Forget auto-exposure, or even light meters. Back in the day this booklet and some mental arithmetic was all that was required to achieve the perfect exposure when taking photographs.

The Exposure Time Tables pocket reference book by Zeiss Ikon was the go-to guide for working out how to snap your shot. As Peta Pixel explains:

There are 5 steps (i.e. things you need to consider) on the path toward determining your proper exposure time: time of day/year, lighting, object to be exposed, dryplate/film, and aperture. Each step provides you with a number. After the 5 steps, add up all your numbers and then look it up on the chart to figure out your exposure time!

The process was, apparently, developed by a photographer named Dr. Max Leo and has long since been abandoned. Still, if you fancy finding out a little more, you can read a PDF of the whole manual here. [Camera Manuals via Peta Pixel]