The rolling shutter effect is usually seen as a damaging defect but even this can be used creatively, with enough imagination. Photo by Jim Kasson, Fujifilm GFX 50S Click! goes the camera and in that fraction of a second the shutter races to end the exposure.
But, although it's quick, that process isn't instantaneous. Whether you're syncing a flash, wondering why banding is appearing in your image or deciding whether to use your camera's silent shutter mode, the way your shutter works has a role to play. This article looks at the different types of shutter and what effect they have.
At their most basic, cameras capture light that represents a fragment of time, so it shouldn't be surprising that the mechanism that defines this period of time can play a role in the final outcome. It's not nearly as significant as the exposure duration (usually known as shutter speed or time value), or the size of the aperture but, despite great effort and ingenuity being expended on minimizing it, the shutter behavior has an effect.
Mechanical Shutters
There are two main mechanical shutter technologies: focal plane curtain shutters and leaf shutters. The majority of large sensor cameras and nearly all ILCs use focal plane shutters while the majority of compacts use leaf shutters.
Focal plain shutters
Focal plan curtain shutters are what you probably think of when you think about shutters. At the start of the exposure a series of horizontal blades rises like a Venetian blind and, to end the exposure, a second series of blades rises up to cover the sensor again.
The first curtain lifts to start the exposure, then a second curtain ends the exposure. The shutter's movement is shown as the blue lines on the graph. The time taken to open and close the curtains (red) is defined by the shutter rate, the exposure time is shown in green.
These blades move quickly but not instantly. We'll call the amount of time it takes the shutter to move across the sensor the shutter rate. This is not the same thing as shutter speed, which is the amount of time that elapses between the bottom of the first curtain lifting and the top of the second curtain passing that same point.
Leaf shutters
Leaf shutters work slightly differently. These are built into the lens, right next to the aperture, and usually feature a series of blades that open out from the center, then snap shut again to end the exposure. Because each blade doesn't have to travel so far, these shutter rate can be much faster.
Leaf shutter still take a small amount time to open and close but they're very fast. And, because they're mounted so close to the aperture, they progressively increase or decrease illumination to the whole sensor, so there's no difference between the slice of time seen by the top and bottom of the sensor.
However, since the same blades that start the exposure also end it, the maximum possible shutter speed is more closely linked to the shutter rate (because you can't end the exposure until the shutter is fully open).
In addition, the distance the shutter blades need to travel depends on the aperture you're shooting at (on some cameras, the shutter acts as the aperture). Consequently, it's not unusual to encounter cameras that can't offer their maximum shutter speed at their widest aperture value.
Electronic shutter
But why do we need mechanical shutters at all? Unlike film, digital sensors can be switched on and off. This reduces the number of moving parts (which both lowers cost and obviates the risk of shutter shock) and means you get a totally silent exposure, so why not use that?
The answer is that you can. However, there is a restriction: while you can start the exposure to the whole sensor simultaneously, you can't end it for the whole sensor at the same time. This is because with CMOS sensors, you end the exposure by reading-out the sensor but, in most designs, this is has to be done one row after another. This means it takes a while to end the exposure.
Fully electronic shutter
This need to read out one row at a time has a knock-on effect: if you have to end your exposure one row at a time, then you have to start the exposure in a similarly staggered manner (otherwise the last row of your sensor would get more exposure than the first).
Electronic shutter tend to be comparatively slow in terms of shutter rate (red), leading to rolling shutter (note that exposure for the top of the sensor has already finished even before the bottom of the sensor has started. This is despite the use of a faster shutter speed (green)
This means that your shutter rate is determined by your sensor's readout speed. Lower pixel count sensors have an advantage in terms of readout: they have fewer rows and each of those rows has fewer pixels in it, both meaning they can be read out faster.
Smaller sensors also have an advantage in this respect: less physical distance to travel means rows can be read-out quicker. This is why we saw 4K video in smartphones, then compacts, then larger sensor cameras and why cameras such as the Canon EOS 5D IV struggle with rolling shutter, even when only using the central region of their sensor. However, newer sensor designs are constantly striving to reduce the read-out time (and consequently increase the shutter rate).
Note from the diagram that even an exaggeratedly slow shutter rate doesn't stop you using fast shutter speeds. In fact, the beginning and end of the exposure can be controlled very precisely, allowing super-high shutter speeds.
However, although each part of the image is only made up from, say, 1/16,000th of a second, the slow shutter rate means each part of the image is made up from different 16,000ths of a second. Essentially, you're capturing the very short slices of time that your shutter speed dictates, but you're capturing many different slices of time. And, if your camera or subject have moved during that time then that distinction becomes apparent. This effect, where the final image is made up from different slices of time as you scan down it is known as the 'rolling shutter' effect.
The same thing happens with any shutter that isn't immediate, which includes focal plane mechanical shutters. However, these tend to be fast enough that the rolling shutter effect isn't usually noticeable.
Electronic first curtain
Electronic first curtain shutter is an increasingly common way for cameras to work. As the name suggests, these work by using the fast mechanical shutter to end the exposure and then syncing the start of the electronic shutter to match its rate.
An electronic first curtain shutter avoids the risk of shake from the first curtain's movement but avoids the downsides of fully electronic shutter.
This requires a mechanical shutter where the second curtain and be operated independently of the first curtain. But, in those circumstances, you get many of the anti-shock benefits of electronic shutter while retaining the speed benefits of a mechanical shutter.
Global electronic shutter
Sensors do exist that can read-out all their rows of pixels simultaneously to give what's called a 'global shutter. ' However, while these are great for video, the more complex technologies used to achieve this add both noise and cost. The added sensor noise limits dynamic range, so they are not yet common for those video or stills applications where image quality is critical.
Find out about flash sync and working under artificial lights
. dpreview.com2017-5-22 13:00
