This Voigtländer Bessa 6×9 from around 1947 has an embossed metal table on the back which allows the photographer to calculate the depth of field based on the aperture and the focus distance. It’s still as accurate today as it ever was, provided you are using a 110mm lens.
If you’re not sure how to read the table, here’s an example. With the aperture set to f/5.6 and the lens focused at 12′ away, the whole distance between 10-15′ will be in sharp focus. You can see that focusing will be tricky with the lens wide open at f/4.5 and the camera focused at 3’5″. The depth of field is only six inches and this Bessa has no focusing aids such as a rangefinder or ground glass. You just have to read the scale and guess the distance (or use a tape measure).
The circle and triangle symbols at the bottom are to do with the hyperfocal distance. To compromise and simplify operation for the photographer, you set the aperture to f/8 and turn the focusing ring to the circle. Now everything between 20′ and infinity will be in reasonable focus, which is probably fine for all landscape photography. Likewise if to turn the ring to the triangle, everything 8-16′ away will be reasonably sharp which is probably fine for most portraits.
The Bessa is in pretty bad shape with leatherette peeling off, rust on the bodywork and the shutter sticking at slow speeds. It still works though! This Bessa is equipped with a Voigtar F=11cm 1:4.5 lens which was the bottom of the range at the time, but still nice to use.