This problem goes a long way to explaining why so many of us have gone to digital so we can let Photoshop deal with problems like this.
Actual exposure is a relatively simple and fixed quantity, light is also normally a relatively fixed and consistent parameter for any given scene, (the sun is a remarkably consistent illuminator). It's in understanding how that light is modified by the atmosphere, the subject's position and how the light is reflected off the subject and how that produces the different contrasts and feel in images that takes experience.
The problem most beginners have is that they try to understand exposure in terms of how the camera works which leads many to think in terms of the camera being fixed and the light being infinitely variable. Which isn't actually the case. It also leads many to think of exposure as a fixed value such as f8 at 1/125sec whereas it's really a range of different brightnesses (or exposures) falling on a media that has fixed limit on the range it can record.
It becomes quite simple when you begin to view it in terms of the nature of light and how film reacts to it. The problem with cameras is that they use hideously complicated automation to try and ensure consistency in exposure which is basically only working on averages and the rule of thumb.
Film requires a fixed minimum amount of light to fall on it to begin the process of reaction and produce the first measurable change in density on the developed negative. This is measured as it's film speed. Increasing the amount of light hitting the film produces greater density in the developed negative until you reach the amount of light that produces the maximum density in the film.
The exact amount of light it takes to do this is consistent for all film of that stock at a fixed development, it is predictable. So for each exposure you are looking to record the exact same range of density on every negative which requires the exact same range of light intensities from shadow to highlight values to fall on the film for each exposure.
To do this you have only two controls, shutter speed and aperture. As they are both of equal calibration (as in each stop halves or doubles the amount of light) gives you a range of equivalent options such as f8-1/125, f5.6-1/250, f4-1/500 etc. This you can vary by one or two stops either way before you start running into problems with either the shadows or highlights falling outside the film's ability to record them. The range of possibilities is quite definitely finite and restrictive with film.
With roll film the development must be the same for all exposures on the roll, it is a fixed quantity and therefore the exposure required to produce those densities is also fixed. For instance Caucasian skin is a tone that's generally one stop lighter than middle grey. To reproduce that tone consistently the film [
or the area of the negative that represents Caucasian skin] must receive exactly the same amount of light on all exposures. And as the meter, (or spot meter), is calibrated to indicate the correct exposure to reproduce that tone as middle grey to get the lighter tone you need the negative to record one stop more density, or give the film one stop more exposure than indicated when taking a reading of Caucasian skin.
When I used to photograph my friends children playing in the garden, for instance, I would just take a reflected reading off the palm of my hand in both sunlight and shade, it was rarely more than a stop or two different and wouldn't vary much for an hour or two. One stop exposure above indicated gave me all I needed. I would then just move the aperture ring accordingly depending on whether the sun was behind or in front, in shadow or sunlight with my full manual Nikon F2. The point being was that I didn't worry about infinite variables for each and every shot as the light was consistent and the exposure needed to reproduce skin tones was fixed. All I needed was a one or two click adjustment depending on the nature of the light and it's direction relative to the subject.
It's actually very simple, and if you develop the film yourself you get the full latitude it offers rather than the somewhat reduced one if you use the marked box speed and send it away to be developed. (
To gain a useful increase in speed films are generally overrated which means usually slightly under-exposed and over-developed which generally blocks deep shadow [no recorded density in the film] and the over-development increases density in the highlights to compensate leading to increased contrast and slight steeping of fall-off in the highlights).
