 14MP will produce less grain than a smaller 14MP sensor.
Exposure time: Long exposures can introduce static, which can also be a cause of digital noise.
Shadows: If you are shooting in broad daylight at a higher ISO, the grain might not be so obvious…unless you look at the shadow areas. Grain shows up more against darker subjects or backgrounds. It gets even worse if, using image-editing software such as Adobe Photoshop, you lighten an image. Then the grain in the shadow areas will become even more obvious.
The photo above was shot at ISO 4000 with a Canon 7D DSLR at the Kennedy Space Center, Cape Canaveral, Florida. While the image technically has more noise than is considered acceptable, I don’t think the noise level interferes with the image. Look at the 100% detail, right. What do you think?
How can you avoid digital noise?
Shoot at a low ISO: Since higher ISOs produce more noise, choose the lowest ISO you can while still getting a good exposure. For a compact camera, that might mean only shooting at ISO 100. For a DSLR, you might get away with ISO 400-800, or even higher on some recent models. If you shoot only at low ISOs, however, you will likely need a tripod or a flash for low-light photography.
Get a camera with a larger sensor: Give your compact camera to your kids and move up to a DSLR, and you’ll see a major improvement in image quality. DSLR too big or heavy? Consider one of the new breed of smaller interchangeable-lens (MILC) cameras.
Use your camera’s on-board noise-reduction: Many modern digital cameras, both compacts and DSLRs, have built-in noise reduction, and many compact cameras will apply noise-reduction to JPEG images as a default. DSLRs tend to have the option of noise reduction on, off, or on at high or low settings. If you definitely want noise reduction off, shoot in RAW mode.
Use noise-reduction software on your computer: If your camera doesn’t have noise reduction built in, or you prefer shooting in RAW and adjusting the noise levels after the fact, use noise reduction software such as Nik Dfine. The advantage of using noise reduction software over in-camera noise reduction is that you have more powerful tools such as reducing grain selectively to parts of the picture where it’s needed while leaving other areas alone.
Shadow play: Digital noise can be seen more readily in shadowy areas of shots. In this photo shot indoors at ISO 2000 with a Canon 7D at the Kennedy Space Center (full shot above) notice in the 100% how the grain is more obvious in the darker areas of the image than the lighter ones.
How can I find out how well my camera handles digital noise?
DxO Labs' DxOMark service tests digital cameras for overall image quality, but they also do specific tests to determine the camera’s noise levels at all ISOs. You can search the Sensor Rankings database for your camera model. The DxOMark Sensor Score Sports (Low-Light ISO) will tell you the highest ISO at which DxO measured acceptable noise levels (which they define as 30dB or higher). Click on the “SNR 18%” tab to view a graph that shows the camera’s signal-noise ratio, measured in dB.
DxO only measures cameras with RAW capabilities, so they won’t have info for most compact cameras. You can assume, however, that noise will start to increase on a typical compact camera by ISO 200.
The good news is that the latest generation of digital cameras does a better job producing images with lower noise levels. As technology evolves, future generations may reduce noise levels even more or eliminate it altogether.