Camera Types
Viewfinder Cameras
The simpliest camera design is the viewfinder one. The camera has two independent optical systems, a finder and a taking lens. Today, almost all viewfinder cameras are in 35mm format, but there are also many old medium format viewfinder cameras. Normally, you only have three distances to select from, or even a fixfocus lens. Many of these cameras have slow zoom lenses (slow in the meaning of the proportion of light passing through the lens, requiring more sensitive film which has worse image quality), and their optical quality isn't sufficient in most cases.Rangefinder Cameras
The rangefinder design is very sophisticated, but not very common any more. In additional to the finder window, there is a rangefinder window the image of which is being superimposed on the normal finder image. You have to match these two images to determine the distance of the object focused onto. Most rangefinder cameras are coupled (you focus on the lens to change the rangefinder image), but some older ones are not (you determine the distance and then focus the lens onto that distance). These cameras are still rather small and light, but often offer high-quality interchangeable lenses and a very sturdy design. Actually they deliver more exact focusing than the reflex design due to the broader triangle base, especially at low light. Most rangefinder cameras are 35mm, but there is has been a certain trend on medium format rangefinders the last years.
Reflex Cameras (TLR and SLR)
The most common design is the reflex design. There are twin-lens reflex cameras (TLRs), where you have a coupled pair of lenses (taking lens and viewing lens), and the image of the taking lens can be viewed from the top of the camera with a mirror. There are many old medium format TLRs. Far more common today is the single-lens reflex (SLR) design. Here you have only one optical system with a moving mirror. You see the image in the finder, and when you release the shutter, the mirror slaps up, you have a blackout in the finder, and the photo is made unto the film. This system has the advantage that you exactly see what you get (especially important in close-up photography), and that you can focus on sight. The problems of the SLR design are the sound and vibration of the mirror slap. The cameras are also much bulkier than viewfinder or rangefinder cameras. Nevertheless the biggest camera systems today are 35mm SLR ones, such as those by Canon and Nikon. SLR is most common in 35mm, but also in medium format.
Ground-Glass Cameras
The ground-glass design is common in large format photography. You have a matte screen on which the image is upside down and mirrored. You compose and focus, and then you place a film holder in place of the screen. After taking, you remove the holder, and again place the screen on the back of the camera. Some medium format cameras also offer a ground-glass viewing option.