- Film grain
Film grain or granularity is the random optical texture of processed
photographic filmdue to the presence of small grains of a metallic silver developed from silver halidethat have received enough photons.
Granularity, or RMS granularity, is a numerical quantification of film-grain noise, equal to the
root-mean-square(rms) fluctuations in optical density, [cite book | title = Handbook of Image Quality: Characterization and Prediction | author = Brian W. Keelan | publisher = CRC Press | year = 2002 | isbn = 0824707702 | url = http://books.google.com/books?id=E45MTZn17gEC&pg=PA12&dq=%22rms+granularity%22&lr=&as_brr=3&ei=vNe8SIODKYa4jgHw59TzBw&sig=ACfU3U0BZlbNUBLuhoh4cgdiYwA08xKVAA ] 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). [cite book | title = Basic Photographic Materials and Processes | authors = Leslie D. Stroebel, John Compton, Ira Current, and Richard D. Zakia | publisher = Focal Press | year = 2000 | isbn = 0240804058 | url = http://books.google.com/books?id=BRYa6Qpsw48C&pg=PA264&dq=microdensitometer+film+grain+noise&lr=&as_brr=3&ei=0MW8SNmsFomkiwHBnezzBw&sig=ACfU3U0Jvob8U9WVhCYJDT0oqx77xSehJg ]
Granularity is sometimes quoted as "diffuse RMS granularity times 1000", [cite book | title = Langford's Advanced Photography | author = Efthimia Bilissi and Michael Langford | publisher = Focal Press | year = 2007 | isbn = 0240520386 | url = http://books.google.com/books?id=DJ9bLcB3eeMC&pg=PT107&dq=%22rms+granularity%22+1000&lr=&as_brr=3&ei=jd28SPCLCIbujgGo2LzzBw&sig=ACfU3U0G1mEb9UuFdUzkVBeUbITBlpMa7A ] 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.Fact|date=December 2007
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. [cite book | title = Silver-halide Recording Materials | author = Hans I. Bjelkhagen | publisher = Springer | year = 1995 | isbn = 3540586199 | url = http://books.google.com/books?id=tiGP9MrVs90C&pg=PA20&dq=%22rms+granularity%22&lr=&as_brr=3&ei=vNe8SIODKYa4jgHw59TzBw&sig=ACfU3U1tIgZDOoxPo1oApzuccVuCUedwKw#PPA20,M1 ] [cite book | title = The Manual of Photography | author = R. E. Jacobson, Sidney Ray, Geoffrey G. Attridge, and Norman Axford | publisher = Focal Press | year = 2000 | isbn = 0240515749 | url = http://books.google.com/books?id=HHX4xB94vcMC&pg=PA417&dq=selwyn%27s-law&lr=&as_brr=3&ei=AeG8SKCiLI6UiAGBlbj0Bw&sig=ACfU3U1704AE7mqHCpGWLYIHkAKK8JoHcA ]
Grain effect with film and digital
Film grain is often considered an artistic effect, and can be found in some digital photo manipulation software such as
Photoshopas something that can be added to an image after it is taken.
In digital photography,
image noisesometimes appears as a "grain-like" effect.
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