Film grain or granularity is the random optical texture of processed photographic film due to the presence of small grains of a metallic silver developed from silver halide that have received enough photons.
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RMS granularity
Granularity, or RMS granularity, is a numerical quantification of film-grain noise, equal to the root-mean-square (rms) fluctuations in optical density,[1] measured with a microdensitometer with a 0.048 mm (48-micrometre) diameter circular aperture, on a film area that has been exposed and normally developed to a mean density of 1.0 (that is, it transmits 10% of light incident on it).[2]
Granularity is sometimes quoted as "diffuse RMS granularity times 1000",[3] so that a film with granularity 10 means an rms density fluctuation of 0.010 in the standard aperture area.
When the grains are small, the standard aperture area measures an average of many grains, so the granularity is small. When the grains are large, fewer are averaged in the standard area, so there is a larger random fluctuation, and a higher granularity number.
The standard 0.048 mm aperture size derives from a drill bit used by an employee of Kodak.[citation needed]
Selwyn granularity
Film grain is also sometimes quantified in a way that is relative independent of size of the aperture through which the microdensitometer measures it, using R. Selwyn's observation (known as Selwyn's law) that, for a not too small aperture, the product of RMS granularity and the square root of aperture area tends be independent of the aperture size. The Selwyn granularity is defined as:
where σ is the RMS granularity and a is the aperture area.[4][5]
Grain effect with film and digital
The images below show an example of extreme film grain:
 Rallycross car pictured on an Agfa 1000 RS slide |
 Detail of the same photo |
Digital photography does not exhibit film grain, since there is no film for any grain to exist within. The effect of film grain can be simulated in some digital photo manipulation programs, such as Photoshop, adding grain to a digital image after it is taken.
In digital photography, image noise sometimes appears as a "grain-like" effect.
See also
References
- ^ Brian W. Keelan (2002). Handbook of Image Quality: Characterization and Prediction. CRC Press. ISBN 0824707702. http://books.google.com/books?id=E45MTZn17gEC&pg=PA12&dq=%22rms+granularity%22&lr=&as_brr=3&ei=vNe8SIODKYa4jgHw59TzBw&sig=ACfU3U0BZlbNUBLuhoh4cgdiYwA08xKVAA.
- ^ Leslie D. Stroebel, John Compton, Ira Current, and Richard D. Zakia (2000). Basic Photographic Materials and Processes. Focal Press. ISBN 0240804058. http://books.google.com/books?id=BRYa6Qpsw48C&pg=PA264&dq=microdensitometer+film+grain+noise&lr=&as_brr=3&ei=0MW8SNmsFomkiwHBnezzBw&sig=ACfU3U0Jvob8U9WVhCYJDT0oqx77xSehJg.
- ^ Efthimia Bilissi and Michael Langford (2007). Langford's Advanced Photography. Focal Press. ISBN 0240520386. http://books.google.com/books?id=DJ9bLcB3eeMC&pg=PT107&dq=%22rms+granularity%22+1000&lr=&as_brr=3&ei=jd28SPCLCIbujgGo2LzzBw&sig=ACfU3U0G1mEb9UuFdUzkVBeUbITBlpMa7A.
- ^ Hans I. Bjelkhagen (1995). Silver-halide Recording Materials. Springer. ISBN 3540586199. http://books.google.com/books?id=tiGP9MrVs90C&pg=PA20&dq=%22rms+granularity%22&lr=&as_brr=3&ei=vNe8SIODKYa4jgHw59TzBw&sig=ACfU3U1tIgZDOoxPo1oApzuccVuCUedwKw#PPA20,M1.
- ^ R. E. Jacobson, Sidney Ray, Geoffrey G. Attridge, and Norman Axford (2000). The Manual of Photography. Focal Press. ISBN 0240515749. http://books.google.com/books?id=HHX4xB94vcMC&pg=PA417&dq=selwyn%27s-law&lr=&as_brr=3&ei=AeG8SKCiLI6UiAGBlbj0Bw&sig=ACfU3U1704AE7mqHCpGWLYIHkAKK8JoHcA.
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