I find cameras are often a lot like cars to people using them: they know when it works, they know what it does and can use it. But they are often clueless when it comes to the mechanics of HOW it works. To some of you, this may be a boring topic and I suggest you instead check out some great travel perspectives from Pam Mandel at Nerds Eye View.
First, a diagram with a bunny:
This diagram shows where the shutter sits in your DLSR. It is behind the mirror (and the light meter on some camera) and typically hidden from view. Let me show you what it looks like with the mirror out of the way.
That’s my thumb in the corner, in case you are wondering. What you are looking at is the first curtain of the shutter mechanism. In the picture you can see the individual leafs of the curtain. Your camera may have more or less than this Canon 7D, which has four and is what I diagram below. No matter the number, the mechanics are the same.
When activated by pressing the shutter release on your camera (or remote) the aperture on your lens is stopped down to the appropriate setting, the mirror flips up and out of the way and then the shutter magic happens. The diagram below shows a longer exposure in order to exaggerate the movements.
Looking at this diagram you may wonder why there needs to be two curtains. That is because, as shutter times get faster and faster, the mechanics of a single curtain fail to keep up with both dropping out of the way and springing back up. Also, it would allow a disproportionate amount of light to hit either the top or bottom of the sensor as it traveled. This is why two curtains are employed.
If you set your camera on a shutter time of one second, you can hear all of this going on inside. The initial slap of the mirror moving out of the way and the first curtain click are often very close. Then after the one second you will hear the second curtain activate followed by the mirror dropping and the whole mechanism resetting.
When things get faster, it looks like this:
With faster shutter speeds the speed of the shutters is increased and necessitates a narrow gap between them to help only allow so much light in.
Shutter and camera technology is constantly changing and improving. Some cameras now have a pass-through mirror and no actual shutter. Most P&S cameras don’t use a shutter curtain at all (scanning the pixels the way an old TV used to project the image by scanning). But for now, for those with DLSRs with traditional shutter curtains, this system still holds true.
Lastly, this may also help explain to some of you what is meant by “second shutter sync” when setting your flash. Basically the flash on your camera (or external flash) can set to fire in sync with the first curtain activating or the second curtain. With very fast shutter speeds, this matters little, but as speeds get slower, say 1/20 second, the decision to of when to fire the flash will make an impact on whether a moving object comes before or after the blur caused by movement.
If you want blur behind a moving object, use a second curtain sync (so the flash fires just before the exposure is ended). If you want blur in front of a moving object, set your camera to first curtain sync so the flash fires with the first curtain and then allows the object to continue moving before closing the second shutter, ending the exposure.
I hope this has been informative to those wanting to know a bit more about what goes on inside their camera.