- its ability to achieve extremely fast speeds provide a new levels of exposure and action stopping capabilities
- having a shutter incorporated into the body of the camera eliminated the need for and expense of buying lenses with built-in shutters (making lenses lighter and more affordable).
Light path through a typical DSLR
As a review, the picture above illustrates the path that light takes to reach the camera's sensor. Entering the lens, light passes through the aperture and then into the body of the camera. When the camera is not in the process of taking a picture, the mirror (shown in yellow above) is in the down position. This allows the image to be reflected into the viewfinder where you see and compose the photography. As you push the shutter button, the mirror moves up and out of the way. This allows the light to pass through the shutter (shown in blue) and strike the elements on the sensor to record the image. It's the shutter in the above picture that will be discussed in this primer.
Focal-Plane shutter in the closed position
The focal-plane shutter inside your DSLR is an electro-mechanical wonder but works in virtually the same fashion as the original Leica version. To load film in a traditional 35mm film camera, the back is opened. With the back of the camera open, the focal-plane shutter is easily seen. (It's that thing we were told NEVER to touch when loading film.) However, the backs of DSLRs can't be opened, and the photographer rarely sees the shutter. So, the picture above is an inside look at the hidden shutter in your camera. Physically the shutter is situated just in front of the camera's sensor.
Functionally, the shutter acts like a door inside your camera. Door closed: no light enters. Door open: light is allowed to strike the camera's sensor. However, this door is really quick. Depending on the DSLR camera model, this "light door" can be open for as little as 1/8,000th of a second. Well.... sort of.
Here's how your camera's focal-plane shutter works.
What isn't apparent when looking at a focal-plane shutter is that it's actually made up of two separate "curtains". The curtains in modern DSLRs are made of extremely light and durable metal leaves (seen in photo above).
Side, cutaway view of vertical focal-plane shutter
The two curtains are shown in the side, cutaway view of a typical vertical focal-plane shutter diagram above. The first curtain (shown in green above, and also in the focal-plane shutter photo) covers the shutter opening when the camera is at rest to seal out light.
When exposure begins, the first curtain instantly moves out of the light path and to the top of the shutter housing
When you press the shutter button at speeds from the camera's slowest shutter speed to 1/60th of a second, the first curtain (shown in green) rapidly opens and retreats to a position in the upper shutter housing. This action allows light to pass through the shutter opening and onto the sensor. When the exposure is complete, the second curtain shown in orange moves rapidly upward to cover the opening and, once again, prohibit light from striking the camera's sensor. Immediately after the exposure, both curtains are returned to their original position in preparation for the next exposure. (In traditional 35mm film cameras, "cocking" the camera not only advanced the film by one frame, but it also reset the shutter by returning the two curtains to their ready position.)
Walking through an example should help. In this example, imagine your eye is substituted for the camera's sensor:
1st curtain closed, prior to exposure
Let's say you determine that the proper exposure for a picture of Portland's skyline should be f/11 at 1/60th of a second. The shutter button is pressed and for a brief instant (1/60th of a second to be exact) you would see the entire picture through the shutter opening as shown below:
Portland skyline seen through focal-plane shutter at 1/60th of a second
From the slowest shutter speed to 1/60th of a second, you would always see this same FULL FRAME image as you watched the shutter open, pause for the required time, and then close. But once you reach 1/60th of a second you have reached the maximum speed at which the shutter can FULLY open and close.
So, what if your exposure calls for a shutter speed of 1/500th of a second?
At speeds faster than 1/60th of a second (see footnote below), the shutter action is quite different. To understand how the shutter creates exposures faster than 1/60th of a second, it is important to remember: A shutter's function is to control the length of time each INDIVIDUAL pixel element on the camera's sensor is allowed to "see" the scene.
To achieve speeds of 1/125th and up (reaching as fast as to 1/8,000th of a second on some cameras), the two curtains work in tandem to precisely reduce the time the sensor "sees" the scene:
Curtain action above 1/60th of a second
As seen in the diagram above, soon after the FIRST shutter curtain begins its travel, the SECOND curtain begins to follow. The result is a SLIT that passes in front of the camera's sensor. The smaller the slit, the faster the effective shutter speed:
Seeing the curtain-created slit
Replacing the camera's sensor with your eye again and freezing the action of the slit, the same Portland skyline would appear as shown below. And eventually this slit will travel the entire height of your sensor's image area.
Shutter action frozen in time to reveal the curtain positions and slit exposure at 1/500th of a second
As the camera's shutter speed increases above 1/60th of second, the space (slit) between the two curtains decreases to reduce the amount of time that each INDIVIDUAL sensor element is allowed to see the scene.
Your DSLR automatically determines the timing between the beginning of the first curtain's travel and the start of the trailing curtain's travel. Precisely controlling this timing determines the size of the slit that travels across the sensor.
Note: This explanation of focal-plane shutters uses 1/60th of a second as the maximum speed that the curtain mechanism can achieve. Many modern DSLRs have broken that decades-old barrier and are now able to attain 1/125th or even 1/250th of a second before employing the slit method. To find the highest speed your shutter can achieve before beginning to use the slit process, look in your manual under "electronic flash". You will find your camera's highest "synch" or "synchronization" speed listed. This is the highest speed at which the first curtain is FULLY open BEFORE the second curtain begins its travel. This synch speed will become very important in future Hub's Camera posts covering electronic flash.
So, what if your exposure calls for a shutter speed of 1/500th of a second?
At speeds faster than 1/60th of a second (see footnote below), the shutter action is quite different. To understand how the shutter creates exposures faster than 1/60th of a second, it is important to remember: A shutter's function is to control the length of time each INDIVIDUAL pixel element on the camera's sensor is allowed to "see" the scene.
To achieve speeds of 1/125th and up (reaching as fast as to 1/8,000th of a second on some cameras), the two curtains work in tandem to precisely reduce the time the sensor "sees" the scene:
Curtain action above 1/60th of a second
As seen in the diagram above, soon after the FIRST shutter curtain begins its travel, the SECOND curtain begins to follow. The result is a SLIT that passes in front of the camera's sensor. The smaller the slit, the faster the effective shutter speed:
Seeing the curtain-created slit
Replacing the camera's sensor with your eye again and freezing the action of the slit, the same Portland skyline would appear as shown below. And eventually this slit will travel the entire height of your sensor's image area.
Shutter action frozen in time to reveal the curtain positions and slit exposure at 1/500th of a second
As the camera's shutter speed increases above 1/60th of second, the space (slit) between the two curtains decreases to reduce the amount of time that each INDIVIDUAL sensor element is allowed to see the scene.
Your DSLR automatically determines the timing between the beginning of the first curtain's travel and the start of the trailing curtain's travel. Precisely controlling this timing determines the size of the slit that travels across the sensor.
Note: This explanation of focal-plane shutters uses 1/60th of a second as the maximum speed that the curtain mechanism can achieve. Many modern DSLRs have broken that decades-old barrier and are now able to attain 1/125th or even 1/250th of a second before employing the slit method. To find the highest speed your shutter can achieve before beginning to use the slit process, look in your manual under "electronic flash". You will find your camera's highest "synch" or "synchronization" speed listed. This is the highest speed at which the first curtain is FULLY open BEFORE the second curtain begins its travel. This synch speed will become very important in future Hub's Camera posts covering electronic flash.
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