The Shutter Conundrum
20 Nov, 2018

The Shutter Conundrum

This is the tale of the shutter in large sensor dedicated digital cameras – a mechanism used to implement a given exposure time selected by a photographer. But it's also the tale of a mechanism that is heading for the history books.

For many years design limitations of Digital sensors made them fairly complicated to use in large sensor dedicated cameras. Greatly simplified, once a sensor is turned on, it starts collecting light in each pixel. Once each pixel has received enough light it becomes “full”, and no more information on light is gathered. To prevent just running full – and control exposure time for our image - a mechanism was needed to stop more light from entering the pixel. There is also a need to read out the current light values, and be able to reset it to start over.

Sensor designs only had one unified electronic way of handling all of this, and that is called readout. Basically, we read the light values simultaneously from every pixel in an entire row across the sensor. Once the read is done, it also resets the pixels in that row, so they start gathering light anew. Then we move on to the next row of pixels, until we have gone through all the pixel rows on the sensor. This is done very quickly – but still not quickly enough. Once a sensor is turned on, the lowest row of pixels gets more exposure time than the upper row when doing the readout – because of the line by line approach. To add bad to worse, sensor readout is done at a fixed speed, so for photography it becomes even more useless as a mechanism to end exposure. Just to illustrate, many sensors do a full readout in fx. 1/60th of a second. How can you do an exposure speed of 1/1000th of a sec if the sensor takes 1/60th of a second to complete readout from the first to the last row? It would create massive overexposure of the lower part of the image.

In mirrorless this gets even more complicated: Mirrorless cameras have no optical viewfinder, so to provide the viewfinder image, the sensor is always on and the rows are read many times a second – over and over, top to bottom. This creates a situation where there is no common starting time for taking an image by turning the sensor on. The exposure of the upper pixel rows are not done at the same relative time as the lowest row. With fast moving subjects this creates artifacts in the image being collected, since the subject has moved between the upper and lower rows light gathering, readout and reset. Collectively this is called “rolling shutter” – because it's “rolling” down the sensor. While it is a BIG problem in video, its fixed speed makes it completely useless I photography.

A final problem is that sensors takes time to turn on – and that time varies with temperature and lots of other factors. How do you include the “turn on” time in your exposure calculations?

"Once each pixel has received enough light it becomes “full”, and no more information on light is gathered"

The mechanical shutter

Because of this sensor behaviour DSLRs and Mirrorless Large Sensor cameras was forced to adopt the mechanical moving shutter curtains of the old analogue cameras to manage when - and for how long - light reached the sensor. For DSLRs this is no biggie, we turn on the sensor, and the mechanical shutters open/close cycles determines how much light gets in, just like old times 😊

With Mirrorless though, you have to have the sensor always turned on - without a mechanical shutter in front - to provide the viewfinder image. So, when taking the photo you first have to close the shutter, re-read all pixels to clear their values, then roll the shutter open/close cycles, read out the photo, and then remove the shutter to once again provide the viewfinder image.

A mechanical shutter curtain actually creates a rolling shutter itself when using VERY fast exposure speeds, but it’s not even in the same ballpark as the sensor readout problem so we can ignore that for now.

"DSLRs and Mirrorless Large Sensor cameras was forced to adopt the mechanical moving shutter curtains of the old analogue cameras"

Advancing towards the electronic shutter

Then, several years back improvements in sensor design started allowing us to globally “reset” the pixels on the sensor without doing a readout, and that introduced a very important new camera feature: Electronic first curtain shutter.

Basically, that means we can control in one single step when all the sensor pixels start gathering light anew. For DSLRs we can now open the mechanical shutter, wait for the shutter vibrations to settle, and then start the exposure by doing a electronic reset of the pixels . We avoid the first shuttercycle and the vibrations that it introduces.

For mirrorless this means we no longer need to close the mechanical shutter and open it again to start taking a picture. So it removes the first full close/open shuttercycle and its vibrations, and It also further shortens the blackout time in the viewfinder.

The "reset" is really done on a pixel row basis, fx: all rows at once which is a global reset, or staggered one line at a time with the speed needed to simulate a mechanical shutter. The staggered approach is currently necessary because otherwise you would overexpose the lower part of the image when the mechanical shutter that stops the exposure moves across the sensor - allowing more light to the last pixels it covers. The electronic first curtain shutter needs to counter this discrepancy by simulating a mechanical shutter when starting the exposure.

"We avoid the first shuttercycle and the vibrations that it introduces."

The global shutter

We are now on the cusp of a yet another sensor advancement entering the marked. It’s called the Global Shutter, and it promises to be a REAL game changer. Very simply said, it allows us to simultaneously readout ALL pixels on the sensor and then reset them. So just like the electronic first curtain shutter allows us to start an exposure without rolling a mechanical shutter, this allows us to end an exposure without rolling a mechanical shutter. And this one can have massive implications for your camera and your shooting:

  • Cameras can all together remove the physical/mechanical shutter which cuts costs. The shutter is a fairly expensive and very sensitive piece of precision mechanics.
  • No moving parts in the camera will improve reliability and longevity. Note: DSLRs still have their mirror mechanism which is mechanical.
  • For Mirrorless with both the mirror and shutter gone, the camera will become completely silent.
  • Release cycles for a camera can now enter millions without service and shutter replacements
  • There is no longer the mechanical shutter chock issue introducing vibrations and blurred images. While a nice feature, it's only a part win in DSLR, as that still have the Mirrorslap to deal with.
  • In Mirrorless, AF can now run 100% continuously during shooting (No shutter in the way)
  • In Mirrorless, the Viewfinder blackout can be completely removed during shooting
  • The implications to Flash Photography are just staggering. You can completely decide when during the exposure the flash should fire (without artifacts from a moving mechanical shutter). You can run with real flash sync times all the way up until the about 1/10000th of a second a flash takes to fire its energy.
  • For mirrorless it can potentially bring much higher frame rates since no mechanics is interfering.

"It’s called the Global Shutter, and it promises to be a REAL game changer"

Our cameras will soon loose the 50+ years old solution of using a mechanical shutter – about time some may say. It will certainly also redefine the high end dedicated camera marked, as Mirrorless designs will completely take over. Also, let not forget to mention the change in noise levels at the big political press conferences :-)

The question is: Are you ready to shoot silently and with no tactile feedback from the “click” of your mirror/shutter doing a cycle?