ISO, film size and grain size

I have been pondering the question of equivalent grain on different black and white film sizes. I'm not sure there is a mathematical formula that could be applied, but maybe someone has already done the homework. Here is the hypothesis: Given the same developer, what would the equivalent grain be between, say, a shot on 120 (6x9) and 135 (24x36) format? What I am trying to determine is if I shot 6x9 on a 400 ISO film, would the grain look the same as 135 shot on the same film underexposed by 2.5 stops and developed accordingly, both negs being enlarged to the same size print? Or is grain size vs ISO not linear at all? I guess I could do a test and see for myself, but I'm not that into testing, especially if there is someone who has come to the conclusion already.

Grain size generally decreases with the ISO, but it's not mathematical. The grain size is determined by the manufacturer, so two films of the same ISO can have different grain sizes.

If you were comparing the grain sizes in the same film with two different size formats, say 35mm vs 4x5, you could mathematically estimate the grain size based on the amount of magnification to make a print of a set size. The grain in the print from 35mm would be roughly about eight times larger.

Pieter12 said:

Or is grain size vs ISO not linear at all?

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That would be the quick summary for sure. To a large extent, grain is inherent to the film - not what speed it's shot at. You can modify the way the grain looks a bit in various ways, but it'll still be the same film. Btw, decisions like how much contrast boost you'll need in post/printing will have a very profound influence on the apparent granularity.

The whole subject is very tricky because "grain size" as such is not necessarily a meaningful parameter. What you're interested in is how grainy a picture looks. That depends on many things, and the actual physical size of silver agglomerations is just one of the contributing factors.

A couple of weeks of online argumentation will take a lot longer than loading two rolls of film in two cameras and just give it a go.

xkaes said:

If you were comparing the grain sizes in the same film with two different size formats, say 35mm vs 4x5, you could mathematically estimate the grain size based on the amount of magnification to make a print of a set size.

Click to expand...

Yeah, that would get you close, at least ceteris paribus.

hehe quite so, yes!