I posted this on another forum, but I think I will have more informative responses here:

Is pushing a film beyond its true speed just a delusion?

I, for one, suspect it is!

An emulsion has a definite speed (with a certain developer and a certain time) and while it is true this speed can vary a bit with altered developing times, I do not think there can even be as much as one stop true speed change.

For example, should you find Tri-x to actually have 400 speed, you really cannot push Tri-x to 800, and certainly not 1600 – all you are doing is underexposing and overdeveloping.

What I said here concerns PLACING ZONE III PROPERLY using the exposure index you think will work.

The zone III shadow detail will not be there – and shadow detail is what film speed is all about! And you certainly aren’t going to get a speed of 3200 out of Tri-x (based on shadow detail and usual 400 speed), I don’t give a damn what you do to the film!

It seems only 35mm photographers use pushing – and maybe some medium formatters do too – but I have never heard the large format guys talk about doing this, and THEY would be the fellows who sure could use faster film! If this were truly a viable process, I guarantee they would use it and speak of it often.

Now, I know you guys running around shooting high contrast, dimly lit scenes and pushing your film to outrageous speeds will SWEAR it works great.

OK – if you are pleased with what you have, then that is good. But, I’ll bet your negatives are thin and very high contrast and even your minor shadows are as black as black can be…therefore, you REALLY ARE NOT gaining film speed. You are only underexposing and overdeveloping to get printable densities in the mid-tones and high values.

Again, FILM SPEED depends on proper shadow values! It does take a certain amount of light to excite the emulsion – if the emulsion doesn’t get that minimum amount of light then there is no density when you develop.

THERE MAY BE certain developers that actually give, say 800 speed, to Tri-X – but probably at some ‘cost’ such as increased grain or other unwanted effects.
But that IS the speed in that developer, and I think there can really be no ‘pushing’ done to significantly alter this established speed.

I realize that a very flat scene can be photographed using a higher exposure index and overdeveloping and yield a good negative (essentially N+ development) – BUT, this would necessitate placing zone III on IV or higher (and what you’ve done here is actually place it properly with the proper film speed!) - again, if the important shadow value is actually placed on zone III and you UNDEREXPOSE by using a higher than actual film speed, that shadow detail is not going to be there as it should.

I’d be interested if anyone can tell us about actually pushing a stop or more and preserving the shadows (basing exposure on proper shadow value) and tonality of a scene – that is, a full range of zones.

So, tell me about it. Set me right if I am wrong…