Using folding cameras requires some care and forces you to make some serious decisions before taking a picture. The shooting procedure is fully manual, which means that all parameters have to be set up by hand: You have to adjust the distance, the aperture, and the exposure time, allowing the time to make sure they are set up correctly. Optimal exposure requires the use of an external exposure meter, which delivers the range of aperture and exposure time combinations available for the given light and personal demands. With their archaic film advance, the uncoupled rangefinder, the front cell focusing, and the lack of an exposure meter, most medium format folding cameras are no point and shoot cameras. However, slowing down by carefully adjusting and controlling the decisive photographic parameters increases the amount of satisfying pictures and revaluates each single shot.

But using vintage folding cameras can be highly problematic: Particularly, Agfa folders feature bellows manufactured from poor plastic material. Constant opening and closing over many decades leave tiny pinholes, particularly in the corners. Besides the problems with light leaks, the lubricants used by the camera manufacturers prior to the 1970s or 1980s are another weak point. They often cause frozen or stiff focusing rings and retarding shutters, the latter resulting in strong overexposure. Dust in the shutter mechanism and fungus or haze between the lens elements are further items. For proper function, the lenses and shutters usually must be disassembled, cleaned (sometimes adjusted), and reassembled. There are only few experts with sufficient know-how.

Some tips for using vintage folding cameras based on my experience until now:

1. Folding cameras are normally fitted with either 3-element lenses (triplets) or 4-element lenses (Tessar types). Particularly, the triplet lenses show their best overall performance when stopped down to f/8-f/16 or even to f/22, whereas f/32 shows significantly lower contrast and resolution as a result of diffraction. Tessar-types perform better at wider aperture setting, in particular in the central image area. In the heyday of folding cameras their negatives were hardly used for enlargements but for contact prints, where some blurring as a result of low lens performance and/or camera shake went unnoticed.
2. Whenever possible, the original lens hood should be used (usually push-on types). Special care is needed for pre-war cameras with uncoated lenses. Direct sunlight falling on the lens usually causes low contrast and lens flares. Manufacturer catalogs published in the fifties often contain a listing of the appropriate accessories.
3. Folding cameras without integrated (uncoupled or coupled) rangefinder need to apply one of the following methods to adjust the relevant distance: guessing, infinity focusing or zone focusing. The closer to the subject, the more important accurate focusing becomes. At close distances an external rangefinder helps a lot.
4. Most folding cameras have no parallax correction marks in their viewer window. Basically, it is wise to incorporate some space around the frame to avoid cutting off important parts of the image. Parallax correction should be considered in particular, when photographing subjects at close distances. I recommend to check out the amount of parallax shift.
5. Old leaf shutters tend to be slower than the adjusted exposure times, particularly the longest and the shorter times. Even after cleaning, lubricating and adjusting (CLA), adjusting for example 1/500 sec may actually result in 1/400 sec. This can indeed be considered when calculating the relevant exposure value, but is no problem when using b&w or color negative film. Such film is somewhat tolerant with regard to slight overexposure.
6. Compur-Rapid and Synchro-Compur shutters use a strong extra spring to achieve their shortest shutter speeds. The top speed should not be set after cocking the shutter, and the shutter should not set off the top speed once cocked. So do not change shutter speeds when the shutter has been cocked. Otherwise, the shutter mechanism can be damaged.
7. Folding cameras are compact and light relative to their format and focal length. As a result, they are prone to camera shake (6×9 folders even more than 6×6 folders). The moving parts of the shutter release mechanism cause some friction and some vibration. For hand-held shooting, exposure times longer than 1/100 sec (or even shorter) often produce blurred pictures. One can bypass the body shutter release mechanism by using the small lever directly on the lens, but this is less convenient. A tripod is another option for creating sharp pictures, although folding cameras were designed for hand-held shooting. Some cameras allow the shutter to be released using a cable release connection directly on the lens, which also avoids using the body shutter release mechanism. Pressing the body shutter release button or the cable release gently definitely helps to avoid camera shake. In this context, one must take into account that the faster exposure times set on mechanical shutters are actually much slower, increasing the risk of camera shake (see above).
8. Film flatness is another issue when using folding cameras, more so in the case of 6×9 folders. Some photographers recommend to open the front of the camera before advancing the film to ensure film flatness. Others recommend to advance the film immediately after each exposure to prevent double exposure. The latter is more relevant for cameras with no double exposure prevention mechanism. My advice is to always open the camera front carefully by releasing the opening button with the hand holding the camera body and to simultaneously catch the front standard by the other hand. This prevents from popping out, suctioning air from behind the film plane and bending the film out of focus.
9. After having unfolded the camera, one should always make sure that the lens board and its struts are locked in position. Folders possibly suffer from issues with the lens board not positioned parallel to the film plane, even with the lens board locked in position. As a result, different amounts of blurring around the borders at wider apertures are noticeable. But Novars in general and Tessars show anyway some blurring around the edges at wider apertures, possibly masking issues with non-parallel lens standards. Particularly, Zeiss Ikon folders are often preferred for their rigid folding mechanism. But due to the comparatively large spread of standard factory models in the fifties and before, we should not expect too much precision of a riveted mechanism.
10. Cameras with no automatic film advance but a red window at the camera back for watching the film advance enable another practice to ensure film flatness: Winding the film after each exposure just before the next frame number appears in the middle of the red window and exactly aligning the frame number immediately before shooting. Or aligning the frame number exactly and retightening the film immediately before shooting by turning the film winding knob a trifle without actually moving the film. But possibly, there remains still some unevenness of the film surface. Small apertures ensure more depth of focus and help to receive sharpness across the full frame.
11. The implementation of a double exposure prevention mechanism made folding cameras easier manageable, but it often does not work smoothly, particularly when a vintage camera has not been cleaned and lubricated. Some suggestions based on my experience with Agfa and Zeiss Ikon folders: If the shutter release button is pressed hesitantly, the double exposure prevention becomes active before exposure and the button is blocked. In this case, the shutter can be released by the little lever at the lens shutter assembly.
12. Another possible consequence of hesitantly pressing the shutter release button is that the automatic film transport is blocked after exposure, because the double exposure prevention did not disengage properly. In this case, I cover the lens opening carefully (e.g., by using my smartphone cover), cock the shutter again and press the shutter release button more strongly. In bright light, I close the aperture and set the shutter at the shortest exposure time (e.g., 1/500 sec) before releasing the shutter again to avoid fogging. The dilemma is that pressing the shutter release button too strong can result in camera shake. Finding the right amount of pressure to trip the mechanical shutter of folders can be challenging.

Note: There are numerous reviews of vintage folding cameras out there in the web, discussing cameras which have not been cleaned, lubricated and adjusted before, sometimes additionally using expired film. This makes it quite difficult for the reader to judge the performance of a specific camera or lens, respectively. For example, low contrast of a vintage lens and the effect of some haze between the lens elements can hardly be distinguished. Also, lens standards nonparallel with the film plane, lack of film flatness and uncalibrated focusing can absolutely pretend bad lens performance and should be taken into account when judging a lens.