One of the most precise mechanism in every camera whether mirrorless or conventional DSLR is the focal-plane shutter. This is the mechanical part of a camera that controls the amount of time the light is allowed to pass through and expose the sensor.
The sound that you hear when your camera takes a shot is not the audible sound you hear in your cellphone but its actually the movement of mechanical pieces opening and closing the window just before the sensor. Focal-plane shutter is one of the three exposure controls you can adjust when controlling the amount of light passing through your camera.
Shutter speed is not dependent on your lens but your camera body. This means that even if you change from one lens to the next, you’ll still have the same shutter speed options. Those camera with no focal-plane shutter like point-and-shoot or those camera mounted on your cellphone controls the amount of exposure time by turning the sensor on and off electronically.Looks like the electronic shutter is still not as accurate as the mechanical counterpart. A mechanical shutter can control the time of exposure by 8000th of a second accurately and consistently. You can divide a second into about 30 options under the shutter speed for you to accurately get the right exposure.
Since this mechanical shutter is design for precision, camera manufacturer warrants the number of clicks the shutter can perform with accuracy. A normal entry level camera is estimated between 40 to 60 thousand clicks while mid to pro models can shoot between 80 to 150 thousand clicks problem free. This also means that in every click, you’re reducing the life of your shutter.
The actuation count or the shutter count is normally checked by second hand camera buyer. It tells you how much shots were taken by the camera and therefore dictate its worth. Even if the shutter reaches its life expectancy, the shutter will continue to work but the possibility of crashing down on you during an event coverage is very much possible.
A friend of mine who owns a classic Canon 5D has more than doubled its life expectancy. There was a time that he was shooting with his camera upside down because the focal-plane shutter has totally deteriorated but can still shoot decently when used upside down.
When shooting strobe, the function of the shutter to control the light through time exposure is useless. Strobes are fast and the light duration is around 2000th of a second. So even if you shorten the amount of exposure, it wouldn’t matter because the strobe already disappeared before you can even control it.
So Instead of three settings in your camera to control light, you’re down to two, ISO and aperture when using strobe. However, you must consider that the additional artificial light or strobe can be adjusted as to power. So you have an additional control to replace as your third control.