How exactly does grain work?

I know the larger grains should react first because they're more sensitive than the smaller grains, but are the types of grain exposed relatively evenly? [Number of large grains ~ Number of small grains] Or do they always favor one side over the other depending on the film emulsion? Or does it just always favor the larger grains because they're more sensitive?

what about when you overexpose [assume same dev time as box speed]; are the larger grains already "spent" and the added density just come from the smaller grains reacting?

Does essentially the same thing happen if you overdevelop? I understand why the grain increases if you underexpose and overdevelop...I'm just kinda interested if overexposing [with normal dev time] gives you similar results to shooting at box speed and overdeveloping in terms of grain.

I know, weird questions...I'm a weird person

My explanation,it may only be good in parts.
The pre-developed emulsion contains micro grains with surface defects and patches of sensitizer randomly distibuted so the larger the micro grain the greater the chance of the presence of these resulting in absorbtion of light quanta and later developing.It is reported to take 2-20 quanta to make a micro grain developable so the smaller micro grains only get the required dose at higher exposure.
If the emulsion receives a very large exposure the active surface sites are saturated. Quanta can still be trapped inside the grain but since the developer does not initially reach them they have no effect as the grain would have developed anyway.
During development the micro grains also clump together to become visible grains.

So essentially it does do the same thing just for different reasons? I should really just test this but I don't know how much longer I'll have access to a darkroom.

Where is PE when you need him...

Poohblah said:

Where is PE when you need him...

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Slightly delayed; tights and cape are at the dry cleaner's...

Regarding the part of your question asking if overdevelopment will give the same result as overexposure and normal development, I think it possible to deduce that the answer is no from the following quotes from the Film Developing Cookbook by Anchell and Troop:
p61'Modern films give best printed results with a richer negative and Crawley notes "modest overexposure does not increase grain".'
p91'Pushing film means underexposing and overdeveloping.....the result is... vastly increased grain and contrast.'
Hence, larger grain will be obtained by overdevelopment than by overexposure.This could be attributed to micro grain clumping increasing as development time is extended.

MrMushroomMan said:

I know the larger grains should react first because they're more sensitive than the smaller grains, but are the types of grain exposed relatively evenly? [Number of large grains ~ Number of small grains] Or do they always favor one side over the other depending on the film emulsion? Or does it just always favor the larger grains because they're more sensitive?

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As I understand modern B&W film manufacture, there are a number of layers in the emulsion. The layers vary in sensitivity and grain size. I think I read somewhere that there are usually 2-5 image forming layers. The days of a single thick image forming layer are long gone.

But to answer your question, the film grain in the top most layer will be exposed first. Be that smaller grain or larger grain depends in large part to how the film was manufactured.

Beyond that, it depends on the image. Where there are photons, there is an opportunity to capture the photons. The more photons there are in a given area, the deeper they will penetrate into the film. It's not really a question of big grains reacting before small grains. If there are enough photons, grains in all layers will react regardless of size.

MrMushroomMan said:

What about when you overexpose [assume same dev time as box speed]; are the larger grains already "spent" and the added density just come from the smaller grains reacting?

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As I understand it, density is built up by a number of mechanisms. I know I can't explain it quickly and accurately, so I'll just refer you to Tim Vitale's excellent exposition on the subject.

Basically, graininess is caused by the formation of grain clumps. Grain clumps are largely caused by overlapping grains as seen by looking through all the layers of the film. IOW, the emulsion is not a 2D object, but is a 3D object with non-negligible depth. The overlapping occurs as you go deeper into the film.

MrMushroomMan said:

Does essentially the same thing happen if you overdevelop? I understand why the grain increases if you underexpose and overdevelop...I'm just kinda interested if overexposing [with normal dev time] gives you similar results to shooting at box speed and overdeveloping in terms of grain.

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Graininess is directly related to density. The reason that push processing results in increasing graininess is because it results in a higher Dmax than a normally exposed and processed film would. That's how you get the increased contrast index, by raising the endpoint of the curve and thus increasing the slope.

If you under expose and over develop one sheet of film, and over expose and under develop another sheet of film, and both sheets miraculously end up with the same Dmax, you'll find that both sheets have very similar graininess. Else, the sheet with the higher Dmax will exhibit more graininess, all other things being equal.

There is reason to believe that as well as the 3D effect the clumping of micro grains also occurs.
Normal development in Rodinal to a given density gives a less grainy result than stand development to the same density.
The number of grains activated by light quanta and their initial 3D distribution in the emulsion is the same in both cases.
The most likely explanation of increased graininess with Rodinal stand development seems to be that the alkali swollen emulsion and long development time allow the micro grains to clump together.

Bruce Watson said:

I'll just refer you to Tim Vitale's excellent exposition on the subject.

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Not to be too much of a stick in the mud... but upon quickly scanning the linked pdf, I doubt the "excellence" of some of the claims in that paper.

To oversimplify, you can obtain lower grain in 1. longer, 2. more dilute, 3. colder development and more in the opposite. The original film matters too and for such things the opinions are pretty widely available on the internet.

SamWeiss said:

Not to be too much of a stick in the mud... but upon quickly scanning the linked pdf, I doubt the "excellence" of some of the claims in that paper.

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care to expand?

Here is the pH and granularity for 2 developers where the sulfite is low enough not to have a solvent effect (film was old Tri-X):
Microdol-X 1+3 pH~8 granularity =42
Beutler pH~11 granularity =51
The higher pH causes an increase in granularity due to the clumping of micro-grains.
If sulfite is high:
Microdol-X 1+0 pH~8 granularity =37
The high sulfite reduces granularity due to solvent effect.
Granularity data from 'Controls in Black and White Photography, R.Henry,p227.

you know, while we're at it...does paper have a grain of it's own? If I remember correctly, paper is usually rated at 1ASA? is it just that the grain is so fine that it's negligible?

And thanks for all the posts so far, they've been a good read.

Paper is based on a photo-sensitive emulsion so it has grain. I think most people rate paper at 6 ASA. But like any silver-based light-sensitive material, it experiences reciprocity.

MrMushroomMan said:

you know, while we're at it...does paper have a grain of it's own? ... is it just that the grain is so fine that it's negligible?

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You don't 'enlarge' the image from the paper, so you don't see the grain. If you make paper negatives and try to make enlargements from them, the grain is there hiding among with the paper fibers and who knows what else the other noise in the image may be from. (Paper negative are neat-o tricks, but my experience is that they're good for contact prints only.)

Assuming that the printed paper grain and the film grain of an image were the same size, if you hold the film up to the light and look through it unaided you don't normally 'see' the grain with your naked eye.

I can't say for sure, but my gut feeling is that the paper grain is actually larger than the film grain if had a set of calipers that small to measure. Of course, I haven't ever looked at prints through a microscope to see if one could actually measure the grain.