Why are so many films multi-layer ?

[J 3]

A lot of film stock is coated with multiple layers of different speeds (grain size). With color you'll have multiple cyans, magenta and yellows (e.g. 2 of each). With b&w you'll have multiple layers with different grain sizes. This gives greater tonal range I've heard.

Does anyone know why they do it this way rather than just mix the differing emulsions of the same color together?

Do the grains pack better this way? Is it easier to control the mix this way?

For some reason they don't seem to do this for b&w paper emulsions as you can buy these in jars. Even with Rollie's black magic where they have a two component multi-grade emulsion.

 

[Stanislav Boščík]

AFAIK B&W film emulsions have several layers today - an antihalation on the back, sensitizers are added as a separate layer in some cases. There is also sometimes a special layer put between film base and emulsion and a protection layer on top of the emulsion is sometimes present. So usually there are more layers in B&W films today and it is no big deal to add one (when the capacity of the producing machine is sufficient of course).

Having two silver emulsion layers is used to increase exposure latitude (~ "dynamic range") of the film. Note that for Berger Pancro 400 those emulsions are also made of two types of Ag halides (silver bromide and silver iodide) and mixing them together would probably not lead to the same results as having two separate layers.

 

[Lachlan Young]

You might want to read this. The principles are the same for colour - with the complications of added couplers. The re-equilibration problem is likely lower in some multigrade papers & more of the problems relate to the sensitising dyes wandering from the intended emulsion (reducing available contrast) - which can be sorted by ballasting those dyes more heavily to prevent them unlocking themselves from the intended emulsion.