by Ian Burley
The answer to this question is down to what you define as a shutterDuring our press briefing on the new Olympus Pen E-P5 it was mentioned that the camera is the first compact system camera (CSC) to sport a 1/8000th top shutter speed. It was only later that I realised that Samsung had already released a CSC, the NX20, with a shutter that could go as fast as 1/8000th. So I queried this with Olympus and their response was rather interesting.
It turns out that Olympus claims the E-P5 is endowed with the first conventional electronically controlled two-curtain mechanical focal plane shutter. Samsung, apparently, equipped the NX20 with a hybrid mechanical shutter that only uses one curtain. The function of the first curtain has been replaced by what is called an 'electronic first curtain'. Electronic first curtain shutters do have undeniable advantages but there could be concerns about image quality compared to a conventional shutter, like that of the E-P5.
CurtainsTo understand all of this it's essential to understand how a focal plane shutter works. First invented by Leica in 1925, the conventional focal plane shutter lay in front of the film frame and operated horizontally. The curtains were made of of a thin fabric made opaque thanks to the cloth material being treated with rubber.
The exposure is made by the first curtain moving to one side until the whole frame is exposed. The second curtain then follows at the same steady speed until the frame is covered once more. This ensures that the whole frame is evenly exposed. To vary the exposure you simply change the speed at which the curtains move - up to a certain speed limit.
This speed limit is a practical one. Accelerating the mechanically-driven curtains accurately and consistently gets more and more difficult the shorter the exposure becomes. This was especially so before electronic control was introduced in the 1970s. The solution was to limit the maximum speed at which the curtains would travel, but trigger the second curtain before the first had fully traversed the frame. This results in a gap or slit formed by the two curtains moving over frame. The sooner you trigger the second curtain to follow the first the narrower the gap between the two curtains becomes and so the slit is smaller and so the exposure is progressively reduced.
In modern cameras the curtains are no longer rubberised cloth, but very low mass ultra-thin metal blades and the curtains operate vertically because this reduces the distance required for the curtains to travel.
The live view scenarioWith compact system cameras that have reflex viewfinder system the view from the camera's rear screen or electronic viewfinder comes directly from the main imaging sensor - the sensor that records the picture you take. So until the moment you take your picture the sensor has to be uncovered in order to provide the live view for framing.
As soon as you press the shutter release home the shutter needs to close fully, the sensor is then reset, and then the exposure is made as described earlier. On cameras that don't have live view modes the shutter remains closed all the time - until an exposure is made. So for live view cameras the shutter is working twice as hard - closing and then re-opening for each exposure. This potentially reduces shutter responsiveness and increases the audible noise of the shutter mechanics in operation. With precision photography the extra complexity of the shutter's operational cycle can mean extra unwanted vibrations.
The electronic first curtain shutterIf you examine the live view focal plane shutter exposure cycle and think about it, the elimination of the first curtain is a natural quest. It minimise the mechanical noise of the shutter, along with vibration, and the simplified mechanics should mean a cheaper and more robust part.
Cameras with live view already have sensors that can record stills and motion pictures without the need of a mechanical shutter at all. This is how live view works, after all. Exposure time is achieved by shifting exposed photosite charges to masked (light insensitive) storage locations adjacent to each photosite. Exposure starts as soon as the photosite charge is reset and stops when the charge that builds up from receiving light is shifted off the photosite ready to be read by the camera's analogue to digital conversion circuitry.
The theory behind the electronic first curtain shutter is that instead of using a physical curtain, you progressively start exposure of adjacent columns of photosites on the sensor followed by the motion of the physical 'second' curtain, which completes the exposure.
If electronic first curtain shutters work, why do we need a second physical curtain?
We have already covered the fact that you can take both stills or motion video without the need for a physical shutter that is separate from the sensor. In fact Nikon's 1-Series J model compact system cameras have completely done away with a focal plane shutter and several Panasonic Lumix Micro Four Thirds camera models offer a 'silent shutter' mode that operates without the focal plane shutter. It could well be that in the future it will become the norm for cameras without mechanical focal plane shutters.
However, for now, there are limitations and disadvantages for completely solid-state electronic shutters.
1. Rolling shutter distortion. Most current CMOS sensors can only process one row or column of pixels at a time. Without a physical shutter to stop exposure when required, time and action continue and are recorded progressing along row by row. The result is a time distortion characterised by objects in the frame that have moved fast enough to be visibly distorted, especially straight lines. This is most evident in movie footage where fast movement of the camera view results in an unpleasant wobbling or 'jello' effect. A still image simply freezes this effect in one frame. The use of 'global' shutter sensors, where the sensor can shift the entire array of charges in one go, will solve this problem but such sensors are not currently in common use. In the mean time camera manufacturers are resorting to reading date off the sensor as fast as possible in order to reduce the unwanted effects of rolling shutter artefacts.
2. Speeds. Where all-electronic shutters have been implemented there has usually been some limitation imposed compared to when using a conventional mechanical shutter. This has been in the form of reduced resolution or a narrowed ISO sensitivity range.
3, Flash. Panasonic does not enable flash to be used with its Lumix Micro Four Thirds cameras when silent shutter is used. If you think about it, the way rolling shutter works it is unlikely to record a whole frame during the very short duration of a xenon electronic flash.
There are likely to be other problems with solid state shutter cameras of today but the above are the most obvious.
Are there disadvantages to electronic first curtain shutters?The simplest and most honest answer I can give is - I am not sure. I do have a Samsung NX20, (which has an electronic first curtain shutter) at the moment and I will be comparing it with some Micro Four Thirds models to find out as conclusively as possible if there are any interesting differences between this and cameras with more conventional shutters. However, I have read reports from NX20 owners that there are some strange artefacts in some of their photos when using very fast shutter speeds, like banding, which can be explained in theory by the use of an electronic first curtain shutter.
Conclusion for now...Olympus is right to claim that the Pen E-P5 is the first compact system camera with a conventional focal plane shutter that offers a fastest shutter speed of 1/8000th second. But Samsung can certainly claim to be the first to deliver 1/8000th top speed to a CSC. While electronic first curtain shutter mechanism shutters are theoretically more robust,quieter and more vibration-free than typical conventional focal plane shutters, let me point out that the E-P5 already has a remarkably quiet and well-damped shutter. If you have used an OM-D E-M5 then the shutter feels and sounds much like the E-M5's, which is already pretty impressive.
Discuss this story:
Is the Olympus Pen E-P5 the first CSC with a 1/8000th top shutter speed?
Four Thirds User editorial team Is the Olympus Pen E-P5 the first CSC with a 1/8000th top shutter speed?
You might have assumed from messages coming out of Olympus that the new Pen E-P5 is the first compac... (more)
John Perriment Re: Is the Olympus Pen E-P5 the first CSC with a 1/8000th top shutter speed?
Thanks, Ian, for explaining with clarity the difference between conventional focal plane and electro... (more)
Ian Re: Is the Olympus Pen E-P5 the first CSC with a 1/8000th top shutter speed?
I think you have summed things up perfectly, John :) Ian... (more)