This is an internet legend.
Originally Posted by Diapositivo
There actually ARE distinct grains (and those are called "grains"; "graininess" is the non-numberable entity resulting in from having grains); those work as the light-sensitive "sensors" in the film. There is a simple explanation; unlike pixels in a digital sensor, these grains exist in a very random three-dimensional matrix with a thickness comparable to 10-30 grains. Therefore, when you look at the finished product under a transmission microscope, you see a composite image of these grains overlapping. (In addition, most BW developers dissolve grains together and change their shape.)
The grains are just so small (in the range of 0.1 - 2 Ám or so!) that you need an electron microscope to see them clearly; so this is an another reason, normal visible light cannot render sharp images of grains and they are blurred. And, finally, you have to remove the grains from the thick emulsion for microscopy if you want to see them separately. See the attached image --- image courtesy Robert L. Shanebrook, Making Kodak Film, 2010 Recommended reading!
So, the original question is perfectly valid, and I can see it would be quite fun to talk about giga- or teragrains to a digital megapixel person . In larger formats than 135, I bet it would quickly become petagrains.
The number will be truly huge, because in film, a single grain works along with hundreds of others to form smooth gradation. (Although, contrary to an another internet legend, a single grain is NOT limited to on/off state but can develop partially. Still, the development is an amplification process with a factor in the range of 1000000, so that for a really smooth gradation, a high number of active development centers (or spots of latent image; a few Ag atoms) is needed, and this happens in multiple grains.)
What would be the way to count the number of grains? First approximate their size distribution (e.g. from electron micrograph), then create a simple mathematical model for their weight, and use some known amount of silver in the film?
Last edited by hrst; 05-16-2012 at 12:01 AM. Click to view previous post history.