I'm using a Nikon FM2 SLR camera. Suppose I use Kodak TMax 400, and I push to ISO 1600 on the camera.
Let's say the current meter says shutter speed 1/250 at f/2.8 is the correct exposure (i.e. meter at sign 'o').
Because we do pushing, should we adjust the shutter speed or aperture? For example to 1/500 or 1/125. Similarly for aperture to f/2 or f/4?
When you set the ISO on camera, that is the adjustment to compensate for increased development time. Any further changes to shutter speed or aperture would change the exposure further.
In the scenario you describe, you want to process ISO 400 film at ISO 1600. That is a two-stop increase in development. So exposure has to decrease by two stops. Based on the exposure triangle, you can distribute a two-stop drop among ISO, shutter speed, or aperture.
The catch is ISO. Unlike digital, ISO is tied to the film, which you've already chosen, ISO 400. So you can meter at ISO 400 and make manual changes to shutter speed and aperture to compensate for development. You can also change the camera ISO to 1600. This will cause the camera to "compensate" for the "higher" ISO by recommending increased shutter speeds or smaller apertures. But since you didn't really change the film, the exposure is effectively decreased by two stops, which you'll compensate for later during development.
Stops – Photographers tend to think in "stops". A stop is a change in a setting that halves or doubles the amount of light that is captured. ISO and shutter speed halve and double when changing stops. Aperture has the following pattern, which you just have to memorize: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. These numbers are marked on manual lenses.
Density – When using film, what's desirable is "good" density. You can learn what that is by seeing film shot at "box speed" that's developed normally (by the book). Detail should be seen in both shadows and highlights. Midtones should be spread out between the two. It should look something like this (The Online Darkroom: How to read a negative):
C = X + Y + Z – This is the basic equation that is used for everything. The components and units might change, but the way it's used is the same. C usually stays constant. X, Y, and Z are given to you somehow (often by the camera or meter). Since C is constant, you don't need to know exactly what it is. Only that, if X changes by a stop, Y or Z also have to change by a stop to keep C the same.
Exposure Triangle – Final Exposure = Exposure(ISO) + Exposure(Time) + Exposure(Aperture)
To keep Final Exposure (C) constant, changes to ISO, shutter speed, or aperture have to be accompanied by complementary changes in the others. Since you are using film, the ISO in this equation is the ISO of the film, which is different from the camera ISO setting.
Push/Pull Processing – Final Density = Density(Exposure) + Density(Development)
The "ideal" Final Density (C) is constant. When you decrease exposure, density also decreases (X). To keep the Final Density the "same", you have to compensate by increasing development to bring density back up (via Y). Similarly, if you change Y, you have to make a complementary change in X.
See also:
If you've put T-Max 400 in the camera and set the ASA/ISO dial to 1600, when the camera's meter indicates '0' at 1/250 @ f/2.8 you're already underexposing by two stops. This is because the meter thinks you've got 1600 speed film in the camera, but the film in the camera is still 400 speed. It doesn't magically change its chemical properties just because you changed the ASA/ISO dial.
That's why you need to "push" the film in development: Your film will already be two stops underexposed if you follow the meter's recommendation when the ASA/ISO dial is set two stops faster than your film really is.
If you change the shutter time from 1/250 to 1/125, the meter will show one stop of overexposure (because you've told it there's 1600 speed film in the camera) when the exposure value of 1/125 @ f/2.8 will still be one stop underexposed for 400 speed film in those lighting conditions.
If you also open up the aperture to f/2, you've increased exposure another stop and the meter, set for 1600, will show two stops overexposed at 1/125 @ f/2. Your film will be exposed "properly", based on the fact the meter thinks your film is two stops faster than it really is. If you set the ASA/ISO dial to 400, under the same lighting conditions the meter would say '0'.
No matter what the ASA/ISO is set to and what the meter says, exposing at 1/125 @ f/2.8 in the same lighting conditions will always result in the same actual amount of light hitting the film.
Think of it this way: The ASA/ISO dial does nothing to the film in your camera. What it does is change the amount of light your meter will read as '0'. The faster your meter is told the film is, based on the position of the ASA/ISO dial, the less total light, determined by the combination of aperture and shutter time selected, it will tell you the correct exposure is.
If you are then going to compensate your shutter time and/or aperture to make up those two stops, you won't need to "push" development because your film will not be underexposed. You're then using the same shutter time and aperture that you would have used if the ASA/ISO dial was set to 400 and you lined up the meter at '0'. In which case it's a whole lot easier to just set the dial to 400 and follow the meter's recommendation.
