The Benefit of Having a Mechanical Shutter?

With the latest series of leaks we’ve learnt a lot more about the upcoming EOS (yet we still don’t have an exact Megapixel count); the latest of these leaks showed off a video of the EOS’s mechanical shutter in action. So I thought I’d take a plunge into the world of imaging and see why a mechanical shutter is a better option than an electronic shutter (of course I just read up on this so my facts aren’t 100% but I thought it was worth a share).

First off some background info, most phones such as 920, 820 and others have electronic/digital shutters which rather than physically closing when an image is captured instead just “turn off” the sensor. On the other hand most SLRs/DSLRs and some camera phones such as the N8 and 808 have mechanical shutters; which physically close and block the light from reaching the sensor while capturing an image. As you can see in the cinemagraph above the 808’s shutter visibly opens and closes when capturing an image, the 920 on the other hand has no “shutter lens” meaning that the camera is visible “open” as seen below.

EDIT: Camera Guru Damien Dinning was kind enough to chip in a comment down below here it is:

Hi everyone, there are so few factually correct statements in the original piece or related comments (no disrespect intended)  I felt compelled to help explain. 🙂

Please note, my comments are ONLY addressing the general topic of mechanical shutters – no more.

Keeping it simple, the main reason for fitting mechanical shutters is for use with xenon flash. Typically CMOS sensors read light across the sensor from left to right and top to bottom. The time each pixel is ‘read’ is the effective shutter speed. This is OK in most cases and OK with LED flash as the light is effectively constant/continuous. LED flash in most cases being the equivalent of turning on a torch before the exposure and turning it off after the exposure has been made, effectively increasing the amount of light in the scene more or less for the duration of the picture.

In the case of xenon, the flash fires a very short ‘pulse’ of light. This pulse can be as short as approximately 1/25,000 (hence why xenon can freeze high speed movement). With a typical CMOS sensor the time difference between the 1st pixel being ‘read’ and the last is greater than this time. The result would be some pixels would be correctly exposed whilst others would be dark or even potentially black. To overcome this, the pixels are effectively read all at the same time. But to achieve this all pixels are turned on, the shutter opens, the flash fires, the shutter closes and the pixels turned off. And that’s why typically mechanical shutters have been needed in products such as n8, n82, n808. In some cases some latest generation sensors can read all their pixels at very high speed (note: again don’t ask me to comment on speculation or rumour) allowing xenon to be used. In some cases e.g. Nikon 1 series these later generation sensors are allowing for electronic shutters whi
ch can provide potential advantages in high frame rate scenarios which mechanical shutters would not be suitable for.

In some cases a hybrid approach maybe used e.g. a SE product of a few years back which featured xenon only used the mechanical shutter for flash but not other situations, which meant in that case it didn’t provide the following potential advantage….

With mechanical shutters, because the pixels are effectively read all at the same time it overcomes the motion skew effect which can typically occur with CMOS sensors due to the time difference between the first and last pixels being read. As the read time from CMOS sensors is increasing (shorter read times) this is becoming less of an issue in some cases.

Mechanical shutters do require additional space, there are no space advantages to them.

As for dust protection there is some theoretical advantage to them but in practice (at least in my experience) I have seen dust penetration in all cameras, there is a fundamental limit to what can be done to prevent dust penetration.

Hope that clarifies things.

So what does this all mean in terms of performance in a camera? and more importantly a camera phone? Well first off there’s the undeniably cool effect of having your shutter pop open and close ; I think this engadget comment captures it perfectly:

On a more serious note the main advantage of having a mechanical shutter on a camera (not a phone) is the ability to use it with an optical viewfinder (rather than a live feed/viewfinder on screen); the benefit of this is a more realistic look at what you’re capturing, plus saving on battery life (because you’re not using the screen – or really anything else). The second benefit comes in terms of production, although using a mechanical shutter is more expensive than just switching off the sensor it does allow for the usage of cheaper less complicated sensors (that can focus on doing their job), further more mechanical shutters take up less space inside a camera module than electronic sensors (for reasons I can’t understand); this is kind of important when trying to keep a camera phone as compact as possible (although I doubt the difference between the two can be *that* great).

In terms of actual photographic results mechanical shutters provide slightly better results by completely blocking off any light to the sensor; preventing any overexposure of pixels and ghosting of images while the first one is being processed:

Once the mechanical shutter is closed, circuitry is then used to shift the charge from each pixel into a storage area. Since the pixels on the sensor remain “live” during readout, if the shutter remained open, light would continue to alter the charge accumulated by each pixel during the shifting operation which could result in blur or ghosting.

Of course mechanical shutters also have the added benefit of providing a “dust barrier” to the lens, protecting it from scratches and other nasty stuff.

On the other hand Mechanical shutters are slower to react than electronic ones, meaning they limit your shutter speed, as well as throw off the synchronized timing of the flash firing and the image being captured.

Honestly the topic is pretty complicated, and I’m an imaging newbie, but I thought it would be an interesting read; for a more detailed explanation check out this great article over here:

http://www.steves-digicams.com/knowledge-center/why-digital-cameras-have-mechanical-shutters.html#b

If anyone would like to add something to this please feel free to chip in down below 🙂

And of course who wouldn’t his camera to look like this?

Edit:

Here’re some useful points/corrections that Werner Ruotsalainen pointed out in the comments:

Why not? 🙂 See below.

“Honestly the topic is pretty complicated, and I’m an imaging newbie, but I thought it would be an interesting read; for a more detailed explanation check out this great article over here: http://www.steves-digicams.com/knowledge-center/why-digital-cameras-have-mechanical-shutters.html#b”

Well, this article doesn’t mention a LOT of things: blooming and, also very importantly, rolling shutter. Both affect electronic shutters in most cases really bad.

“On a more serious note the main advantage of having a mechanical shutter on a camera (not a phone) is the ability to use it with an optical viewfinder (rather than a live feed/viewfinder on screen); the benefit of this is a more realistic look at what you’re capturing, plus saving on battery life (because you’re not using the screen – or really anything else).”

??? Are you sure you did mean “if you have a mechanical shutter, you don’t need to power / sample sensor at all because you can also use the optical viewfinder?” This isn’t really true.

1, if you use live view on mirror cameras, the sensor will be sampled all the time.
2, if you don’t, the system may decide to entirely power down the sensor to save power / keep the sensor cool.
3, nevertheless, mechanical shutters – and this is the most important part! – are always open even on DSLR’s with true mirrors, that is, an alternate way of light. They only close immediately before taking an image and AFTER setting exposure, based on the previous sensor readout (the optical way of light can only be used for PDAF, not for exposure / ISO setting). That is, there’s no difference between purely electronic and mechanical systems at all in this respect.

“Of course mechanical shutters also have the added benefit of providing a “dust barrier” to the lens, protecting it from scratches and other nasty stuff.”

VERY few mechanical shutters double as retractable / automatic lens protectors. Most of them are either in the lens (most P&S models) or directly in front of the sensor (all current ILC’s). The reason for this is simple: retractable / automatic lens protectors are prone to stuck (because of liquids / dust) / damaged by the user AND, in general, there are points in the line of light where a much smaller shutter can do, while the front lens, generally, is of much higher diameter than some of the inner lens.

If the lens protectors get stuck, the camera itself can still take images. However, if they also double as mechanical shutters, the camera itself becomes useless as it won’t be able to take pics at all.

Category: Lumia, Nokia, Symbian, Windows Phone