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.
Granularity is a bit complex and, ultimately, not very telling. For consumer film stocks Kodak have moved to a more meaningful concept: the Print Grain Index. Print Grain Index takes into consideration image enlargement and is entirely based on perception from a fixed distance (14 inches, or around 36 cm). It is out of the scope of this article, but to illustrate the fine grain of modern stocks: for example, a Kodak Ektar 100 negative can be enlarged to 6×4″ (15x10cm) from a 35mm source, or to 10×8″ from a medium format source without any perceptible grain when observed from the fore-mentioned control distance.
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.
You can read the previous parts of the Cinematic Look series here:
Part 1: Aspect Ratio, Sensor Size and Depth of Field
Part 2: Frame Rate and Shutter Speed
Part 3: Dynamic Range
This entry was posted by cpc on September 17, 2012 at 8:24 pm, and is filed under Uncategorized. Follow any responses to this post through RSS 2.0.You can leave a response or trackback from your own site.
I’ve been enjoying this series immensely. Your articles should be required reading for any film enthusiast or aspiring filmmaker. Thank you.
As you mentioned sensor noise tends to look more organic these days. But it would be interesting if there is a way to capture light digitally the same way film stock does, instead of trying to mimic it later in post. I think Fuji is on the right path. On the other hand, people in the market for such a camera could afford to shoot film anyway.
A true simulation of film’s randomness would require sensor photosites varying their positions. Then there is the purely philosophical question whether the current clean, ordered and formal digital aesthetic is inferior, or simply something we need to embrace and adjust to.
That seems possible, but I am unaware if this can even be engineered, or if anyone is even looking to it.
Philosophy wise, I don’t think its a matter of inferiority but rather aesthetic choice these days. But like anything else, change is inevitable and soon the collective conscious will probably err on the side of digital. Although, I hope these choices are only made in favor of the narrative.
It might be possible to use a very small basic photosite size and reallocate these around in consequtive frames, changing the effective size of the readout pixel. Offsetting rows/columns a bit would also inject another variable of pseudo-chaos. Problem is, the end result will need to be resampled for display on monitors/TVs/digital projectors, and they are standard rectangular matrices. One would need a pretty high display resolution to preserve the original structure. Not impossible though.
Choice is about to disappear soon, with Fuji getting out of motion picture film business and Kodak in turmoil. But yes, it is really mostly a matter of philosophy and aesthetics. In terms of convenience digital is way ahead, and recently also taking the lead technical quality-wise.
Yea, I can see how that would work now. You’re definitely right about requiring the high resolution. From first hand experience anything uploaded on Vimeo, either a fine grain or real film scans, results in ugly pixelization due to the low resolution.
I think after awhile our definition of what constitutes the “film look” will evolve. Although the debate is still pretty much on going from what I saw in the documentary Side by Side. Which I think you would like if you haven’t already.
Would love to get your opinion, feedback on Vincent Laforet’s post
I have mixed feeling about such tools. On the one hand it could be argued they don’t do anything that couldn’t be done otherwise using software like After Effects. On the other I applaud the effort to help create a more ‘cinematic’ the definition of which is going to be hotly debated for some time to come. Having just watched ‘The Master’ (both shot AND projected in 70mm) I hope we never see the day when everything becomes digital.
Vincent has messed a bit the way densities and grain work, but I find the samples quite nice. Although Vimeo’s compression certainly doesn’t do them justice.
The different sensor profiles for colorimetry and gammas don’t really make that much sense to me. Well, maybe for raw video. But for baked video they need a profile for each combo of camera curve + color settings. Nevertheless, this is indeed necessary for any true simulation, so it is the right approach.
Then there is the thing that modern film (modern as “in the last 10 years”) doesn’t really preserve its native colorimetry on its way to theaters, what with the DI and the extensive grading most movies receive. In this respect, matching film stock colors is more of a curious toy, than a practical thing. For digital only projects, that is. For mixed film/digital it can be useful for matching different sources before grading, but then again – there are LUTs for this anyway.
As for the ideology behind such tools – I am all for having the options. Smart use, or no use is then a matter of taste and good measure.
The Master and The Dark Knight Rises form an interesting trend with their larger format source. Cinematographers with affinity to shooting film are looking for something that can set their image apart in technical terms. Something that equals (or betters) the high-end S35+ sized digital sensor. Very much like how shooting medium format for stills is for many photographers the way to catch up with the definition and high megapixel detail of APS-C (or larger) sensors.
I believe the concern with film grain is akin to the constant use of
Planar Focus. There was a time when shallow focus was a necessity;
you could only move so fast and playing catchup with the scene could be used, by master hands, as stylistic element.
But others prefered DOF and worked to acheive it.
When video came along the smaller handicams tended to be shaky and now the MTV shake has to be in every shot.
I prefer clear, bright, smooth shots with great DOF; 100 years from now, in a world of 4d surround cinema, they’ll be trying to reproduce the “flat” look.
(This is a reply to Matt’s comment above. Apparently the software messed up.)
It is certainly evolving all the time. Future generations may very well consider film technically inadequate, the same way some people find black & white + mono sound unwatchable. Appreciating the past is often an acquired taste and requires a conscious effort to first get to know it, and just then to start liking it.
It is also intresting how visible grain is similar to motion blur and strobing, in the sense that technically they all are defects. But they are defects that are deeply imprinted in the notion of what is filmic. At least for some generations. On the other hand, being defects, they are understandably under some pressure to get disposed of.
Thanks for the heads up. I am interested in Side by Side since I saw the trailer, but I haven’t got the chance to see it yet.