The term ‘noise’ in the way we are using it here originally came from the world of radio… more specifically old fashioned High Frequency radio use. The transmitted signal would be received but those long wavelengths were also coming from lots of other sources; car engines, refrigerators, overhead power cables, the atmosphere, thunderstorms and even outer space… indeed almost any electrical effect, natural or man made. If these other sources were strong enough they would swamp the transmission and the strains of the BBC World Service would fade into the hiss.
The effect of image noise is similar but a better analogy is to remember our days of using wet film. In order to shoot in low light we used fast film that had large crystals of the light sensitive silver halide in the film emulation, big enough to react quickly to the low light levels. The disadvantage was that these fast films produced grainy, blurred negatives. When we wind up the ISO setting of our digital cameras we get a similar effect, albeit for a different reason.
When we increase the ISO setting of our cameras we are asking the electronics to amplify the light signals received by the sensor so that we can get a viewable image. This amplification (increase in sensitivity) has a down side. The millions of light sensitive pixels on our sensors cannot all be made perfectly. Some will react to light a little more than its neighbour and some a little less. As we amplify the output of the sensor these differences become more apparent and errors appear. We see this as noise.
An acceptable level of noise will depend on our personal tolerance to imperfections. As a general rule however, if the level of noise exceeds the level of detail in the image it will probably be unacceptable and visa versa.
This exercise asks us to shoot an identical image throughout the ISO range of our cameras, varying the speed to obtain the same EV (Exposure Value) but going no lower than ½ sec to avoid introducing long exposure noise. Below are my images with areas of the ceiling, the dark doorway and the bust details magnified. The images were shot in RAW but haven’t received any additional processing, just a basic conversion to JPG.
At ISO 100 the ceiling looks smooth, the dark areas unblemished and the detail is clear.
By ISO 400 a small amount of noise is appearing in the smooth ceiling area but not enough to affect the natural texture, indeed without the ISO 100 shot to compare it might well have gone unnoticed. The door and bust details are generally unaffected.
For a lot of cameras, ISO 1600 would be almost unusable but I am fortunate to be using the Canon 5DII and even at this level the noise isn’t too noticeable, until you examine the smooth ceiling which is now quite obviously suffering from a rash of speckles.
By ISO 3200 the ceiling is developing it’s own patterns caused by the noise that has appeared. The frame of the door has also become quite speckled, however the texture and colours mixed in with the bust do a good job at disguising the effect.
ISO 6400 is the practical maximum of my camera. It does have specialist levels of H1 (ISO 12800) and H2 (ISO 25600) but these should be used in conjunction with the High ISO speed noise reduction C.Fn settings which would skew the findings of this exercise. At ISO 6400 noise is apparent in most areas of the image and it gives the entire image a mottled, rough textured finish. The overall image is still quite acceptable at a reduced size but close up it definitely suffers.
High ISO settings will always degrade an image to one degree or another, particularly in areas of smooth texture that will become artificially grainy. However, if the situation demands a speed or aperture setting that forces the use of high settings then it is the lesser of the evils. In some situations, the grainy quality introduced by a high ISO can help to create a period feel to an image but usually when combined with a form of monochrome to complete the illusion.
The images above had only the most basic RAW to JPG conversion but with a little effort even a high ISO image can be corrected to become quite acceptable. Below are two magnified images taken from the set above, one is from the ISO 100 photo and one from the ISO 6400 one.