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All you need to know about noise

2. Juni 2007, 20:04:47 Uhr:

It's well known that images from digital cameras show higher noise when using higher ISO settings, and that small digicams also show higher noise than DSLRs. As a photographer you can accept this as a fact, but if you're technically interested the following questions may come up:

  • Why is there more noise at higher ISO settings?
  • Why is there more noise with smaller digicams?
  • Why is there more noise with longer exposures?
  • What can be done against noise?
To understand this you first need to know how image sensors work in general.

Short introduction to image sensors

In the old days, our image sensors were composed of light-sensitive particles, embedded in gelatine, pasted onto a cellulose triacetate carrier. We called them “film”. Light falling onto these particles caused a chemical reaction, and it was amplified and fixed during processing.

Digital image sensors work similar. There you have light-sensitive, isolated areas on a silicon carrier, called photo sites. When light falls onto these during exposure, electric charge builds up in the photo sites through the photoelectric effect. After exposure, the charge of each photo site is read out, amplified, digitized and stored on a storage medium.

The problem with these photo sites is that charge is not only caused by light falling onto them. Other factors also cause charge buildup, mostly temperature, but also cosmic rays, radioactivity, electricity leaking from neighboring photo sites, etc., and the amplifiers and A/D converters introduce their own noise. This minor part is – on average – more or less constant, i.e. not depending on exposure, and – looking at a single photo site – random. For example, a photo site may be hit by a radioactive particle during exposure, or it's not. This part of the total charge is what we see as noise.

Higher noise at higher ISO

So if noise is constant, why do we see more noise at higher ISO settings?

Now here's a fact that may surprise you if you did not yet dive into this topic before: You can not change the sensitivity of an image sensor. The sensitivity is fixed when the sensor is designed and built, determined by the layout of the photo sites, the materials used and by support structures like microlenses. Changing sensitivity means building a different sensor.

So, you might ask, what is the ISO setting of your camera doing if it's not changing sensitivity? The ISO setting is doing two things:

  1. It changes exposure metering, meaning that at higher ISO settings the camera chooses to expose the sensor less, and
  2. it cranks up the signal amplifiers that sit between the sensor and the A/D converters, to compensate for the smaller exposure.

Of course, the amplifiers can not distinguish between the charge of the photo sites caused by light and that caused by other sources, so it amplifies both. Since noise is more or less constant in absolute terms, it grows relative to the desired signal.

This is also illustrated in the following picture:

On the left side we see the charge of a photo site. The red part is noise, caused by sources mentioned earlier, and the green part is the signal caused by photons, i.e. light. The middle column is the same situation, only at a higher ISO setting. Noise is the same, as mentioned before. However, the signal is smaller, because at higher ISO settings exposure is reduced by the camera. To get the same apparent brightness, the amplifiers compensate for reduced exposure, increasing both the desired signal and noise.

In real life, noise is of course a lot lower compared to the signal. The illustration is only qualitatively correct, not quantitatively.

Higher noise with smaller sensors

With smaller image sensors, like those used in small digicams and cell phones, you also get higher noise. This is not because noise is inherently higher with these. These small sensors contain also relatively small photo sites. The smaller a photo site, the lower its sensitivity. So to reach their nominal ISO rating these cameras have to amplify the signal from the sensor more than other cameras with larger photo sites. This higher amplification is what is causing higher noise.

There are also minor factors responsible for higher noise. Typically the signal processing in these cameras is less sophisticated, and additional noise is actually coming from the amplifiers and A/D converters instead of the sensor.

Higher noise with longer exposures

If you compare photos taken with long exposure times to photos taken with short exposure times, you also often see higher noise. This is true even though total exposure of both is identical and the same ISO setting was used. Why is this?

The reason is again the different sources of signal and noise. The signal is caused by light falling onto the photo sites, and the charge caused by this is the same, no matter if you expose briefly with much light or for a long time with little light. But noise is not caused by exposure but by independent sources. The longer the exposure time, the more opportunities for these other sources to cause noise. A 2 second exposure contains twice as much noise as a 1 second exposure, because temperature etc. can influence the photo sites twice as long.

How to avoid noise

If you want to avoid noise the first thing to do is use a camera equipped with a sensor having a high native sensitivity (typically, a DSLR with a large sensor and large photo sites), and use a low ISO setting. This keeps amplification at a minimum.

You can also use noise reduction, if your camera supports it. With this feature the camera first makes the intended exposure, followed by another exposure of the same length, but with the shutter closed. The second image contains only the noise that occurs at the current temperature with the current sensor. The data of the second image is then subtracted from the first image, resulting in an image with strongly reduced noise (noise is not zero; because of the random nature of noise, the second image is not 100% identical with the noise of the first image). This noise reduction technique is called “dark frame subtraction”, and many cameras offer it for longer exposures.

You can also use software to suppress noise that still ends up in the image. But suppressing noise instead of avoiding it in the first place, and at the same time not losing image detail is a tricky task. You need sophisticated software and know how to use it well.

Other than this, there is not much the photographer can do.

For sensor makers, there are a few things more that can be done. Thermal noise can be avoided by cooling down the sensor. However, this is not practicable for photo cameras, but only for sensors attached to telescopes etc. But camera makers can at least avoid heating up the sensor too much.

Sensor makers can also increase native sensitivity, on one hand by making photo sites large, and on the other hand by increasing quantum efficiency, i.e. increasing the fraction of photons actually causing a charge increase. But quantum efficiency is already high, around 70%. So even if quantum efficiency is increased to 100% (which is unlikely), sensitivity can at most increase by 1/3. And photo sites tend to shrink instead of grow, because of the never-ending demand for higher pixel numbers.

Sensor makers can also improve their signal processing chain, making sure that noise introduced there is as small as possible. Moving towards CMOS sensors instead of CCD sensors is one way, because with CMOS sensors the processing electronics is closer to the individual photo sites.

References and links

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