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I Shutter to Think...

You press the button to take a picture and hear the click of the shutter activating. Why is it that so many digital cameras still feature mechanically activated focal plane shutters? Why not simply turn the sensor on briefly and then back off to end the exposure?

There once was a time that cameras were completely mechanical. My ancient Nikon FM2n film camera could work just fine with no battery power at all. Yes, the light meter required a battery to function, but so long as you relied on other means to determine proper exposure, you could shoot just fine without electrical power of any kind. Those days are long gone now. My old trusty Nikon N8008s came not only with auto-exposure but also auto-focus and wouldn't function at all without batteries. This was a somewhat electro-mechanical era where the batteries powered not only the electronics including the LCD display panels, but also drove the film advance motor that wound the film onto the sprockets of the canister, frame by frame. And they also temporarily lifted the SLR mirror out of the way and powered the mechanical shutter opening and closing.

Modern digital cameras have dispensed almost entirely with the mechanical components of older models. Film has given way to digital sensors that record information electronically and don't need to be pulled forward to prepare them to shoot the next frame. Modern cameras have basically evolved into very specialized computers, and computers require electrical power to operate. Even as the SLR mirror and mechanical shutter remain as standard equipment.

My cell phone camera does have an electronic shutter. I have the option enabled for it to produce a satisfying "click" sound when I take a photo, not so much because I long for the days of yore, but because it lets me know that I really pressed it without looking. But there isn't really anything in there that clicks — it's just a sound effect. A new breed of mirrorless cameras also makes do without mechanical parts. For one thing, this lets them be more compact than is possible with single lens reflex models.

Digital advances have improved photography in many ways, from improved metering and focus, to expanded dynamic range exposure and improved image resolution. And yet mechanical shutters have yet to yield to electronic ones, most notably among "professional" cameras.

A typical mechanical shutter on an SLR camera actually consists of two shutter curtains. At slower shutter speeds, the first curtain opens to expose the sensor, and the second shutter curtain closes behind it to end the exposure. It takes just a fraction of a second for either curtain to go from one side of the sensor to the other, but at short enough exposures, there simply isn't time to accomplish the traversal before the exposure is supposed to end. To go beyond normal sync speeds, the second shutter curtain actually starts closing before the first one has fully opened. This results in the image effectively being exposed by a rolling "slit" created between the two moving shutter curtains. By narrowing the gap between them, extremely short exposure times can be achieved. Once the light has been physically blocked, the image data can be read from each photosite (pixel) without risk of further exposure altering the image.

Most electronic shutters work the same "rolling" slit method as does the high-speed mechanical shutter. The circuity essentially scans the frame from one side to the other, offloading the data to be stored on your memory card. There isn't enough bandwidth to read it all at once. But with no mechanical shutter to physically block the light, the fastest possible shutter speed is limited by the speed with which the image data can be read from the photosites. As fast as this is, it's just not possible to achieve the same high speeds as the pair of travelling mechanical shutter curtains can go. Without a way to physically block the light, the shortest exposure time is limited by the sensor read out time.

Even at more moderate and comparable shutter speeds, the digital shutter image can suffer when compared to the mechanical one. The faster start and end to the exposure interval created by the moving mechanical shutter curtains can help to minimize image artifacts caused by objects that move during the exposure. Put simply, when one side of the frame gets exposed at a slightly different time than the other, moving objects can appear blurred or skewed. A moving object gets exposed at different places at different times across the frame. The faster the subject speed, the worse the resulting skewing problem.

An alternate electronic shutter design avoids this problem, but at the cost of adding a different one. Rather than employing the rolling transfer technique, it is possible to add additional circuitry to each photosite to store the accumulated charge even once power has been cut and the exposure has ended. This would thereby allow the data to be offloaded at a more leisurely pace, just the same as if the sensor were blocked by a physical, mechanical shutter. But this second circuit for each photosite can not only add considerable cost to the overall design, it can also degrade image quality simply by occupying space. More circuitry on the sensor doing something other than sense incoming light means less surface area actually recording your image. And less ability to sense light increases the likelihood of noise. All this extra circuity isn't free either and can result in increased prices for cameras that use this technique.

Cameras that include a "Live View" function actually employ both mechanical and electronic shutters. The mechanical shutter has to be held open for the image to be visible in the Live View display, so the camera must rely on an electronic shutter to avoid exposure. When the shutter release is pressed, the electronic shutter turns off, and the mechanical shutter controls the recorded image exposure.

Some newer cameras actually have a hybrid shutter design, utilizing an electronic shutter for the first curtain, and a mechanical shutter for the second curtain. With this approach, the exposure starts electronically, and ends mechanically. In the end though, light is still blocked after the exposure to allow for data to be read from the sensor without skewing or other potential problems.

Video recording always relies on a rolling electronic shutter, constantly offloading data across the frame while recording the next frame from areas already offloaded.

Clearly, electronic shutters do have certain advantages. Fewer moving parts can mean greater reliability and smaller camera designs. Being silent, they won't alert wildlife to your presence. Having no moving parts, they impart no camera shake. But for now at least, there are still too many hurdles to overcome to use fully electronic shutters in all cameras. This remains one area where the tried and true mechanical design still prevails.


Date posted: October 25, 2015

 

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