A DSLR shutter controls the amount of time light hits the image sensor, determining the exposure. It works through a precise sequence of movements involving the camera's mirror and one or more shutter curtains positioned directly in front of the sensor.
When you press the shutter button, a series of mechanical actions occur rapidly. First, the reflex mirror, which normally directs light from the lens up to the viewfinder, flips up and out of the way.
According to the reference, after the mirror pops up, you see the shutter. Before it even goes down, before the shutter curtain goes down and exposes the light to the image sensor, the shutter mechanism is visible. This description highlights the two key components involved:
- The Mirror: Moves up to allow light to reach the sensor.
- The Shutter Curtain(s): Opens and closes to control the duration of light exposure on the sensor.
Most DSLRs use a type called a focal-plane shutter, which typically consists of two curtains (a front curtain and a rear curtain) located just in front of the sensor plane.
Here's a simplified breakdown of the exposure process described:
- Mirror Up: The mirror lifts out of the light path.
- Shutter Visible: After the mirror is up, the shutter mechanism is seen.
- Front Curtain Opens: The first curtain (front curtain) moves to uncover the sensor, allowing light from the lens to begin hitting it. This starts the exposure.
- Exposure: Light exposes the sensor for a set period (the shutter speed).
- Rear Curtain Closes: The second curtain (rear curtain) follows the first, covering the sensor and stopping the exposure.
- Mirror Down: The mirror returns to its original position, reflecting light back into the viewfinder, and the shutter curtains reset.
At slower shutter speeds, the first curtain opens completely, and then after the set time, the second curtain closes. At faster shutter speeds, the second curtain starts closing before the first curtain has finished opening, creating a moving slit across the sensor to achieve a very short exposure time.
This intricate mechanical dance, initiated after the mirror lifts, is crucial for capturing the image by precisely timing how long light is allowed to reach the digital sensor.
