Electronic Front Curtain Shutters (Sony SLT).pdf

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Electronic Front Curtain Shutters

September 19th, 2011 - 09:51:47 PM:

What is it, and why use it?

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

Traditional focal plane shutters

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

There are two sets of shutter blades (or two pieces of shutter cloth in older cameras) that cancover 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 isidle. 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 (therear curtain) starts to move and cover the image area again, moving at the same speedas the front curtain.

3. When the rear curtain has fully closed, exposure is complete. After that, film cameras canstart winding the film to the next frame, andDSLRs can read out the image from thesensor and process it.

4. For the next shot, the same action can take place in the opposite order and direction, orthe camera may move both shutter curtains into their initial starting position.

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


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Shutter movement of focal plane shutter

This shutter construction was used for film SLRs (and many rangefinder cameras), and it worksthe 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*1and other brands'DSLRs. Then the shutter is openbetween shots so that light can reach the image sensor. But when taking a still image, theshutter 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 sensorwithout disturbance by further exposure. So in these cameras the shutter sequence becomesmore complicated:

1. The shutter first has to close. When it's fully closed, the image is cleared from the sensorby draining all photo sites.

2. Then the front curtain and rear curtain perform their dance as in earlier DSLRs. When therear 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 mechanicalfront curtain, the image is cleared from the sensor, pixel row by pixel row, in the samedirection and with the same speed a with a mechanical shutter. Exposure of each row ofphoto sites starts immediately after it is cleared. This is possible, because you are notinterested in the image that was stored in the photo sites before clearing them, only in theimage that is createdafterwards.


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2. After some delay, which again is the exposure time, the (mechanical) rear curtain starts toclose, trailing the clearing of pixel rows with the same direction and speed. Pixel rowscovered 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 thesensor and process it.

4. Then the mechanical shutter can open again.

The advantages of this system are obvious:

There are fewer mechanical parts. Specifically, you need only a single set of shutter

blades.*2

For one shot, the shutter blades have to move only twice, not four times as before. Thisroughly doubles the lifetime of the shutter compared to the earlier design, withoutspending extra efforts in mechanics and materials.Since there is no movement of any parts for the front curtain, vibrations are reduced.The delay from release to exposure is reduced. You can start exposure immediately anddon't have to wait for the shutter to close first.

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

Not really. If the rear curtain would move across the frame, clearing pixel rows like the frontcurtain, we would erase the image that we've just taken, and the entire procedure would bepointless. The point of the mechanical rear curtain is to stop exposure of the sensor to lightwhile preserving the image that is already in the sensor. Whatever kind of rear curtain one mayinvent, 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 nevercloses in video mode (or Live View, which is basically the same), and still the camera canproperly record the frames that make up the video.

In video mode, the camera goes through the pixel rows of the sensor in regular intervals. Eachrow is read out (and cleared in the process) to form part of a frame, and after that the same rowis exposed anew, to be read out and cleared later for the next frame. Obviously this works justfine. 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, andthe frame rate is at most 60 fps (and therefore exposure time is 1/60 s). The camera caneasily read and process a row of 1920 pixels during the time it can spend for each row atthe given video frame rate. That's different with still images. For example, the A77 with a60004000 image and minimum exposure time of 1/8000 s would have to clear a row of6000 pixels and have it read out and processed 1/8000 s later. If it took longer, the pixelsof the next row would be exposed longer, and the minimum exposure time could not be


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Display filter re-designed Followup on Electronic Front Curtain Shutters

reached. The timing would also have to be absolutely constant, i. e. no pixel row would beallowed to require more than 1/8000 s of processing. If it required less, processing of thenext row would have to be delayed precisely to the next 1/8000 s slot. With a mechanicalsecond 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 resulin reduced image quality. While this would be hardly noticable in (comparable) low

resolution video, it might be in high resolution still images.

Footnotes:

1. Earlier Sony DSLRs used a secondary sensor in the viewfinder for Live View, so this does not apply to them.2. The A77, A65, NEX-7 and NEX-5N actually still have shutters with two sets of blades, and you can switch

between mechanical and electronic first curtain.

Update from September 20th, 2011:

The question came up why the cameras mentioned above have the option to turn off theelectronic 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, theghosting of a blurred area may occur, depending on the subject or shootingconditions. 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 originalauthor 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 meansnothing but lots of light reaching the sensor. So in this situation ghosting of a blurred area mayoccur, whatever that means. My take on this is that clearing the pixels while they are exposed isless clean, and when they are only exposed for a short time afterwards the resulting image willbe of lower quality. Clearing the sensor image while it's in the dark, covered by a mechanicalshutter, seems to avoid this problem in this situation. I remains to be seen how severe theproblem is in real life.

When a Minolta/Konica Minolta lens is used, set this item to [Off]. If you set this itemto [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 conjunctionwith exposure metering and shutter action, and these parameters are not delivered by Minoltalenses but only by Sony lenses. It's unclear if this is really about Minoltalenses, or A-mountlenses in general (as opposed to E-mount lenses). This could be some obscure problem withadapted lenses on E-mount cameras and may not apply to the implementation in A-mountcameras. Again, what this means in real life remains to be seen.

Categories:Articles, Camera TechnologyArchive of all entries

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2011 Michael Hohner; This page was last changed on 2014-09-06

Readers' comments

#1: Comment posted by Pauon September 21st, 2011 - 09:09:17 PM:

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 withLive View (with the exception of some 1D series models) and largely discussed and

documented by macrophotographers and astrophotograpers for its vibration freefeature, 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: Comment posted by Chris Jankowskion October 20th, 2011 - 02:50:48 AM:

Re. Minolta/Konica Minolta lenses that may cause incorrect or uneven exposure whenusing electronic first shutter curtain.

A plausible explanation for this behaviour is in timing and efficiency of aperture closingmechanism 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 processof 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. Insuch 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 AFsettings.

One could also work with fully open lens (not often desirable), of course.

Michael Hohner answers:

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 explanationactually becomes lessplausible. The aperture mechanism has not changed for Sony

A-mount lenses, and I don't have the impression that their aperture closes any fasterthan 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?


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#3: Comment posted by Jan Brittensonon January 1st, 2012 - 12:31:07 AM:

The first electronic curtain is likely rate limited by the clock it shared with readout. So itcan'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 aslit 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 async 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 panningwith 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, turnit 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 practicaldifference that matters here.


I think there are some errors in what you write. First, the second curtain has to moveacross 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 howyou 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 theexposure 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 implementthe first curtain.

#4: Comment posted by Jan Brittensonon January 8th, 2012 - 12:53:12 AM:

What I'm saying is that the EFC likely can't clear an entire frame in 1/240s, because it'srate 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 biasvoltage 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 thesecond 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.


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 (ormechanical) front curtain and mechanical rear curtain. So I don't understand what

you're trying to tell us here.


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#5: Comment posted by Badb0yon January 8th, 2012 - 11:18:14 AM:

So finally worth efcs to use or not :) ?


In the situations that it does work (and these are most), I think it's worth using it.

#6: Comment posted by Greg Beethamon August 1st, 2012 - 02:48:16 AM:

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 knowhow 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


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 withthe EFCS.

#7: Comment posted by Greg Beethamon August 4th, 2012 - 07:27:56 PM:

Some Sony lenses are 5 pin? I didn?t know that, but I guess there is not much point inhaving a focus distance encoder in a wide angle lens for example.

I?m wondering now about SSS, (Pentax have image stabilization that moves in threedirections, one would think that would make the design engineers life rather difficult

when taking shutters into account). What exactly happens with the moving sensor atthe 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 timeregardless 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 forthe duration of the shutter one would think.

Perhaps the electronic first curtain actually reduces the apparent ?cycle time? quite alot 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 havingreached 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 wasused. 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 whichcameras. (Some Sony advisories leave a lot to be desired)

Greg


Of course, SSS keeps working during exposure, otherwise there would be nostabilization effect.


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#8: Comment posted by Greg Beethamon August 5th, 2012 - 02:02:54 AM:

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 inconflict 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


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-upon the topic?

#9: Comment posted by Jeff Thompsonon December 19th, 2012 - 05:22:58 AM:

Does anyone really know what's going on with the Canon 60D EFSC? There is somemechanical 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) becauseit'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: Comment posted by Karsten Meyeron January 2nd, 2013 - 09:33:41 PM:

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.


It seems like you can not use flash when the EFCS is enabled. I consider this a severelimitation.

Panasonic probably uses a variation of the video shutter as described above.

#11: Comment posted by Mark Pon January 11th, 2013 - 11:08:22 AM:

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: Comment posted by Mark Pon January 11th, 2013 - 11:15:21 AM:

sorry, just realized your remark abt the lack of flash use was for panasonics..


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#13: Comment posted by John Bon January 26th, 2013 - 03:38:33 PM:

Has anyone ever captured an adverse effect of using the efcs, for example whenintentionally 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?


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: Comment posted by Gordon March 21st, 2013 - 03:47:51 PM:

With a lens that must use an adapter you have to manually focus and set the f-stop,theNEX 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.


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#15: Comment posted by Gregg Leeon April 20th, 2013 - 05:55:10 PM:

I just got an SLT (A99) for the first time so I haven't paid attention to this subjectbefore.

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 implementationswhere 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'tsee 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 curtainand end of first.)

Traditional SLR

shutter mechanism parks between exposures with first curtain closed and second

curtain open

image recording beginsfirst curtain starts to open,

second curtain starts to close

first curtain reaches fully opensecond curtain reaches fully closedimage recording ends

second curtain opensfirst curtain closes

SLT EFCS=off

shutter mechanism parks between exposures with first curtain open and secondcurtain open.

first curtain closesimage recording begins

first curtain starts to open,

second curtain starts to closefirst curtain reaches fully opensecond curtain reaches fully closed

image recording endssecond 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" shutterrather 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 curtainclosed or second curtain closed."

Gregg Lee


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#16: Comment posted by John Easthamon September 21st, 2013 - 06:29:42 PM:

The hot spot or over exposer created by wide open Minolta lenses is caused by thereflection 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 1.4and 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.


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

examplehere.

#17: Comment posted by tesilabon October 21st, 2013 - 04:30:08 PM:

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 notproven, this is due to the EFSC.

#18: Comment posted by RobertLon December 4th, 2013 - 02:58:55 AM:

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 isthe goal?


I'd expect the opposite. The mechanical shutter may induce extra vibrations, and

electronic shutters avoid that.


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#19: Comment posted by RobertLon December 4th, 2013 - 10:52:29 PM:

That is almost certainly true to some extent, but flipping up a mirror assembly in anDSLR 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 frontcurtain into the A7r? It must take up more space after all and be more difficult and

expensive to manufacture than an equivalent electronic version.


In a DSLR you can use mirror pre-fire to let these vibrations die down before the actualexposure, 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 frontcurtain 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 notdesigned the A7R sensor with an electronic front curtain, I have no idea.


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 (butnothing there really explains why Sony removed the setting option).

R.


I can absolutely notconfirm the underexposure problem that they talk about there. TheA77 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 iftrue, it would not only affect Minolta lenses, but also Sony lenses.


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.


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#22: Comment posted by Dan Con December 17th, 2013 - 11:47:46 PM:

I have a very bad underexposure problem happening with my new Tamron 24-70 f2.8whenever 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: Comment posted by JimSon December 18th, 2013 - 04:37:24 AM:

A curiosity comes to mind when talking about the total time of exposure when highspeed 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 rightto 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 exposedportion of the sensor moves down.


That's called the rolling shutter effect, and it can lead to distorted images, like this

one.

#24: Comment posted by Tord S Erikssonon July 31st, 2014 - 11:48:09 PM:

The Nikon V1, V2 & V3 all have this type of shutter available, but also a conventional

mechanical (which has a much higher flash synch speed).

These cameras are excellent for macro work, and there is no shudder from any shutterat all. Also works very well with long focal lengths, like the 70-300CX, the sharpest 70-

300 I've come across.

The shutter can also be fully electronic, but that mode gives a lower resolution image.

#25: Comment posted by Heiko Herrmannon August 16th, 2014 - 08:55:55 PM:

Electronic rear curtain now finally has been implemented in the A7s.


with the danger of severe image distortions.

#26: Comment posted by Heiko Herrmannon August 16th, 2014 - 09:47:36 PM:

The reason why it is not implemented on the a7r is: 36mp cant be cleared out in

1/160s.


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#27: Comment posted by Heiko Herrmannon August 17th, 2014 - 12:44:24 AM:

"? with the danger of severe image distortions."

Sure it has its limitations, but it opens new situations: photographing at concerts withquiet parts (classical for example), weddings, sleeping babies, etc. ...

And since it is optional, you can always switch to rear curtain mechanical shutter or

even first and rear shutters - adapting to the situation you are using your camera :)...

#28: Comment posted by Heiko Herrmannon August 17th, 2014 - 12:50:22 AM:

By the way, you should add this page to the "Engl. Artikel" menu on this page, as it isinteresting :)...

Documents

Electronic Front Curtain Shutters (Sony SLT).pdf