On Sensor Noise & ISO
Every so often, the question of noise comes up on the forums. While the basic concepts of light control (aperture and shutter speed) are discussed to death, the question of digital noise on a DSLR is rarely granted more than a cursory sentence indicating that higher ISO means higher noise.
I’ll try to explain here, some of the concepts behind the digital sensor and what we see as “noise”. I’d also like to note, that while some of the concepts are similar, the article below is NOT applicable to photographic film grain when using chemical recording mediums. (like 35mm film).
The sensor in your camera is, at it’s most basic, a light amplifier and collector. It is in charge of collecting photons that are focused upon it by the lens system, and subsequently transforming this inherently analogue data into a stream of digital bits to be interpreted by your camera’s processing circuitry.
Natively, the sensor in your camera operates at a particular sensitivity. This sensitivity or “noise floor” is the lowest energy photon that the sensor will register as significant, and then pass along as a piece of digital information. (The native sensitivity of most DSLR sensors ranges from around ISO 50 to ISO 200.) Photons with a low energy level will not meet this minimum level and will be disregarded by the sensor circuitry.
The rub, comes when using ISO ratings higher than the sensors “native” level. As you dial up the sensitivity rating, the sensor begins to amplify the photons it receives in order to have them meet the noise floor. It is for this reason, you are then able to take pictures in lower light levels. The lower energy photons being reflected from your subjects are now amplified enough that the sensor considers them valid information and records them. This is almost directly analogous to how an audio amplifier works. It takes a weak signal and using electrical tricks, boosts the level to something more perceptible to humans.
It is through this audio analogy that we find the first common cause of noise. Much like how turning up the volume on a cassette tape increases the white audio noise along with the music, turning the “volume” up on a digital sensor increases the photon noise being recorded by your camera.
Photons of light are always bouncing everywhere. Even in very dark locations, there are uncountable photons bouncing off everything. Normally these photons are of such low energy, they cannot be perceived by the human eye and also fall well below your sensor’s noise floor. These photons can be of any color of light and are bouncing erratically like pin balls from object to object until they’re either absorbed or translated into another form of energy. (such as heat).
It should now be obvious why raising the ISO on your camera increases the amount of noise present in the image. Those errant photons that would normally be far too weak to register, have now been boosted into the realm of significant information. This is why noise appears as tiny dots of color or white against otherwise dark objects. There is little you can do to eliminate high ISO sensor noise on your camera short of upgrading it. Manufacturers have made incredible gains in the electrical circuitry of sensors, allowing them to produce useful photographs at sensitivity ratings unheard of in years gone by.
The second common cause of sensor noise is the Long-exposure noise. This is a little easier to explain. Along with those low energy errant photons bouncing around, every now and then, there’s a random high energy photon mixed in. Much like a rogue wave on the ocean, these photons have enough energy to register on the sensor, even at low sensitivities (low ISO numbers). Since most photographs are taken with exposures well under 1 second, the chances of encountering one of these “rogue” photons is relatively rare. As exposure gets longer, the odds of encountering one (or many) increases exponentially.
Interestingly, the chances of encountering a rogue photon also increase significantly as the distance to your subject matter decreases. Without something to bounce off (and then into your sensor) most of these rogue photons are absorbed or deflected by the atmosphere. Long exposure noise of a distant landscape will generally be significantly less than that found in an enclosed room. For a scene with no movement taken from a tripod, the choice of high ISO vs long exposure mainly comes down to your distance from subject matter and the specific high ISO capabilities of your camera.
Hopefully by now, you understand the basics of digital sensors and why higher noise is produced in some situations.
