gray scale covers the gamut from complete black to absolute white. Ansel Adams was a master at capturing the gray scale with his "Zone System". It was all an issue of deciding what was the magical 18% gray and metering to ensure he had detail across the board, then developing the negatives to ensure it happened. The Zone system in short was one where after you shot the film to expose for the shadows and develop for the highlights.
This image was from my archives and dates back to November 1975. I was then very much into Adams and his zone system. I metered off an 18% gray card that came with my Kodak Professional Photographers Photoguide. I still carry it today to meter off of in extreme situations when I am shooting on film. (Yes, I still do my own B&W silver work AND I refuse to call it "analog photography" like some hipsters I know).
So what about the world of digital photography? You can still use the zone system. You set your ISO on your camera and use manual settings. The setting will come from the exposure details given to you from a handheld meter focused on a gray card. Use the recommended exposure settings from that reading and et voila you are working in the zone system in the digital world. If you are not sure how to set your DSLR on manual, you can always set it to bracket shots 1 stop apart and shoot of the cameras average meter in semi-auto mode.
Enjoy "Ozark Farm". The original was shot on Tri-X, developed in D76. The negative was scanned to positive with an Epson V600 Photo scanner. It does click to embiggin.