If you are trying to learn how to better control your exposure and development for Zone System, BTZS, and other exposure systems, then you will probably need a densitometer. This device reads the densities of your negatives, and depending on the model, the prints too. This video goes over the basics of how to use one in your darkroom.
Different makes of densitometers include X-Rite, Tobias, Macbeth, Eseco, Heiland, and Kodak.
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