Do We Still Need Focal Plane Shutters in DSLRs?

Started Nov 17, 2013 | Questions
WilbaW
WilbaW Forum Pro • Posts: 11,426
Do We Still Need Focal Plane Shutters in DSLRs?

I guess film SLRs use FPS simply because we need to shield the film when the lens is removed. (Right?) So we don't need a shutter with a digital sensor for that reason.

Are state-of-the-art electronic second curtains good enough that we don't need a physical shutter at all?

So if you were designing a modern interchangeable lens camera system from scratch, no legacies, would you include a physical shutter? What type and why?

Two recent examples - Nikon 1 cameras have electronic shutters (AFAIK), but the Canon EOS M has a physical second curtain. How do we make sense of that?

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Steen Bay Veteran Member • Posts: 6,974
Shutter shock

WilbaW wrote:

I guess film SLRs use FPS simply because we need to shield the film when the lens is removed. (Right?) So we don't need a shutter with a digital sensor for that reason.

Are state-of-the-art electronic second curtains good enough that we don't need a physical shutter at all?

So if you were designing a modern interchangeable lens camera system from scratch, no legacies, would you include a physical shutter? What type and why?

Two recent examples - Nikon 1 cameras have electronic shutters (AFAIK), but the Canon EOS M has a physical second curtain. How do we make sense of that?

Think that most recent Sony interchangeable lens cameras (except the 7R) have an (optional) electronic first curtain shutter. Probably a good idea. Lots of threads on the mFT forum about 'shutter shock' and blurred images at 1/60 to 1/125 sec or so. Guess that shutter shock is an issue on most/all cameras with a focal plane shutter.

mosswings Veteran Member • Posts: 8,284
Re: Do We Still Need Focal Plane Shutters in DSLRs?
3

WilbaW wrote:

I guess film SLRs use FPS simply because we need to shield the film when the lens is removed. (Right?) So we don't need a shutter with a digital sensor for that reason.

Are state-of-the-art electronic second curtains good enough that we don't need a physical shutter at all?

So if you were designing a modern interchangeable lens camera system from scratch, no legacies, would you include a physical shutter? What type and why?

Two recent examples - Nikon 1 cameras have electronic shutters (AFAIK), but the Canon EOS M has a physical second curtain. How do we make sense of that?

To implement an electronic shutter requires a means of transferring the photon generated charge to a light-shielded storage node, preferably simultaneously to avoid certain effects that crop up with focal plane shutters and moving subjects (the jello wobbles, caused by the moving slit of the focal plane). This is called a global shutter, and exacts several penalties for its use. The first is added complexity per pixel, and lowered sensel area per pixel site as compared to a standard sensor with a mechanical shutter. Thus electronic shuttered sensors usually don't offer as good an image quality as standard sensors. If the electronics could be vertically stacked in some way (I'd MFRS don't call it planar for nothing), then the sensel areal efficiency could match current designs.

There are assuredly several other concerns over the use of an electronic shutter, but this is perhaps the biggest one for DSLR users.

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(unknown member) Senior Member • Posts: 1,053
Yes.

The change state for the sensors is still too slow.

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WilbaW
OP WilbaW Forum Pro • Posts: 11,426
Re: Do We Still Need Focal Plane Shutters in DSLRs?

mosswings wrote:

... electronic shuttered sensors usually don't offer as good an image quality as standard sensors.

I'd always wondered about things like that but hadn't found any detail, thanks.

So, would we be better off with leaf shutters in the lenses?

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hjulenissen Senior Member • Posts: 2,077
Re: Do We Still Need Focal Plane Shutters in DSLRs?
1

"what if"

The quantum/jotta/Eric Fossum sensor ever made it into reality. The one where you read a high density (spatial/temporal) sensor detecting for presence of photons. Assuming that all of the sensor/physical challenges were solved. In order to make a traditional image file, you would have to do some downsampling (spatio-temporal). What would be the temporal downsampling characteristic preferred by most of the photographers, most of the time? A rectangular function of time? Gaussian? Asymmetric? Would it be synchronous across the frame, "tearing" (like todays electronic shutters) or involve radial symmetry like the leaf shutters?

What is done in rendered computer games? (I assume that they are free to do anything as long as it looks "good" and does not spend too much computing time).

-h

Chas Tennis Contributing Member • Posts: 788
Believe that mechanical shutters needed to reduce Jello Effect

I have a Casio FH100 super compact camera. It has an electronic shutter burst mode with a frame rate of up to 40 fps and records up to 30 pictures. It has a mechanical shutter burst mode at up to 10 fps. The user's manual says that to avoid distortions to use the mechanical shutter. While researching the issue I believe that I found references and threads saying mechanical shutters in DSLR's were necessary to reduce distortions.

I tested my Casio FH100 in burst mode.

The pictures of the black disc show

1) Electronic Shutter. Disc not rotating with no motion blur or Jello Effect. 1st of Disc

2) Electronic Shutter. Disc rotating with Jello Effect distortions. 2, 3, & 4rd of disc

3) Mechanical Shutter. Disc rotating with mechanical shutter - a leaf shutter - showing little distortion but motion blur. 4b& 5th of disc.

http://www.dpreview.com/forums/post/39393614

For details click "gallery" for captions describing each picture.

DSLR cameras have a slit shutter (not a leaf shutter) that produces a somewhat similar distortion to that of the Jello Effect from the rolling shutter.  The leaf shutter exposes all parts of the frame at the same time while the DSLR slit shutter scans across.  Rolling shutters (electronic) read one line at a time and scan down the frame.   The electronic shutter is not as fast as the mechanical shutter.

Chas Tennis

falconeyes
falconeyes Senior Member • Posts: 1,437
+1
1

Yes, electronic global shutters implemented in the analog domain have a negative impact on image quality, as far as quantum efficiency (small effect), read-out noise (medium effect) and dynamic range aka full well capacity (large effect) are concerned.

This is why mechanical shutters can't be avoided in top of the line still cameras.

E.g., the Nikon 1 just accepts jello effects in stills and doesn't that great of an IQ anyway.

However, an electronic global shutter implemented in the digital domain would avoid all such negative effects. Pixels would have on-chip read-out and ADC electronics (like what Sony sensors already do today) and digital accumulators (aka pixel register). The pixel register would store an intermediate readout (or snapshot) as a number rather than electrons in an analog charge well. Point is that today, to embedd 20 million pixel registers on chip is very expensive. Many times more expensive than a focal plane shutter. But of course, this is going to change over the next decade ...

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WilbaW
OP WilbaW Forum Pro • Posts: 11,426
Re: +1

Interesting, thanks.

So.. no-one wants to take a punt at the leaf shutter question?

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mosswings Veteran Member • Posts: 8,284
Re: +1
1

WilbaW wrote:

Interesting, thanks.

So.. no-one wants to take a punt at the leaf shutter question?

I'll kick, but be forewarned that this just a semi-informed opinion.

Falconeyes elucidates the problem nicely.  The real advantage of electronic shutters is in their speed -but all implementations of electronic shutters possible today have similar speed problems to any mechanical shutter and a few problems that mechanical shutters don't.  The per-pixel direct conversion scheme Falconeyes posits probably requires a 3D IC processing technique because with planar IC processes (all current technologies) additional circuitry just reduces the pixel area available to the sensing well, lowering quality.  And one thing that mechanical shutters have over electronic ones is that you can absolutely shut off all light from the sensor.  With an electronic technique the isolation isn't perfect. Also, the sensor is always on and just as important is going full tilt everywhere on chip. The line-oriented architecture of current sensors, including the EXMOR variety, saves a lot of power. One of the advantages of pixel level conversion, though, might be that you can digitally compensate for device variations at that level rather than at the line level, so banding and other imperfections could be even less than with today's best sensors. Speaking of power, mechanical shutters don't need much and only when you capture.

Back to leaf shutters: they need to be in the lens.  They will by virtue of their mechanicalness introduce rolling shutter effects, just in the radial direction. They get slower the brighter the lens becomes because the shutter leaves have to move across a larger maximum aperture (focal plane shutters' speed is independent of lens aperture).  I for one don't want anything more to be stuffed into my lenses; VR and AF have made them quite large.

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Chas Tennis Contributing Member • Posts: 788
Re: +1

mosswings wrote:

....................................................................................................

Back to leaf shutters: they need to be in the lens. They will by virtue of their mechanicalness introduce rolling shutter effects, just in the radial direction. ..................................

As the leaf shutter first opens I believe that it somewhat resembles a pinhole then an increasing aperture. All elemental detectors of the sensor could receive light at the first opening. The exposure time is a sequence of an increasing aperture - a time with full aperture - and then a closing aperture.

The mechanical shutter test that I showed for the Casio Fh100 showed motion blur but no rolling shutter-like distortion which is consistent with the above view of how the leaf shutter works.

Mechanical Leaf Shutter Test Casio Ex FH100

Chas Tennis

mosswings Veteran Member • Posts: 8,284
Re: +1

Dumb, dumb, dumb, I stand corrected.  That is the clear advantage of a leaf shutter. Another advantage of a leaf shutter is that it can flash sync at ALL shutter speeds it's capable of, because the entire frame is exposed at once.  The big disadvantage of a leaf shutter is that it can't offer as high an effective shutter speed as a focal plane shutter because it can't create a narrow moving annulus in the same way as the focal plane shutter creates a moving slit.

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WilbaW
OP WilbaW Forum Pro • Posts: 11,426
Re: +1

Thanks chaps, you have satisfied my curiosity. 

Just found this - Kowa SETR - The last leaf-shutter SLR - which gives an interesting historical perspective.

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Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,940
Good find

WilbaW wrote:

Thanks chaps, you have satisfied my curiosity.

Just found this - Kowa SETR - The last leaf-shutter SLR - which gives an interesting historical perspective.

Now that's an exotic piece of camera history. Never ever heard of this thing.

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Horshack Veteran Member • Posts: 5,803
Re: Shutter shock
1

Steen Bay wrote:

WilbaW wrote:

I guess film SLRs use FPS simply because we need to shield the film when the lens is removed. (Right?) So we don't need a shutter with a digital sensor for that reason.

Are state-of-the-art electronic second curtains good enough that we don't need a physical shutter at all?

So if you were designing a modern interchangeable lens camera system from scratch, no legacies, would you include a physical shutter? What type and why?

Two recent examples - Nikon 1 cameras have electronic shutters (AFAIK), but the Canon EOS M has a physical second curtain. How do we make sense of that?

Think that most recent Sony interchangeable lens cameras (except the 7R) have an (optional) electronic first curtain shutter. Probably a good idea. Lots of threads on the mFT forum about 'shutter shock' and blurred images at 1/60 to 1/125 sec or so. Guess that shutter shock is an issue on most/all cameras with a focal plane shutter.

The shutter shock issue affects MILCs more because they have to first close the 2nd curtain before starting the exposure, meaning the residual resonating vibration from the 2nd curtain is occurring during the exposure, whereas DSLRs  only have the 1st curtain's vibration to contend with.

barryhk Junior Member • Posts: 41
Re: Do We Still Need Focal Plane Shutters in DSLRs?

hmm... I read the whole thread but still couldn't understand why do we still need a focal plane shutter.....

I am sorry that I am too dumb to understand the explaination if it is already answered.... probably a nice diagram can help.

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Chas Tennis Contributing Member • Posts: 788
Re: Do We Still Need Focal Plane Shutters in DSLRs?
1

barryhk wrote:

hmm... I read the whole thread but still couldn't understand why do we still need a focal plane shutter.....

I am sorry that I am too dumb to understand the explaination if it is already answered.... probably a nice diagram can help.

I believe - but am not certain - that the reason is that the mechanical focal plane shutter can scan in front of the sensor faster than the sensor can be read out electronically using the 'rolling shutter' process. I believe that it would be time consuming finding that conclusion stated on the internet. The scan time for DSLR forcal plane shutters is often roughly 1/250 second. Finding electronic scan times for mega pixel DSLRs would also be difficult. For your diagrams try - Google: DSLR focal plane shutter Youtube

Reply discussing electronic shutter, focal plane shutters and leaf shutters and the distortions produced.

http://www.dpreview.com/forums/post/50993374

There have been threads discussing this issue but I could only find one.

http://www.dpreview.com/forums/post/29298847

Chas Tennis

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