David, this is easier if you say it like this:
The meter does NOT know what you are pointing it. It only measures the light falling on its photodetector. Also, it does NOT know how dark or bright you want the object to be on the film. He ASSUMES (it is calibrated so) that you want to make it 18% gray.
If you point it to a black cat and use the reading, you will get a 18% gray cat on the neg. If you point it on a white cat and use the reading, you will get AGAIN a 18% gray cat on the neg. They will be exactly the same. Why? Because the meter only measures the light reflected from them and calculate the f/stop and shutter time to make them 18% gray on the film you use.
If you want a black cat to be BLACK and not gray, it needs LESS exposure than the meter indicates, thus, you have to underexpose it (shorten exp.time or stop the lens down).
If you want the WHITE cat to be WHITE it needs MORE light, more exposure than indicated by the meter, thus you will have to overexpose compared to the meter's reading.
If you find a cat that is 18% gray naturally 🙂, your meter will show you EXACTLY what you have to use to get its true brightness/darkness/grayness on the negative - 18% gray.
Of course, this all excludes the next step, the printing, where you can compensate for under/overexposure and make the two cats again the same looking 18% gray. Alas, commercial labs always DO compensate at printing, following some whatever logic they have (man or machine), so you will never be able to figure out what's going on and who screwed it up, unless you look at the film directly, and not at the prints.
As an example: stage, black background, bright spotlit musician. Even if you expose the negative perfectly, your lab will make an average gray out of it, which will mean: they expose it shorter than they should, and the big black background (transparent parts on the negative) will be brightened up to murky gray, while the spotlit musicians (the dark spots on the negative) won't have enough light to correctly show up on the print - they will be too bright thus and the facial/hand details will disappear in the blown-out highlights.
There are 3 ways out:
1. Discuss your printing preferences with the lab (expensive and time-consuming).
2: Include big bright regions like reflectors, etc in the frame to compensate for the big dark backgrounds (tricky, rarely possible but this is what i try currently).
3: Don't go to a lab - print your own, digitally or traditionally.
Shooting slides is also ok but not really an option in black and white.
Now i release your hand, but don't fall! 🙂
hope it helps.
