Many people cite SLRs as bulky, noisy and complicated. The latter is not true, at least for anyone who reads the manual enclosed with the camera. The second point might just be true: SLRs are noisy. In fact, I rejoice in the noisiness older cameras. This post is all about the noisiness of SLRs, and why we love it!
You now know about the mirror found between the film and the lens. But I didn't tell you about the focal plane shutter. The FPS is a photographic shutter that stops light from making contact with the focal plane (film or sensor) of the camera.
Almost all SLRs feature horizontal double-curtain shutters. In slow shutter speeds, the opening curtain moves to the left, and after the necessary amount of time has passed, the second curtain (which has remained static) moves to the left side, thus, closing the focal plane again. When the shutter is recocked, both curtains move back to their starting positions, ready to be released again.
In faster shutter speeds, the second curtain starts to close before the first curtain has fully opened. This creates a slit between both curtains through which light passes. As the speed of the shutter rises, the size of the slit becomes narrower and narrower, which allows some cameras to reach shutter speeds of up to 1/8000 of a second.
The Double-Leaf shutter consists in 2 blades that are usually between or behind the lens. This system is used in the Kodak Retina Reflex series of cameras. This type of shutter synchronized with flash at most shutter speeds, and it became the favourite of studio photographers.
All this brings us back to out first point: noisiness. The SLR has to do at least 6 things when the shutter is released: first, the mirror rises, shortly afterwards, the first curtain opens the shutter, and then the second curtain closes the shutter. All this motor moving and closing and opening rattle is what makes an SLR take neat exposures.