Cinematic Look, Part 4: Film Grain
Film grain is possibly the single most differentiating factor of film images when compared to digital images (in both stills and video). It is also the first characteristic of the film look the average viewer would pick if they had to point their finger. This part of the cinematic look series explores some of the properties of film grain and how film grain relates to image perception. We also talk a bit about digital sensor noise, which is the closest perceptual relative of film grain in the digital video world.
What is film grain?
Film grain is often used to describe a few different concepts. For the film savvy viewer, film grain is the random grain-like texture seemingly overlaid on a scene captured on film. It is observed in a paper print, on a display or through projection. In this aspect film grain is somewhat related to film scratches and dirt specks. On a higher level, the grain texture is one element that distinguishes the film image from reality. This is true for stills, but even more so with cinema where the random nature of the grain manifests itself in consecutive frames, and the greater enlargement makes it more pronounced. A more technically inclined person with less sentiment for films would simply call this apparent image graininess noise.
In black and white negatives the light sensitive elements are usually silver halide crystals suspended in gelatin. When photons hit the crystals, they are converted to a latent developable state. Subsequently, lab processing dispenses with the unexposed particles. Film grain is commonly thought of as the remaining silver particles. This isn’t entirely correct. While observable film grain is a result of these image-forming particles, it is distinctly different from the particles themselves. The individual silver particles are so small they can’t be seen. What is perceived as grain is clumps of these particles and, more precisely, micro-variations in areas of relatively uniform negative density. In color film the silver particles are coupled with dyes; silver is removed in processing after development and only dye clouds remain. These dye clouds are the cause of graininess in color film.
Graininess is a subjective visual sensation. And it is highly dependent on scene tones, colors and details. All this makes it a bit hard to quantify. Traditionally, film stock density unevenness is quantified through measurements of density fluctuations. This objective quantity is called granularity. Granularity is measured with a microdensitometer in a small area of uniform density at 1.0 density above base. The microdensitometer usually has an aperture of 48 microns (0.048mm). The standard deviation from the average density gives us root-mean-square granularity. Standard deviation is very small, so it is usually multiplied by 1000 to bring it into whole numbers territory. Where there are very small silver particles, many of them are averaged and fluctuation is small. With large particles, there are less of them getting averaged, and fluctuations are larger. Modern film stocks (like Kodak Vision 3 stocks) have granularities below RMS granularity of 5, which is considered finer than extremely fine.
Film grain and film properties
In order to facilitate the capture of different shades, film uses silver halide particles of various sizes. But each particle needs the same number of photons for exposure, no matter what its size. So larger particles are exposed faster, and smaller particles need more light (or more time) to capture enough photons. This variance in particle size is responsible for the great dynamic range of film. In the dark areas of the image only the large particles are exposed, and in brighter areas particles of all sizes get exposed. That’s the reason grain appears coarser in shadows and low mids. Some cinematographers overexpose a bit in order to get the cleanest results, but this is stock specific.
There is a similar connection with film speed (sensitivity). Slower film is cleaner and finer grained due to its very fine individual grains. Fast films need larger particles to capture light faster, and thus exhibit coarser grain. Different developers can affect graininess, especially with black & white film. More notably, developers containing silver solvents lead to a softer grain look.
Film grain is also connected to image sharpness. While the relation is complex, especially in color film, fine grain stocks generally resolve more than coarser grained stocks. But there is more than resolving power to perceived sharpness. Film grain is noise and can mask image detail out. But it can also enhance tonality and fine detail by modulating tonal changes that are too miniscule for the brain to register. For those not easily scared by terminology: in this case film grain acts as stochastic noise and causes stochastic resonance.
Film grain and the film look
Film stock manufacturers have always considered graininess a defect and have strived to decrease granularity. Filmmakers, on the other hand, often consider low to moderate grain an important aesthetic. Both for the pleasing qualities of its texture and for its subtle veil over reality.
But there are a couple of other properties not so obviously related to the film look. They aren’t as much a result of film grain as they are a consequence of film grains.
A digital image is made of rows and columns of dots (pixels). It is a matrix. So a digital sensor always samples uniformly the image delivered by the lens. This leads to aliasing problems with high frequency detail in the scene. Hence the need for anti-aliasing filters in the typical digital camera. These optical low-pass filters can kill very fine detail and also complicate the use of small symmetrical lenses with digital cameras with short flange focal distance (like Sony E-mount cameras). In contrast, the individual grains of film and, subsequently, the clumps that form visible film grain are placed randomly. This prevents any noticeable aliasing. There is an attempt to mimic this in the recent Fuji X-mount digital cameras. They use a pseudo-random color array for their CMOS sensors, and have dumped the anti-aliasing optical filter.
The other effect concerns movies specifically and is also a result of the random distribution of grains in film. Because of this randomness, each consecutive frame in a film roll captures a slightly different image of the scene (for static or slowly moving scenes). While any single frame may lack some details, all frames as a whole can capture lots of fine detail. When film is projected at 24 fps, the brain integrates the individual frames’ contributions and sees the cumulative result. This lends an organic feel to projected film images.
The closest relation to film grain in digital video is sensor noise. Unlike grain (usually looked at positively or ambivalently), sensor noise was widely considered a detriment to image quality. This is because digital sensor noise lacks grain’s inherent randomness of appearance and variation in size. Sensor photosites (pixels) are placed on a matrix and they are ordered and fixed sized. These properties translate to sensor noise. There are various causes of noise in sensors. Shot noise (photon noise), thermal noise, readout and reset noise, quantization noise, voltage variance noise, etc. all merge in a single combined manifestation. In earlier sensors noise would often manifest itself in patterns, and would appear quite objectionable. Newer sensors largely dispense with the fixed noise patterns and demonstrate a much more random noise structure. In CMOS sensors, debayering acts as partial anti-aliasing on noise and softens it. Video compression can further smear noise: this can be blotchy and ugly with heavy compression, but can be a positive when only slightly affecting noise. As a result, some recent digital cameras produce an organic noise structure that shares characteristics with film grain.
In general, images acquired digitally are cleaner than film, especially at base ISO speeds. Noise is mostly apparent in dark areas or when sensor gain is applied to increase sensitivity. In both cases visible noise is the result of lower signal-to-noise ratio.
Film grain in post
Film grain is often added to images in post in an attempt to get some of the characteristics mentioned above. Usually grain is applied to simply mimic the film look, but there are sometimes technical reasons behind the decision. Digitally acquired images can look clinical, being both clean and sharp. And more so with CG images. Overlaying a bit of film grain dirties them and adds some texture. With low tonal resolution images (such as 8-bit compressed video) film grain can act as dither and help cover banding issues. But adding film grain is not reserved for digital cinematography. Cinematographers routinely add grain in DI to movies shot on film, because latest film stocks with their extremely fine granularity can look scarily clean. Added film grain can be either software simulated, or scanned from actual exposed film stock. While the latter is the preferred method for most, there are some good synthetic noise examples around.