Grain and Noise

By ralph

Three aspects of a “quality” photographic image that we’ve come to look for are full tonality, sharpness and a lack of grain. At first blush it would seem that sharpness would be the principal area where digital images still have to catch up with film but since “file size” as measured in mega pixels,( the index of detail in digital images), keeps increasing due to lively competition, this shortfall is rapidly disappearing. Since each pixel has only one tone you might think that they have an automatic superiority in smoothness over our familiar, more or less grainy film images. This is generally the case when shooting high brightness subject matter but the situation can change radically in low light and with long exposures.

It seems that CCDs (image chips) generate a constant, low level of electronic noise which builds up as the exposure goes on.. That means a short exposure in bright light shows very little, but in a very long exposure in dim light the accumulating noise can be quite noticeable and noise in a digital image looks an awful lot like grain in a film image. Even though they represent old and new technologies grain and noise have many similar behaviors. For instance most digital cameras offer a choice of different ISO settings, a huge convenience because you don’t have to change the roll of film to meet different situations – but it turns out that noise increases as you push the numbers higher, just as grain of a film increases as you either use a higher ISO film or push the same film in processing.

It also seems that there are large differences in noise levels between the various makes of cameras and images chips. The overall size of the chip appears to have some effect. The larger ones, generally near or over 25mm in length have much lower noise levels and in consequence the cameras that use them offer ISO speeds up to 1000 or more. The smaller (well under 25mm.) chips used in the “prosumer” level cameras, often betray their noisiness by limiting ISOs to 320 or less. In the first group you find the “pro” cameras like the Nikon D1x, Canon EOS D1 and D30, and the Fuji S1 (and the newer models). Users of these cameras report very little noise even in exposures of a few seconds. The Kodak DCS 760 seems to do less well despite similar specs, but the lower price Kodak DCS 720 which puts fewer but larger pixels on a similar size chip seems to be the runaway speed champ achieving ISO 3200 and more.

The Olympus E-10 and 20 are examples of prosumer cameras with a 320 ISO speed limit. Sure enough, some users report considerable, even objectionable noise when taking night shots, although the problem is less likely in the E-20 which uses a neat strategy for reducing it. Since noise is totally random a double exposure laid on top of the first exposure would tend to smooth it out as random “high spots” cancel out random “low spots”. So the E-20 makes an extra black exposure to cancel out much of the dark noise. Many other cameras use a similar strategy by taking unexposed pixels around the margin of the chip and use their noise to reduce the noise in the image area.

Multiple whole exposures also would be effective in canceling noise – if you had a rock steady subject like a still life you could shoot it two, four or more times to reduce the noise, and let the software hold the image tones at normal. This is the strategy used by film scanners that permit multiple scans of a given image. The image tones add up but the noise cancels out more and more with increasing scans. Since this opens up more clear, dark tones it effectively expands the overall density range in f stops, (in digital lingo it’s called the dynamic range).

It seems that the noise that arises in the CCDs comes from thermal excitation, either heat from the environment or from its own operation. That opens up another possible strategy for improvement – actively cool the ima
chip. One of the earliest professional digital camera backs, the Leaf scan back had active thermoelectric cooling in addition to relatively large pixels and was able to cover a density range of 11 f stops,(equivalent to a 3.3 dynamic range) and is still the best in the field. Active cooling is used in some of the high end scanning equipment for the same reason.

This factor might help explain the great differences in noise that various photographers have reported from similar equipment. For instance photographer A might shoot a night view of a skyline on a cold night and get a fairly clean image while photographer B with the same camera might shoot a hot but dim interior (with a similarly long exposure) and get a messy, noisy image. This is speculative, since I haven’t seen any data relating specific temperatures and noise levels (how much heat is needed to produce a visible difference), but it’s something to watch for if you get into a similar situation.

The parallels with film and grain continue. I once saw a demonstration where a photo technician took two or more grainy, identical B & W negatives, carefully registered them and printed through the pack to get a finer and sharper image than any individual negative could make. This is the factor that makes the tiny frames of movie film look clear and sharp when the movie is projected on the screen. The constant overlapping cancels out the grain and reinforces the detail of the subject. Astronomers like to keep their film cool (easy enough at night on a mountain top) to increase it’s “sensitivity”. Actually cold doesn’t help the ISO sensitivity of film but increases its ability to hold details in dim light and long exposures.

Let’s face it, creating effective images with long time exposures is still one of the crowning glories of film. True enough when you pass the ten and 100 second mark you might lose a stop or two in ISO speed (that old “reciprocity departure”), but film still keeps on chugging along like the energizer bunny getting more and more detail with increasing time. Once I found myself on a hill overlooking Granada, Spain at night and was disappointed that it looked so dark. I shot anyway and went up to 5 minutes at f 5.6. The result was amazing, the dimmest street lights turned it into an exotic city of light. Recently many photographers have used the same approach to turn the fairly dim New York skyline into a bright city with its Towers of Light.

In digital photography the new kid on the block is the CMOS chip. It is seen as a successor to the CCD because it’s supposed to be cheaper to produce. The downside seems to be that it produces inherently more noise, but in actual production different cameras get different results. The professional CMOST back for 2 ¼” cameras seems to have a lot of it but Canon has used CMOS in some recent cameras with good results. The up-and-coming Foveon Corp. which has apparently created the first true 3 level color sensors, has used mostly CMOS chips, with good results. It looks like they just may be the “next big thing”.