“I shot it at 1600 and pushed it 2 stops! It looks great!”
I’m sure we’ve all heard this. It’s largely a myth. All films are within a third of a stop of the manufacturer’s published ISO rating in most developers.
This is extremely true of C-41 color films because the processing is standardized and the films are manufactured to conform to that processing environment. Given that, we’re not going to talk about color films. If it’s rated for ISO 100, shoot it at 100. If it’s a 400 speed film, shoot it at 400. Any lab that is worth anything can give you a wonderful scan of that film.
For black and white films, this is also largely true based on how the ISO standard prescribes determining the speed of the film which is explained very simply:
Film speeds are based on the exposure required to give a log density of 0.10 above film base plus fog (fb+f).
Keeping it simple
Terminology aside, we can say the speed of the film is determined by the amount of exposure needed to get a certain fixed amount of density above film base plus fog. For the ISO standard, that fixed density is 0.10 log density above film base plus fog. You can use a different fixed density level and you can use a different density scale and that’s totally valid except that it’s not to the ISO standard.
Where the confusion comes in is film density in the shadows or minimum density areas of the emulsion develops at different rates depending on the film developer used. The developer used along with developer temperature and time spent developing affects how much density is developed.
Push that film!
Not so fast. You see, depending on the size of the silver grains in the emulsion, you do in fact need a minimum amount of light to hit each grain so that it will develop anything at all. Believe it or not, that minimum amount of light is amazingly close to the ISO rating of the film.
So what does push processing actually do?
Simply put, push processing develops more density for the silver crystals that did get enough light to develop anything. What this does is make the density difference large enough that you can see it and gives the impression that the film is more sensitive than it actually is. In Adobe Lightroom speak, it’s the equivalent of going into the Develop Module and pulling the shadows up without changing the black level.
It does not make the film more sensitive. Once you go below that minimum exposure level, no amount of processing is going to bring more density into the emulsion. All you’ll do is develop more film fog.
So how come some films still look great pushed?
That’s because normally exposed and developed film typically has its film base plus fog at 6 stops below middle grey. That makes for some awesome blacks. If you switched over to the Exposure Index method of exposing, and under exposed the film by a stop, the film base plus fog effectively moves up by one stop to 5 stops below middle grey. This still results in totally acceptable blacks for most exposures. If you under expose by 2 stops (exposing a 400 speed film at EI 1600) the film base plus fog moves up to 4 stops below middle grey. This results in your blacks starting to look dark grey and a little muddy. This is where the push processing come in. More development time adds more density to what shadows you do have which makes the shadow areas have more contrast which to a degree counteracts the raised film base plus fog. There is a point of diminishing returns because once again, there is a minimum amount of light you need in order to develop anything above film base plus fog.
So that’s it in a nutshell. Pushing film is a myth. If you switch to a different exposure standard (like Exposure Index), then you can get a little more performance out of your film, but at the end of the day, the worst thing you can do is under expose your film.