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Electronic Front Curtain Shutters

19. September 2011, 21:51:47 Uhr:

What is it, and why use it?

When Sony recently announced the SLT-A77, SLT-A65, NEX-7 and NEX-5N, one little detail in their specification lists was mostly overlooked: They are Sony's first photo cameras with an electronic front curtain shutter. What does this mean, how does it work, and why use it anyway? This page provides the answers.

Traditional focal plane shutters

To understand how electronic front curtain shutters work, we have to first get back to traditional focal plane shutters. These work as follows:

There are two sets of shutter blades (or two pieces of shutter cloth in older cameras) that can cover and uncover the image area (with film or a digital sensor underneath). In traditional (D)SLRs one set of shutter blades is unfolded and covers the image area when the camera is idle. When you press the release, the following sequence of events takes place:

  1. The unfolded set of shutter blades (the “front curtain”) starts to uncover the image area, with its edge moving across the frame at constant speed.
  2. After some delay, which is the exposure time, the folded-away set of shutter blades (the “rear curtain”) starts to move and cover the image area again, moving at the same speed as the front curtain.
  3. When the rear curtain has fully closed, exposure is complete. After that, film cameras can start winding the film to the next frame, and DSLRs can read out the image from the sensor and process it.
  4. For the next shot, the same action can take place in the opposite order and direction, or the camera may move both shutter curtains into their initial starting position.

The following animation illustrates the view through the shutter onto the scene and demonstrates shutter movement.

Shutter movement of focal plane shutter

This shutter construction was used for film SLRs (and many rangefinder cameras), and it works the same for DSLRs as well.

Cameras with live view

The situation changes a bit when you introduce Live View with the main sensor, as it's implemented in Sony's SLT cameras*1 and other brands' DSLRs. Then the shutter is open between shots so that light can reach the image sensor. But when taking a still image, the shutter must be closed before exposure to be able to properly clear the image from the sensor, and it also must be closed after exposure so that the image can be read out from the sensor without disturbance by further exposure. So in these cameras the shutter sequence becomes more complicated:

  1. The shutter first has to close. When it's fully closed, the image is cleared from the sensor by draining all photo sites.
  2. Then the front curtain and rear curtain perform their dance as in earlier DSLRs. When the rear curtain has fully closed, the image is read out from the sensor and processed.
  3. The shutter is opened again to be able to take a Live View image from the image sensor.

So with these cameras you have double the shutter movements compared to traditional SLRs. This is slow, noisy and wears out the shutter more quickly.

Electronic front curtain shutter

A solution to these problems is the electronic front curtain shutter. It works as follows:

  1. The shutter stays open at the beginning of the exposure. Instead of having a mechanical front curtain, the image is cleared from the sensor, pixel row by pixel row, in the same direction and with the same speed a with a mechanical shutter. Exposure of each row of photo sites starts immediately after it is cleared. This is possible, because you are not interested in the image that was stored in the photo sites before clearing them, only in the image that is created afterwards.
  2. After some delay, which again is the exposure time, the (mechanical) rear curtain starts to close, trailing the clearing of pixel rows with the same direction and speed. Pixel rows covered by the rear curtain then receive no more light.
  3. When the rear curtain has closed, the camera can read out the entire image from the sensor and process it.
  4. Then the mechanical shutter can open again.

The advantages of this system are obvious:

Electronic rear curtain?

So can we employ the same system also for the rear curtain and get rid of the mechanical shutter altogether?

Not really. If the rear curtain would move across the frame, clearing pixel rows like the front curtain, we would erase the image that we've just taken, and the entire procedure would be pointless. The point of the mechanical rear curtain is to stop exposure of the sensor to light while preserving the image that is already in the sensor. Whatever kind of rear curtain one may invent, it would have to do the same.

And what about video?

At this point you may wonder how the same cameras can record video. The shutter never closes in video mode (or Live View, which is basically the same), and still the camera can properly record the frames that make up the video.

In video mode, the camera goes through the pixel rows of the sensor in regular intervals. Each row is read out (and cleared in the process) to form part of a frame, and after that the same row is exposed anew, to be read out and cleared later for the next frame. Obviously this works just fine. So why not use the same method also for still frames?

The reasons are:

  1. Speed of processing: Even with Full HD video, each frame consists of only 2 MP, and the frame rate is at most 60 fps (and therefore exposure time is 1/60 s). The camera can easily read and process a row of 1920 pixels during the time it can spend for each row at the given video frame rate. That's different with still images. For example, the A77 with a 6000×4000 image and minimum exposure time of 1/8000 s would have to clear a row of 6000 pixels and have it read out and processed 1/8000 s later. If it took longer, the pixels of the next row would be exposed longer, and the minimum exposure time could not be reached. The timing would also have to be absolutely constant, i. e. no pixel row would be allowed to require more than 1/8000 s of processing. If it required less, processing of the next row would have to be delayed precisely to the next 1/8000 s slot. With a mechanical second curtain, only the timing between the first curtain (which requires no processing, only clearing) and the second curtain has to be right. After the second curtain has closed, the camera can process the entire frame in larger chunks and with variable timing.
  2. Image quality: Reading out a row of pixels while they are still exposed will probably result in reduced image quality. While this would be hardly noticable in (comparable) low resolution video, it might be in high resolution still images.


Update from September 20th, 2011:

The question came up why the cameras mentioned above have the option to turn off the electronic front curtain shutter. The manual of the NEX-5N gives some hints:

When you shoot at high shutter speeds with a large diameter lens attached, the ghosting of a blurred area may occur, depending on the subject or shooting conditions. In such cases, set this item to [Off].
First of all, this is badly worded and likely a result of mistranslation. I don't think the original author really meant “large diameter”, but rather “large aperture”, because the physical diameter of the lens can hardly influence the shutter. A high shutter speed at large aperture means nothing but lots of light reaching the sensor. So in this situation “ghosting of a blurred area” may occur, whatever that means. My take on this is that clearing the pixels while they are exposed is less clean, and when they are only exposed for a short time afterwards the resulting image will be of lower quality. Clearing the sensor image while it's in the dark, covered by a mechanical shutter, seems to avoid this problem in this situation. I remains to be seen how severe the problem is in real life.
When a Minolta/Konica Minolta lens is used, set this item to [Off]. If you set this item to [On], the correct exposure will not be set or the image brightness will be uneven.
This is even less clear to me. Obviously the camera uses some lens parameters in conjunction with exposure metering and shutter action, and these parameters are not delivered by Minolta lenses but only by Sony lenses. It's unclear if this is really about Minolta lenses, or A-mount lenses in general (as opposed to E-mount lenses). This could be some obscure problem with adapted lenses on E-mount cameras and may not apply to the implementation in A-mount cameras. Again, what this means in real life remains to be seen.
What is it, and why use it? When Sony recently announced the SLT-A77, SLT-A65, NEX-7 and NEX-5N, one little detail in their specification lists was mostly overlooked: They are Sony's first photo cameras with an electronic front curtain shutter. What does this mean, how does it work, and why use it anyway? This page provides the answers.

Leserkommentare

#1: Kommentar von Pau am 21. September 2011, 21:09:17 Uhr:
Just to point that electronic front curtain shutter, also called "electronic first shutter curtain" EFSC is a feature implemented in almost all Canon EOS dSLR cameras with Live View (with the exception of some 1D series models) and largely discussed and documented by macrophotographers and astrophotograpers for its vibration free feature, altough Canon never publity it like Sony in its new mirrorless models.
Just google for EFSC and Canon.
It's a great feature and would be interesting for many people in this specilized areas, and also for use with microscopes and supertelephoto lenses.
#2: Kommentar von Chris Jankowski am 20. Oktober 2011, 02:50:48 Uhr:
Re. Minolta/Konica Minolta lenses that may cause incorrect or uneven exposure when using electronic first shutter curtain.
A plausible explanation for this behaviour is in timing and efficiency of aperture closing mechanism of the lenses. After you press the shutter release button the aperture of the lens is first closed to the required value. In the old lenses this is amechanical process of the camera body pusing on a little lever in the lens. If the aperture closes too slowly the recording of the image may start before the aperture closed to the preset value. In such case, either the bottom of the image will be overexposed or the whole image will be overexposed.
If this is the case one could, in principle, manually work around it by locking the exposure first, then pressing the depth of field preview button and then pressing the shutter release button. However, all of this is tricky and can play havoc with your AF settings.
One could also work with fully open lens (not often desirable), of course.
Michael Hohner antwortet:
I had the same speculation in the Dyxum forum, but related to the warning in the NEX-5N manual. Since the same warning is in the A65 and A77 manual, this explanation actually becomes less plausible. The aperture mechanism has not changed for Sony A-mount lenses, and I don't have the impression that their aperture closes any faster than that of Konica Minolta lenses. Also, some Sony lenses and some Konica Minolta lenses are both re-badged Tamron lenses, and most current Sony lenses are re-badged Minolta lenses. Why does Sony warn specifically against Konica Minolta lenses, and not also against these other lenses?
#3: Kommentar von Jan Brittenson am 1. Januar 2012, 00:31:07 Uhr:
The first electronic curtain is likely rate limited by the clock it shared with readout. So it can't clear rows faster than it could read them. The second curtain isn't limited by this clock. A mechanical shutter with a sync speed of 1/250 exposing at 1/8000 will move a slit 250:8000 of the height of the frame across it. If the height has 4000 rows, then the size of this slit is 4000*250/8000 = 125 rows. For an electronic shutter, which has a sync speed of say 1/12 (at still resolution; the A77 shoots at up to 12fps, right?) this becomes 4000*12/8000 = 6 rows. At 1/24 it would be 12 rows. So if you're panning with a fast lens wide open, tracking a subject, it's not hard to see how the background could smear.

What they're really saying is that at high shutter speeds and narrow depth of field, turn it off when shooting moving subjects. Sports, wildlife; that sort of thing.

Not sure what the difference in lenses is; maybe newer Minolta and all Sony branded lenses have better defined aperture timing, or can go from one aperture to another without passing through an implicit reset state (wide open). Or some other practical difference that matters here.
Michael Hohner antwortet:
I think there are some errors in what you write. First, the second curtain has to move across the frame at the same speed as the first curtain. Otherwise pixel rows would not receive an identical amount of exposure across the frame. This is true no matter how you implement the first curtain. Remember that the second curtain is always mechanical. The sensor readout speed merely sets an upper limit for the frame rate, but does not directly determine the sync speed. If that wasn't the case, you would see uneven exposure with all apertures and lenses and also with static subjects.

Second, the problem with smearing when photographing fast moving objects is caused by the fact that an exposure using a focal plane shutter can take much longer than the exposure time suggests (it's always sync time + exposure time). So when you, for example, use an exposure time of 1/8000, it takes about 1/240 until the entire process of exposing the sensor is over. This is also independent from the way you implement the first curtain.

#4: Kommentar von Jan Brittenson am 8. Januar 2012, 00:53:12 Uhr:
What I'm saying is that the EFC likely can't clear an entire frame in 1/240s, because it's rate limited by the chip clock and/or the rate at which it can discharge, or some other factor. The second curtain, by contrast, doesn't "do" anything other than remove a bias voltage from a row. So it's not speed limited. The workaround is to block the chip with the mechanical shutter, clear it, then use the first mechanical shutter to expose and the second electronic shutter to stop, because the first mechanical shutter is faster than the first electronic. Other variations are possible as well, such as start from the center by opening both mechanical blades, then use two electronic second shutters.
Michael Hohner antwortet:
First, if the front curtain was any slower than 1/250, then the camera wouldn't have an x-sync time of 1/250. Second, the cameras in question only have an electronic (or mechanical) front curtain and mechanical rear curtain. So I don't understand what you're trying to tell us here.
#5: Kommentar von Badb0y am 8. Januar 2012, 11:18:14 Uhr:
So finally worth efcs to use or not :) ?
Michael Hohner antwortet:
In the situations that it does work (and these are most), I think it's worth using it.
#6: Kommentar von Greg Beetham am 1. August 2012, 02:48:16 Uhr:
The difference between lenses could be referring to the differences between earlier 5pin and later 8pin lenses but the Sony individual who wrote the advisory didn't know how to explain it in English or wasn't told himself in detail by anyone within Sony's engineering dept. who might have been able to explain it too him.
Greg
Michael Hohner antwortet:
This can't be the explanation. 8-pin lenses have been around for a very long time (1991), some Sony lenses also have 5 pins, and in fact 5-pin lenses work just fine with the EFCS.
#7: Kommentar von Greg Beetham am 4. August 2012, 19:27:56 Uhr:
Some Sony lenses are 5 pin? I didn?t know that, but I guess there is not much point in having a focus distance encoder in a wide angle lens for example.
I?m wondering now about SSS, (Pentax have image stabilization that moves in three directions, one would think that would make the design engineers life rather difficult when taking shutters into account). What exactly happens with the moving sensor at the moment of exposure, is the assembly stopped for the exposure moment (iris stop-down and the shutter duration), it?s difficult to believe that it remains in motion. Most Sony shutters range from 1/160sec to 1/250sec fastest mechanical cycle time regardless of the apparent shutter speed that?s set faster than that but what I don?t know is the amplitude and transit times the SSS has that would have to be frozen for the duration of the shutter one would think.
Perhaps the electronic first curtain actually reduces the apparent ?cycle time? quite a lot if the first curtain is equal to the speed of a ?read? but one would also think that it would be coordinated with the movement of the aperture actuator in the camera having reached it?s pre-determined position first before the electronic first curtain begins, and one would be entitled to assume that that would be in effect no matter what lens was used. Maybe some lenses have slow iris response times compared to other lenses, and perhaps Sony should compile a list of exactly which lenses are affected on which cameras. (Some Sony advisories leave a lot to be desired)
Greg
Michael Hohner antwortet:
Of course, SSS keeps working during exposure, otherwise there would be no stabilization effect.
#8: Kommentar von Greg Beetham am 5. August 2012, 02:02:54 Uhr:
If the SSS remains operative during all exposures regardless of focal length (magnification factor) and shutter speed one would think that there would be a grey area at times where the speed of the shutter and the speed of the SSS would be in conflict with the speed of the read and each other depending of course on which one can move the fastest in relation to the other?or not. :-)
Greg
Michael Hohner antwortet:
I'm not sure what you mean with “speed of the read” here and where you see a conflict.

BTW, did you read my follow-up on the topic?

#9: Kommentar von Jeff Thompson am 19. Dezember 2012, 05:22:58 Uhr:
Does anyone really know what's going on with the Canon 60D EFSC? There is some mechanical movement at the time the shutter "opens", and I've yet to see an explanation of it. It's not the aperture closing down (not on my 60D anyway) because it's there when I use a mirror lens. It's subtle, but I've seen claims that it's enough to blur macro images.
#10: Kommentar von Karsten Meyer am 2. Januar 2013, 21:33:41 Uhr:
Thank you very much for this informative report, Mr. Hohner!

I just want to mention that the latest mirrorless Panasonic cameras, the G5 and the GH3, can take pictures totally without the mechanical shutter. There are some limitations, but it's still useful.
I wonder how they made it possible - and I hope, Sony will do the same.
Michael Hohner antwortet:
It seems like you can not use flash when the EFCS is enabled. I consider this a severe limitation.

Panasonic probably uses a variation of the video shutter as described above.
#11: Kommentar von Mark P am 11. Januar 2013, 11:08:22 Uhr:
interesting thread, thanks! so basically except for panning shots efsc is ok?

btw, i'm be able to use the flash on the nex-7 (with the 16mm pancake lens) with no problems (fill, slow and rear curtain)
#12: Kommentar von Mark P am 11. Januar 2013, 11:15:21 Uhr:
sorry, just realized your remark abt the lack of flash use was for panasonics..
#13: Kommentar von John B am 26. Januar 2013, 15:38:33 Uhr:
Has anyone ever captured an adverse effect of using the efcs, for example when intentionally trying to learn about it by experimentation, or accidentally?

I shoot with a lot of MF legacy glass and am just wondering if there are circumstances when I should be turning it off?
Michael Hohner antwortet:
With the A77, I've found a few odd effects of the EFCS, but not related to specific lenses, but related to flash. Flash exposure is quite inconsistent with the EFCS on, or it even does not work in the case of the Macro Ring Flash. When turning EFCS off, everything is just fine.
#14: Kommentar von Gord am 21. März 2013, 15:47:51 Uhr:
With a lens that must use an adapter you have to manually focus and set the f-stop,the NEX cameras will automatically choose correct exposure by adjusting the shutter speed and ISO.It doesn't seem to matter wether the front shutter is used or not.
#15: Kommentar von Gregg Lee am 20. April 2013, 17:55:10 Uhr:
I just got an SLT (A99) for the first time so I haven't paid attention to this subject before.

Regarding your description of "Cameras with live view" I don't see why there would have to be two shutter cycles. Maybe there were two cycles on early implementations where live view was an option with an optical viewfinder design. But for SLTs which are always live view and which end the shutter cycle with both curtains open, I don't see it.

It seems like the curtain sequence with shutter setting faster than sync speed would be as listed below (slower than synch speed reverses the order for start of second curtain and end of first.)

Traditional SLR
shutter mechanism parks between exposures with first curtain closed and second curtain open

image recording begins
first curtain starts to open,
second curtain starts to close
first curtain reaches fully open
second curtain reaches fully closed
image recording ends
second curtain opens
first curtain closes

SLT EFCS=off
shutter mechanism parks between exposures with first curtain open and second curtain open.

first curtain closes
image recording begins
first curtain starts to open,
second curtain starts to close
first curtain reaches fully open
second curtain reaches fully closed
image recording ends
second curtain opens

This is exactly the same series of steps. The only difference is that "first curtain closes" moves from last to first. The parking spot is different.

Note: I think your description gets off track with the first step, that refers to "the" shutter rather than one of the curtains. "1. The shutter first has to close." This is ambiguous. The shutter mechanism can be closed in two mutually exclusive ways. First curtain closed or second curtain closed."

Gregg Lee
#16: Kommentar von John Eastham am 21. September 2013, 18:29:42 Uhr:
The hot spot or over exposer created by wide open Minolta lenses is caused by the reflection from the open censor seen on the rear element of the old film lenses. New sony lenses have antireflection coats on the back of the wide aperature lenses. 85 1.4 and 50 1.4 etc. I think this is what it is about. The rear coated lens was not required for film, as the film was dull.
Michael Hohner antwortet:
That can't be the reason. When you take a shot, the shutter is inevitably open, no matter how the shutter is constructed. You will get the reflection either way. See for example here.
#17: Kommentar von tesilab am 21. Oktober 2013, 16:30:08 Uhr:
With some fast exposures I have seen uneven exposure between the top and bottom of the frame with a visible horizontal line between them. I have assumed, but not proven, this is due to the EFSC.
#18: Kommentar von RobertL am 4. Dezember 2013, 02:58:55 Uhr:
Thank you for explaining this technology Michael!

It left me wondering, is it possible that another IQ advantage of the Sony A7r over the A7 model is the presence of a mechanical front curtain in the former?

Perhaps electronic front curtains are not quite as clean a solution when maximum IQ is the goal?
Michael Hohner antwortet:
I'd expect the opposite. The mechanical shutter may induce extra vibrations, and electronic shutters avoid that.
#19: Kommentar von RobertL am 4. Dezember 2013, 22:52:29 Uhr:
That is almost certainly true to some extent, but flipping up a mirror assembly in an DSLR should do the same too - right?

I guess it really depends on whether any induced vibrations are significant enough to matter (Sony design engineers must have tested this or they are really slipping).

Any thoughts yourself on why Sony may have decided to put a mechanical front curtain into the A7r? It must take up more space after all and be more difficult and expensive to manufacture than an equivalent electronic version.
Michael Hohner antwortet:
In a DSLR you can use mirror pre-fire to let these vibrations die down before the actual exposure, and with SLTs you don't have a flip-up mirror at all.

You don't save space with an electronic front curtain, because the shutter blades are still there for the rear curtain. And with the A7 you can turn off the electronic front curtain and use the shutter blades also for the front curtain.

The electronic front curtain is part of the image sensor. As to why Sony has not designed the A7R sensor with an electronic front curtain, I have no idea.

#20: Kommentar von RobertL am 6. Dezember 2013, 02:33:02 Uhr:
My curiosity aroused (as I have a new Sony A7r myself) I found this page:

http://photo.stackexchange.com/questions/39489/disadvantages-of-electronic-first-curtain-shutter

It sounds like there are certain scenarios where EFCS may cause image blurring (but nothing there really explains why Sony removed the setting option).

R.
Michael Hohner antwortet:
I can absolutely not confirm the underexposure problem that they talk about there. The A77 with EFCS and a 1985 Minolta 50/1.4 does not at all underexpose at 1/8000. They must have seen a different problem.

The blurring is what also the camera manuals warn against. But I've never seen it so far. The explanation they give there for this possible blurring is dubious, and even if true, it would not only affect Minolta lenses, but also Sony lenses.

#21: Kommentar von RobertL am 7. Dezember 2013, 01:27:15 Uhr:
It is still quite early days with the new A7r. Perhaps the real reason for Sony's decision to make it a mechanical shutter only will eventually become public.

There must be many many people asking the same question.
#22: Kommentar von Dan C am 17. Dezember 2013, 23:47:46 Uhr:
I have a very bad underexposure problem happening with my new Tamron 24-70 f2.8 whenever there is a high shutter speed. If I turn the EFCS off it fixes the problem. I am running an A77, with my Dt lens 18-250 there is no problem.
Dan C
#23: Kommentar von JimS am 18. Dezember 2013, 04:37:24 Uhr:
A curiosity comes to mind when talking about the total time of exposure when high speed exposures are taken. If there is a thin vertical line of light moving quickly right to left traversing the entire field of view in the sync time (1/250th). Then even with a 1/8000th second exposure, wouldn't the line get exposed as a diagonal line (top right to bottom left)? Since exposure time is sync speed + 1/8000 and the rear curtain still takes the 1/250th of a second to close, the line will move right to left as the exposed portion of the sensor moves down.
Michael Hohner antwortet:
That's called the “rolling shutter effect”, and it can lead to distorted images, like this one.
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