I was always wondering about this because I don’t understand why I can see random colorful pixels when shooting something in the dark…why is that?
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Okay, so there’s three main elements involved with creating a digital image (or even film). Overall, you have light passing through a lens which focuses the light onto a light-sensitive sensor/film which records the image.
Aperture: How big the hole is behind the lens that allows light past – bigger hole = more light. The tradeoff is that the larger the aperture, the narrower the field of focus is (basically, the range of distance that is in-focus to the camera)
Shutter Speed: How long the image sensor is exposed to the “image”. In traditional film and DSLR cameras, there is a physical shutter that opens to allow light in then closes again. This is done in software with smartphone cameras. Slower shutter speed = more light passing through = brighter image. The tradeoff is you’re more prone to blurry images with a slower shutter speed.
ISO: Sensitivity of the sensor/film – **This is what causes the “noise”**. Basically, the more you crank up the sensitivity, the more light is registered on a particular photosensitive cell, but the more prone to random errors you are.
So, let’s say you’re taking a picture in low light. In order for the image to look just right, you have to balance the tradeoffs between aperture, shutter speed, and ISO. You could open the aperture all the way, but then if you wanted a person and the wall behind them fully in focus that wouldn’t do. You could leave the shutter open for longer, but if you don’t have a perfectly steady hand or if anything you’re taking a picture of moves, the image might get some motion blur. You could crank the ISO up to max, however then you start getting more noise.
In reality, you’re balancing all three based on the situation at hand. Smartphones try to do this automatically but at the end of the day, the more light you have for a photo the better.
The challenge with taking a photo in the dark is obviously that there’s less light for the camera to work with. One way to go about adjusting for this is to make the film/sensor more sensitive to light. In film cameras this was done by actually using a different film with a different chemical composition that was more sensitive, this sensitivity was measured in ISO. With the introduction of digital photography this ISO measurement was carried over, but instead of measuring film sensitivity it now controls gain on the signal coming from the sensor. Kinda like a microphone, turning up the gain boosts the signal coming in making it louder, but too much gain and a loud noise will get all distorted. The same happens for the camera sensor, high ISO on a sunny day can over-expose the image.
The reason for noise in a dark image is because of a quirk of the high ISO. There is essentially less information for the camera to work with in a dark photo, think of it like this: Say each pixel on the camera sensor can tell you how much light hit it on a scale from 0-10. In a dark picture maybe each pixel only measured a 0, 1, or 2. Turning up the ISO is kinda like multiplying each of those values by 3 in the final image. The problem with that is the only values we can get out now are 0, 3, and 6. In a picture taken on a bright day they’ll be a nice gradient between any light and dark spots ranging from 0 to 6. But our boosted image has all these artifacts of sharp brightness changes, and we see noise in our picture.
One addition: the way around this is to increase the length of exposure. A long exposure lets more light reach the sensor, and allows the ISO to be kept low enough to avoid noise.
Edit: another thing. Film cameras also had “noise” at higher ISO but it causes the film to look grainy, not noisy. This is because the light causes actual grains of silver to crystallize on the film when it hits it. The higher ISO film stock works with less light by growing bigger grains of crystals, so it looks grainier.
Transistors and resistors create their own voltage and current noise just by existing and carrying power. These side effects can mimic the desired minute electric output of a photon striking the electronic sensor.
Those random colorful pixels are mainly caused by two effects going on in the camera, one is called dark current, the other read noise. There is also something called shot noise.
The way a digital camera sensor works is by having a load of pixels, each of which have an area sensitive to light on them. When a photon of light hits the pixel it is absorbed and causes an electron to be excited up in energy. At the end of the exposure you measure how much excited charge there is in each pixel, this tells you how much light hit it, and therefore how bright it should be in the final image.
Dark current is where electrons become excited due to something other than light striking them, this could be due to thermal effects for example where the random jiggling of energy in the pixel gives an electron enough energy for it to jump up and be excited. This results in fixed ‘hot pixels’ and randomly brighter pixels throughout the image. This happens in pretty much any electronic circuit with a photosensitive element causing a flow of charge. Since this is a flow of charge (current) being induced without light (dark), we call it dark current.
Next up when the charge is being read off the pixel to be measured the circuitry involved will add some amount of noise, this noise is being added when we read off the values of each pixel and so we call it read noise.
The final thing is shot noise, this isn’t like the others discussed in that it doesn’t come from the electronics or way we detect light, but from the nature of light itself. Light comes in little quantized packets called photons, this means that it isn’t a continuous thing. In the dark you’ll get random pixels which happened to end up with more photons being detected than others around them due to the random and quantized nature of light, this results in a fine variation in brightness throughout the image.
You’re going to get a lot of answers talking about ISO and how having a high ISO makes the image more noisy. This isn’t strictly speaking correct. Turning up the ISO is basically turning up the ‘volume knob’ of the image, it amplifies the signal from the sensor before it gets converted to a digital image. Because the signal is amplified the image becomes brighter, as does any noise in it. This makes the image **look** more noisy because the noise is brighter, but that actual amount hasn’t increased. In fact turning up the ISO decreases the read noise talked about earlier so it actually ends up making the image less noisy.
tl;dr Noise is due to the way we detect light, the way we read off that detection and statistical effects.
Cameras aren’t very sensitive in the dark. The picture you get would ordinarily be very very dark, but the brightness is amplified to get something more viewable. Unfortunately, due to the camera’s insensitivity in the dark, this reveals a lot of noise.
You don’t see that noise in daytime photos because there’s so much light coming into the camera, you don’t need to rely on amplifying the bare minimum that the imagine sensor can handle. The noise is entirely drowned out by the good image signal.
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