Which begs the question, how does one focus without a rangefinder?
Cameras without rangefinders have a distance scale of some sort engraved on the lens collar. Typically, this is in feet or meters, sometimes both.
On 'zone-focus' cameras, the distances are marked with icons representing an approximate distance, such as an icon of a face for a close portrait, a head-and-shoulders icon for, well, you guessed it, and an outline of mountains for infinity. But these zone-focus cameras are more commonly seen on 35mm cameras.
To focus a camera which does not have a rangefinder, one either measures the distance to the subject by estimation or by using a measuring instrument of some type.
When these cameras were popular, accessory or pocket rangefinders were also popular. Search eBay for 'watameter' and 'ideal range finder' and 'pocket rangefinder' for examples. They are simply rangefinders, just like you'd find on a camera, but they are designed to slide into the camera's accessory shoe when needed. You find the distance, then turn the focus collar on the lens to match the distance read out on the pocket range finder.
One only has to be careful to get a pocket range finder that reads out in the same measuring system as the camera - feet or meters - or be prepared to commit math.
The other system, as I mentioned, is estimation, also known as 'guestimation'. One guesses the distance and sets it on the lens focus collar.
This is not unlike setting exposure without a meter - you might manage to get halfway decent at it in time, and if you shoot under the same or similar conditions all the time, you might not ever have to worry about it. For example, if you shoot landscapes, then infinity focus (actually probably hyperfocus) is your friend. Depth-of-Field at small apertures will cover a multitude of errors, just like film latitude covers small exposure errors. It only gets difficult if you are shooting close in, or at wider apertures, in which case you would probably definitely want a pocket range finder.
Like any older manual system, it is not generally difficult to do, but it adds time and some complexity to your work flow. Using an older folding camera means learning about remembering to advance the film using the red window the same way every time (either right after you take a shot or right before, but if you don't have a system, you'll forget and get blank frames and double exposures). It means remembering to cock the shutter, the winder doesn't do that for you on many cameras. It means setting the focus, aperture, and shutter speed way out on the front of the camera, which is most inconvenient compared to modern 35mm SLR cameras.
However, the results can be stunning, especially on a very large MF such as 6x9 (my personal favorite). You can get a neg which you can scan and it will blow away ANY modern consumer digital camera output, and most 35mm film as well.
Whatever you get, check the bellows for light leaks before you go out, put a piece of tape over the red window so you keep out as much light as possible, and have fun. FYI, Agfa folding cameras often have frozen front focusing cells from the type of grease they used then. If you get one, be prepared to spend many a fun hour with a toothpick and degreaser, trying to get it unstuck. They're good cameras, but that dang grease they used then, argh.
