Types Of Shutters

There are two principal types of shutters:

1. Between-the-lens shutter

2. Focal-plane shutter

The between-the-lens shutter consists of a series of thin steel blades set between the elements of a lens. The shutter release button activates a spring that snaps the blades open for the duration of the exposure, then shut. This type of shutter is also called a leaf type shutter.

Types Shutters Camera

Focal-Plane Shutter.

Between-The-Lens or Leaf Type Shutter

Focal-Plane Shutter.

Between-The-Lens or Leaf Type Shutter

The focal-plane shutter is located in the camera, directly in front of the film. It is directly in front of the plane of focus -where the film is. Hence, the name.

Focal-plane shutters offer two advantages:

First, because the focal-plane shutter is built into the camera and not into the lens, interchangeable lenses are usually less expensive. With the leaf shutter, a shutter is part of each lens. With the focal-plane shutter, the lens does not include a shutter so the lens can be less expensive.

Second, focal-plane shutters are capable of much faster exposure times. To understand why, you have to understand a bit about how the focal-plane shutter works. The focal-plane shutter operates something like a pair of roller window curtains. One curtain pulls open first, permitting light to reach the film. Then, after a predetermined fraction of a second, the second curtain follows it and blocks the light. That's how the focal-plane shutter works up to the limits of the speed with which the curtains can move across the film. This limit is around 1/60-second or 1/125-second on most SLR's.

If the curtains cannot expose the entire frame for any shorter time than 1/60 or 1/125, how is the SLR capable of making exposures of 1/500 or 1/1000 or even faster? The answer is, by exposing only part of the frame at a time, as follows:

At very fast exposure speeds, such as 1/500 or 1/1000, instead of the second curtain waiting for the first curtain to open up all the way, the second curtain starts to follow the first curtain before the full frame has been exposed. The film is exposed by a "slit" of light that sweeps across the frame. To see how this works, follow the pictures on this page.





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F Curtain 2

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In these pictures, assume that the two curtains are moving from left to right. In Picture A you see the complete scene as it looks to you in the view-finder. In picture B you see the image that the film "sees" before exposure. It sees nothing because Curtain 1 is blocking the light from reaching the film. Now7 you press the shutter button. In Picture C you see the start of the exposure. Curtain 1 has started to move to the right, exposing a vertical segment of the film. As shown in Picture D, when Curtain 1 is about one-quarter of a frame across, Curtain 2 starts to follow it, exposing the film to a "slit" one-quarter frame wide. In Pictures E, F, and G you see how Curtain 2 follows Curtain 1 across the film. The only light that reaches the film is the light in the opening between the two curtains - the "slit." The film is exposed in one continuous sweep of this slit.

In Picture H Curtain 2 has reached the other side of the frame, and no more light reaches the film. The exposure is complete. How does this arrangement permit exposures of extremely short duration? Let's say that at 1/250, the "slit" is one-quarter of a frame wide. How can we make the exposure even faster? By narrowing the slit. If we start Curtain 2 sooner so that the slit is only one-eighth of the frame wide, then the exposure becomes 1/500. If w^e start Curtain 2 even sooner so that the slit between the two shutters is only one-sixteenth of the frame wide, then the exposure becomes 1/1000th. And so on. That's how the focal-plane shutter achieves its incredible speed. Not because the curtains move so fast but because they can provide exposure through a sliver-thin slit between the two moving curtains.

Your understanding of the way focal-plane shutters operate will help later when you reach your lesson on how to use strobe.

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  • goytiom
    What are the two "principal" types of shutters?
    8 years ago

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