Electronic VS mechanical shutter

In the film cameras age, there was only one kind of shutter, the mechanical shutter. With digital cameras two new kind of shutters have arrived: hybrid shutters and fully electronic shutters, that are going to replace the old mechanical shutter. Let's see how the various shutters works and what are the pros and cons of every system.

The mechanical shutter of a Canon camera, photographed during a fast exposure. As you can see, the upper curtain has already begun closing even though the first (lower) curtain has not opened completely yet.

The mechanical shutter

Even though there are many types of mechanical shutters, those that are used in modern cameras are composed by thin curtains that quickly expose or hide the sensor. Exposure can happen in two different ways:

1) The first curtain opens and it exposes the whole sensor, then the second curtain closes and it terminates the exposure. This is possible only at slower shutter speeds: the fastest shutter speed that allows the camera to work in this way is the "sincro-X" or "sincro-flash" speed, and usually it is around 1/250 of second. When the shutter operates in this way, it is comparable to the global shutter of fully electronic shutters.

2) At fast shutter speeds, the second curtain begins to close while the first curtain is still not fully opened: the result is that the whole frame is not exposed in the same instant, but it is reached by light in slightly different moments (we are talking about extremely small fractions of second!). When the shutter operates in this way, it is comparable to the rolling shutter of fully electronic shutters. This may surprise you, because we usually associate the rolling shutter artifacts with digital cameras, but actually all film cameras used a (mechanical, of course) rolling shutter for photos taken at above 1/250!

So, why we did not see rolling shutter problems with mechanical shutter? Because the mechanical rolling shutter is much, much faster than the electronic rolling shutter used in most consumer cameras, so the artifacts are so small that are almost always impossible to notice.

The electronic shutter

The electronic shutter has several advantages in comparison to the mechanical shutter:

1) it is more robust and reliable, because there are no moving parts.

2) it is completely silent: if you are shooting in situations where the shutter noise may be a problem, for example some wildlife photos, theaters, street and reportage, the electronic shutter allows to be completely unnoticed.

3) it eliminates the risk of image blur at slow shutter speeds created by the shutter vibrations. The mirror vibrations are not the only cause of blur: in macro photography it is clearly possible to see the blur created by the shutter movements, unless the camera has some way to address is as electronic shutter or, at least, hybrid shutter.

4) it allows extremely fast shutter speeds: while most mechanical shutters tops out at 1/8000, electronic shutters can go far beyond.

5) it improves battery life because there are no moving curtains.

6) in mirrorless cameras, it improves autofocus speed and tracking, because the AF sensors (placed directly into the main image sensor) are always exposed to light.

So, why the electronic shutter is not the standard in every camera yet? Actually, most cameras manufactured from 2006 already have electronic shutter: if your camera has live view, it has electronic shutter, but...it uses the electronic shutter only for live view and video, while for still photos it used the mechanic shutter. To understand why, let's see how different electronic shutters work.

1) Rolling shutter is very similar to the mechanical shutter: instead of capturing the whole photo at a time, it 'scans' the sensor from top to bottom. If you take a photo of something that is moving fast, the top and the bottom areas of the sensor will record it in slightly different instants, and the result is that it look distorted or 'skewed'. This problem could be eliminated if electronic shutters became as fast as mechanical shutters to scan the frame, but sadly we are not there yet: even very high end cameras still have rolling shutter artifacts.

2) Global shutter captures the whole photo in a single time, as mechanical shutter only does at slow shutter speed; electronic global shutter, instead, can do it at every shutter speed! Global shutter eliminates the artifacts of rolling shutter and it could effectively replace the mechanical shutter in every situation, but it has two downsides: it is more difficult and more expensive to make and it requires more circuitry in each photosite ("pixel") of the sensor, reducing the area available for light collection, so the images have a little more noise and less dynamic range.

Thanks to the improvements in sensor technology, as microlenses, the loss of image quality can be greatly reduced, but nowadays (2014) the prices of cameras with electronic global shutter are still very high: the Sony F55, for example, cost 30,000 US $.

The hybrid shutter

Many consumer cameras currently on the market have an hybrid shutter, that can be used as only option or activated on request. For example, in Canon cameras the hybrid shutter, also called 'electronic first curtain', is used when you activate the 'silent shooting' mode.

The hybrid shutter begins the exposure using the electronic shutter, but terminates it with the mechanical shutter. It has the advantage of being more silent than fully mechanical shutter, it eliminated most of the vibrations and it has no rolling shutter artifacts. The downside is that it still requires the presence of a mechanical shutter so it has the limitations of this kind of shutter.

A test of shutters at fast shutter speeds. 1) reference photo, the fan was still. 2) Photo taken with mechanical shutter; there are no visible artifacts. 3) Photo taken with hybrid shutter ('electronic first curtain'); again, there are no artifacts. 4) Photo taken with fully electronic rolling shutter; there is huge distortion due to the slow readout time of the Canon 70D. This photo is a still frame from video, since the 70D uses the fully electronic shutter only in video mode.

Misconceptions and curiosities

1) CCD vs CMOS: it is not true that only CCD can have global shutter. Actually, both CCD and CMOS can use global shutter, but while for most CCDs it is the standard, for CMOS it is more expensive and complex. That said, there are already CMOS with global shutter, as the sensor of the already mentioned Sony F55.

2) Shutter Speed vs Sensor Readout: with rolling shutter, taking a photo at, let's say, 1/4000 of second does not mean that you are actually freezing 1/4000s in your photo; it usually takes much more, so your 1/4000s exposure may actually take 1/100s or so. How is it possible?

You can select the shutter speed, that is the amount of time during which each pixel records the light, but not the sensor readout time, that depends by your camera. The readout time is the time required to 'read' the sensor from top to bottom.

In other words, if you select a shutter speed of 1/4000 and the fastest sensor readout of your sensor is 1/10, every pixel will record light only for 1/4000 of second, but the pixels at the bottom of the sensor will do their 1/4000 exposure about 0.1 second later than the pixel at the top. This is what causes the rolling shutter artifacts.

3) Sensor readout time or some cameras currently on the market: the Panasonic GH3 takes 0.100s (1/10s); the new Panasonic GH4 has improved it to 0.050s; Nikon 1 V3 has an excellent (for it price) 0.012s; Red Epic has a time of 0.005s. The equivalent of a mechanical shutter, that eliminates visible artifacts, it likely around 0.001s, so we are still far away.

Nowadays, who is for?

Currently there are few DSLR and mirrorless cameras that offer the option of using the full electronic shutter even for still photos: Sony A7s, Nikon 1 cameras, Panasonic GH4 and few more. All these cameras have CMOS sensor with electronic rolling shutter.

If you have one of these and you shoot mainly landscapes, macro, portrait and other still or slow moving subjects, you can turn on the fully electronic shutter and enjoy its advantages. Instead, if you take photos of sport, wildlife and other fast moving subjects, I suggest to avoid the electronic shutter, at least until global electronic shutters or very fast rolling shutters will become a reality in mainstream cameras.

Risposte e commenti




Paranord

inviato il 20 Luglio 2015 ore 17:51 Can you comment on the speed of the electronic shutters in the Nikon 1 lineup when taking video. The high speed 1200 FPS and 400 FPS modes certainly do seem to give you a full-frame shutter. I've got the Nikon 1 J4. What's the best way to test the electronic shutter speed?



Philip Mok

inviato il 22 Maggio 2019 ore 4:20 Wonder how sensor work when use mechanical shutter? Still scan from top to bottom? Or exposes all the time, only control by the shutter movement? If still scan from top to bottom, how the slow scan speed can match the fast movement of shutter leaf? If it is all time exposes, than why cannot use as a global shutter?