For techno geeks: Want to know why shutter is necessary for DSLR

[mukul]

I have Powershot A450 [CCD sensor] that seems does not have any shutter assembly.And it can capture up to 1/2000 sec.
Mobile cameras also do not have any physical shutter mechanism.

Why shutter assembly is necessary for a DSLR?

If that dslr can do without any physical shutter assembly it can definitely reduce cost, camera shake etc. etc.

techno geeks, please enlighten me

 

[bsb03]

It has to do with the sensor design. The sensors in most dSLRs build up a charge when the shutter is open and then when the shutter shuts it releases the charge. If there was no shutter the sensor would continue to gather light and charge causing a bad image. It is actually cheaper to use a mechanical shutter because it reduces the amount of circuitry in the sensor itself.

 

[mukul]

^^ Then what is the sensor design for mobile cameras etc?

Can you please mention any resources for further reading?

 

[dgatwood]

Short answer: They aren't necessary. When you use your camera in video mode, the shutter stays open.

Long answer: Whenever you obtain image data from a sensor, you don't get all the data at once. Instead, you scan the sensor across one row, then the next, then the next until you reach the bottom. (The actual direction is arbitrary.)

The problem with this approach is that it is not instantaneous. As a result, most sensors exhibit a phenomenon called "rolling shutter", wherein video shot out the window of a moving car appears to be slanted, because by the time the camera scans the last line, the vehicle has moved. In effect, the exposure at the top of each frame begins and ends a fraction of a second before the exposure at the bottom of the frame.

There are three approaches for handling this problem:

• Ignore it. Cell phones and many point-and-shoot cameras don't bother to do anything about rolling shutter, because most consumers don't know the difference. And even DSLRs act this way when used in video mode.
• Use a mechanical shutter. By physically opening and closing a shutter, you expose the entire sensor all at once, then cover it up all at once, so the exposure at the top of the frame occurs precisely at the same time as the exposure at the bottom of the frame.
• Use an electronic shutter circuit that simultaneously dumps the entire contents of the sensor into a buffer elsewhere on the image sensor, which the camera can then read at its leisure. Thus far, I've only seen this technique used on CCD-based sensors, rather than the CMOS sensors that most still cameras use.

 

[jrista]

Most electronic shutters up until recently were progressive shutters...the sensor would be read top down or bottom up row by row. That's relatively fast, however not fast enough to prevent exposure gradients from occurring, where the rows read later are brighter than rows read earlier.

To guarantee that no light continues to expose the sensor during readout, a mechanical shutter is used to block light. That's the primary reason.

Today, more advanced electronic shutter technology called a "global shutter" is becoming more prevalent. A global shutter does a simultaneous dump of all pixels into a per-pixel backing buffer or "memory". This allows the exposure on the image signal to-be-read to stop. That buffered exposure is then read out, row by row. When the next frame of an exposure is ready (in either video or continuous stills), the sensor pixels and the backing buffer are reset to zero charge, and the new exposure starts.

Global electronic shutters are more expensive, as they require more logic per pixel. They need the necessary charge transfer logic and buffer memory, which requires more space. When you factor in shared pixel architecture, it gets more complicated. The transfer of pixel data into the buffer memory does take some time as well, and there is still a per-row activation required to initiate the transfer (although it can happen much faster than a full row readout), so use of a global shutter can still impose minor limits on frame rate when we get into the kinds of pixel counts we have for still photography (at least at price points that are acceptable to most photographers...when cost is no longer an object, you can achieve high quality IQ at exceptionally high frame rates...but it definitely increases the cost of the shutter.)

At some point, I figure electronic first curtain shutter will become the norm for DSLRs. Eventually, the shutter should eventually be dropped entirely for a full global electronic shutter, even if the mirror remains. I don't foresee Canon doing anything with global shutter until they reduce their transistor size. At 500nm, adding a global shutter would decimate their IQ. At 180nm, they would definitely be more capable of adding global shutter technology, but it still takes up die space and reduces fill factor a bit.

 

[Loswr]

Use a mechanical shutter. By physically opening and closing a shutter, you expose the entire sensor all at once, then cover it up all at once, so the exposure at the top of the frame occurs precisely at the same time as the exposure at the bottom of the frame.

That's true at slower shutter speeds, but not at faster shutter speeds. A camera's Xsync (max flash sync) speed is the fastest shutter speed at which the entire sensor is exposed at once. At speeds faster than that, the second curtain starts it's traverse before the first curtain finishes, so the sensor is exposed through a moving 'window' between the shutter curtains; that window gets narrower as the shutter speed increases further.

 

[dgatwood]

Use a mechanical shutter. By physically opening and closing a shutter, you expose the entire sensor all at once, then cover it up all at once, so the exposure at the top of the frame occurs precisely at the same time as the exposure at the bottom of the frame.

That's true at slower shutter speeds, but not at faster shutter speeds. A camera's Xsync (max flash sync) speed is the fastest shutter speed at which the entire sensor is exposed at once. At speeds faster than that, the second curtain starts it's traverse before the first curtain finishes, so the sensor is exposed through a moving 'window' between the shutter curtains; that window gets narrower as the shutter speed increases further.

Fair enough. Focal-plane shutters are definitely less than ideal. They're just better than most non-global electronic shutters.