Firstly the advantages:
A rangefinder camera provides higher image quality than an SLR. This is partly because there is no flipping mirror; lenses can therefore be designed in such a way that the back of the lens doesn’t have to be removed away from the image plane to avoid getting hit by the mirror. When using a wide-angle lens, this enables designers to create lenses whose rear elements have far greater proximity to the image plane. Consequently, wide-angle rangefinder lenses can be sharper, smaller and give less distortion than your average SLR lens.
Also thanks to the lack of a flipping mirror, vibration is far less likely to blur hand-held shots. With speeds of 1/30 – 1/8, SLR flipping mirrors can result in blurred shots even when using a tripod – unless a mirror lock-up is used.
Rangefinder cameras enable greater precision in focusing when using normal or wide lenses.
They are smaller and lighter because they don’t carry the flipping reflex mirrors, focus screens and prisms. Also their lenses are smaller and lighter; wide and normal rangefinder lenses can be tiny because there is no flipping mirror to take up space.
Wide, ultra-wide and ultra-ultra wide lenses are easy to get for rangefinder cameras – even the cheapest ones are usually excellent.
The fact that there are no mirrors flipping up and down during each shot means rangefinder cameras are quieter – all you’ll hear is the click of the shutter, which means they are excellent for wild life photography.
When the mirror flips in an SLR camera in order to take a picture, the viewfinder goes dark. Given that this is the moment at which an image is captured, it can be annoying. With a rangefinder, the viewfinder doesn’t black out, so you always know the subject’s expression as it’s recorded, even when using flash.
During long exposures, it is possible to focus, compose and shoot with both eyes open.
Rangefinders also have no shutter lag – the moment you press the button is the moment you capture. With SLR cameras, the mirror has to move away before the shutter is able to open.
Even in very low light, it is possible to focus a rangefinder camera by flipping the focus until the two images merge.
Now for the disadvantages:
You don’t know exactly how the image will end up.
Because the viewfinder is separate, what you see and what the camera sees are from a slightly different point of view. For normal shots taken at average distances, it doesn’t matter. However when using a long lens, or for macro use, you’ll have no idea what you’re getting because you won’t see any depth of field in the viewfinder – where everything is always in sharp focus.
When using a lens that is wider than around 24mm, a separate, extra-wide viewfinder will need to be clipped onto your hot shoe. Focus exposure need to be set through the main viewfinder, while composition has to be achieved through the clip on viewfinder.
A large lens may be visible in the corner of the viewfinder and this can obstruct some of the image. Cut outs can be used, but many photographers simply avoid big lenses.
You cannot buy a fisheye lens or an ultra-telephoto – instead the photographer simply has to position themselves correctly to achieve the desired composition. Similarly, rangefinders cameras don’t tend to handle focusing close up, say between 0.5-1m.
Given the delicacy of the camera, it will need servicing every so often to ensure it is in an optimum state.