Well as usual, my puzzler is puzzled about quite a few things. One of them is do with photography and the way in which the mechanical shutter curtains coordinate with a flash and how the same thing is achieved with an electronic shutter. You see, my Olympus OMD EM-1 Mark II has both electronic and mechanical shutters.
Most mechanical shutters consist of a front and a rear curtain. We don’t often think about how they operate but if you want to see, there is a high-speed video at this link (https://www.youtube.com/watch?v=ptfSW4eW25g). The link shows the shutter operation for a DSLR , a camera with a mirror but the two curtain system is much the same as that found in a mirror-less camera. In a mirror-less camera, the imaging chip is normally on and open to the light entering through the lens. The image the chip is producing is displayed on the LCD display and/or in the electronic view-finder (EVF). When you press the shutter button the front curtain of the shutter falls and stops light from entering, after this the sensor chip is switched off, and any information that may have been present in the pixels is erased. The sensor chip is then turned back on and the camera opens the front curtain, light pours in forming an image and the exposure is terminated by the rear curtain of the shutter that rises to block off the light. For relatively long exposures, less than say 1/250th of a second, the two shutters move independently; the front curtain is fully open before the rear curtain terminates the exposure. For shorter exposure times the shutters move together, the front shutter moving up with the rear one creating a slit, the width of which determines just how long any row of pixels is exposed to the incoming light. The “shutter rate” is defined by the speed at which the shutter curtains can move and the “exposure time” by how long any row of pixels sees the light. A third factor is “shutter lag” which can be thought of as the ‘thinking time’ of the camera – when you press the shutter button, actually nothing in the way of image formation occurs, rather the camera’s electronics get themselves ready to take a picture, the front curtain falls, and the imaging chip is erased. Shutter lag is important because if you trigger a camera using a laser trigger or other device in order to image a fast-moving object, the trigger pulse is sent to the camera in a few microseconds or less, but the camera doesn’t get round to recording an image for the period of the shutter lag which can be many milliseconds (a typical value for a DSLR is about 50ms). Mostly shutter lag is short enough that it doesn’t matter much. However, it does if you want to catch an image of a speeding bullet. This is because you can trigger the camera as quickly as you like but by the time processes described above have occurred, the bullet has long gone. The solution to that problem is to work in the dark with the shutter open and then form the image with a very brief flash of light. It should be clear that there is a big difference between the ‘shutter rate’ (the speed at which the shutter curtains rise and fall – relatively slow processes) and the ‘exposure time’ (how long a row pixels is exposed to the light). Some point and shoot cameras have such long shutter lag times that even slow moving objects have beetled-off between pressing the shutter-release button and the camera getting round to taking the picture – very annoying!
When you want to take a flash photograph it is important that the whole of the sensor chip is exposed to the light at the same time and thus that neither of the shutter curtains is blocking the light. It is for that reason that the flash ‘sync speed’ (exposure time) is limited to 1/250th of a second or less because for shorter exposures the two shutter curtains have to travel together forming a slit which would lead to only a band of pixels on the sensor chip being exposed to light during the brief flash from the strobe.
There are just a couple of other things to mention about flash photography and shutter curtains. Most cameras allow you to either fire the flash when the front curtain first opens i.e. at the beginning of the exposure or at its end when the rear curtain is about to rise. What difference does that make? Well if you take, a say 1 second exposure in dim light and have the flash go off at the beginning (front curtain sync), an object moving from left-to-right in the frame will be shown frozen on the left of the frame due to the light from the flash and then, by virtue of the ambient light, as a blur to the right as it continues to move across the frame. The opposite will be true if the flash is fired at the end of the exposure (rear curtain sync). Whether you want one or the other depends on how you want the image to look.
Try as I might, I was unable to find out exactly how a flash gun can be synced to an electronic shutter. Nearly all cheap point and shoot cameras have flashes that sync perfectly with their electronic shuttering. Though unknown to many of its owners, the Olympus OMD-EM-1 Mk 2 camera that I own, can also sync a flash to its electronic shutter. That this is little know is probably because that option is buried deep within that camera’s somewhat complex menu system.
I decided I needed to to understand how electronic shuttering works. Normally, a mirror-less camera is in ‘live view’ mode with the images it captures being sent to the LCD and/or EVF. When the shutter button is pressed, the imaging chip is erased and then for a period commensurate with the exposure time, allowed to accumulate information about the light intensity which is then read out and stored. That processed rolls down the sensor chip so once again there is a difference between “shutter rate” – the rate at which the process described above rolls down the sensor chip, and the exposure time – the length of time the pixels on the chip accumulate light.
The well-known “rolling shutter effect” is a direct result of the relatively slow shutter rates of both electronic and mechanical shutters….but which of them has the faster shutter rate? Here are some pictures I have taken to try to demonstrate the rolling shutter effect. Note they also illustrate the difference between exposure time and shutter rate – exposure time here is 1/8000th of a second but the shutter rate is much slower (for various reasons, I think it takes about 1/320th of a second for the pair of shutters to move across the sensor in my camera). It’s pretty clear from the photos that the distortion caused by the shutter rate is much worse for the electronic shutter which I think implies that it takes a lot longer to complete a readout of the whole sensor than it does for the mechanical shutter curtains to travel across the sensor chip. Interestingly, and for reasons I don’t quite understand, the image from the electronic shutter is crisper. I guess the more marked rolling shutter effect when using the electronic shutter is unsurprising because if my suppositions are correct, then for the mechanical shutter all the pixel rows were cleared before the shutter curtains started to travel and are read-out after they have closed. Thus, the distortion is solely the result of the time it takes the slit between the shutter curtains to expose the sensor. Whereas, for the electronic shutter even though the exposure times that it is capable of are far shorter than for the mechanical shutter, the shutter rate is quite a lot slower because row after row of pixels have to be cleared, set to on and read out during the exposure.
So while electronic shutters can manage much shorter exposure times than their mechanical equivalents they have the disadvantage due to their slower shutter rate, of distorting moving objects. The big advantages of the electronic shutter are that nothing moves – it is err umm entirely ‘electronic’ so its operation is silent, there is no mechanical judder and thus no camera wobble to blur the image, and there is no wear and tear as there would be were it mechanical. A further big disadvantage of electronic shutters is the relatively slow flash sync speed.
As mentioned above, it seems a relatively poorly known fact that you can use a xenon flash with the electronic shutter of the Olympus OMD EM-1 Mk2 camera. At first sight this may appear a bit of a paradox because most of the descriptions you find on-line of the operation of an electronic shutter imply the sensor is being cleared, exposed, and read out a line at a time. If only a single line were available to the light at any one time then a brief flash would only be caught by a few rows of pixels. I was puzzled and could not find an answer on-line as to exactly how an electronic shutter operates during brief exposures so, I set out to try to determine the answer by experiment. Since, my camera can at its maximum sync speed, sync a flash regardless of the power setting and thus the brevity of the light output, I reasoned that all the rows of pixels must be available to receive information when the flash fires. The exposure time in the picture below is 1/50th of a second (the maximum sync speed) and the flash duration is about 1/8000th of a second but the frame is evenly illuminated as it is even when the flash duration is reduced to 1/25000ths by turning the flash power down to a minimum. So, I believe that all the camera’s pixels must be cleared and then rapidly set to ‘on’ line-by-line just as they would be behind the mechanical shutter prior to the exposure. After that, they are allowed to accumulate light for 1/50th of a second before being read out very quickly also line-by-line. So, my tentative conclusion is that the sync speed limit of 1/50th second results from the time it takes to set all the sensor’s pixels to ‘on’, and read them all out. If you try to take a picture with a shorter exposure, only some of the rows of pixels are available to to form an image.
But how does an electronic shutter operate when set to faster exposure times? I decided to try to discover the answer by using a camera trigger to fire my camera and then after a delay to allow for the shutter lag, to then trigger a flash set to a very low power so that the flash duration is extremely brief. In this setup, the pixels that are available to receive light should be obvious in the images formed. What the images show is that there is a band of pixels available to receive the light that travels across the surface of the chip during the exposure and that the width of the band is directly proportional to the shutter speed. Presumably, at the leading edge of the stripe, rows of pixels are being made available to receive light and at the lower edge, those pixels are being turned off and the image information read out. The images are similar to those you would expect from a mechanical shutter but I haven’t made a mistake, this is the Olympus’ electronic shutter! Given what appears to be a mode of operation not disimilar to that of the moving slit found in a mechanical shutter, could the exagerated rolling shutter effect associated with the electronic versus the mechanical shutter simply be down to the slower shutter rate? The maximum number of frames that the EM-1 can manage in high speed silent mode is about 60 per second (~17 ms per frame) suggesting a maximum scan rate of at least that. The 50 ms for the minimum exposure time to sync a flash in electronic shutter mode suggests a frame rate quite a lot lower than that when in normal silent shutter mode. However, both rates are significantly slower than that achieved by the mechanical shutter – a frame in less than 4 ms (again note exposure time and shutter rate are different things). So, perhaps it is indeed all down to shutter rate.
If you know better what is going on PLEASE let me know because it is puzzling my puzzler – lots!