External light meter allows you to set the ISO speed at your liking.
Does that mean you can replace that for the need of pushing or pulling your film?
No, it does not.
A given film has a given sensitivity, expressed as ISO. What this means is that, if processed normally, then there's a relationship between the amount of light which falls on a given area of the film and the density of that area after it is processed. This relationship is generally monotonic (more light means darker) but not linear or proportional: in particular it is somewhat 'S' shaped, so that the relationship between light and density flattens off both at the top of the curve and the bottom. So, for instance, however much light you expose the film to it will only go so dark after development.
If more light falls on the film than it is intended for (it is overexposed) & you process it normally the resulting negatives will be very dense, and detail will be lost in the highlights as a lot of the information will be squashed up into the top (shoulder) of the 'S'. To deal with this you pull the film in development which makes the negatives less dense & preserves some detail in the highlights, at the cost of negative quality.
If less light falls on the film than it is intended for (it is underexposed), then if it is processed normally the resulting negatives will be very thin and detail will be lost in the shadows as information is squashed into the bottom (toe) of the 'S'. To deal with this you push the film in development which makes the negs denser, recovers some detail in the shadows, again at the cost of negative quality.
A meter gives you some information about the light conditions, and setting the ISO on the meter will let it tell you what exposure is appropriate for a film of that ISO. It will not change the ISO of the film. If you set the meter, say, to have an ISO a stop more than your film, then your film will be underexposed by a stop. If you set the meter to have an ISO of 1 and use Tri-X, it will be overexposed by about 8 stops.
[Disclaimer: yes, I've oversimplified the film-sensitivity thing somewhat.]
If you use an external light meter, you should set its ISO setting to match the film in your camera. You use the meter to measure the light, and it tells you what aperture and shutter speed to set on your camera. It's as simple as that.
You cannot just set any ISO value on the meter. Because that's not the sensitivity of the film in your camera. If you set the incorrect ISO value on the meter, it will (understandably) give you incorrect suggestions for aperture / shutter speed, and you will expose your film incorrectly.
We use the light meter to gauge scene brightness. We set our camera's exposure based on the light meter reading augmented by experience. Our goal might be reduced contrast -- we pull. Our goal might be increased contrast -- we push. One axiom - expose for the shadows and then develop for the highlights -- it still stands! However I like this modification -- Underexpose to allow over-developing -- overexpose to allow under-developing.
A light meter is a device to indicate the amount of electricity produced by a photocell according to the level of luminance. It correlates light intensity to a numerical index.
The resulting numerical index (reading) is then used with a calculator (mechanical or algorithmic) to indicate a combination of intensity and time settings for an exposure by an electro-mechanical apparatus according to the ISO formula.
Mathematically, you can calculate it all via:
- sISO = ln(ISO / 100) / ln(2)
- sAperture = -ln(Aperture) / ln(√2)
- sShutter = EV + sISO + sAperture
- Shutter speed = 2-sShutter
Taken from Is there a formula to calculate ISO…
Setting the ISO index on a calculator is not altering the speed/sensitivity of the actual material you are using. You could put ISO 100 film in your camera and set it to 1000 on your calculator which doesn't change what is in your camera.
Using the incorrect setting will produce disappointing results.
You must compensate (somehow) for the difference between the actual sensitivity and the wanted results. With film, you can compensate for insufficient photonic energy with chemical energy, by increased (push) processing. With a sensor, you can compensate for insufficient photonic energy with amplification of the signal. The same is true for excessive photonic energy. You must compensate for the deviation in exposure level from desirable.