Question regarding mechanical shutter vs. e-shutter

[bolandross]

Hey folks,

I hope it's okay to ask a question that is not specific to the L-Mount.

Today I had some spare time to think about the technical properties of modern cameras and it became clear to me that I am not fully understanding the concept of the mechanical shutter in cameras which use a digital sensor instead of film.

I understand that when using the electronic shutter, short readout times do reduce the unwanted rolling shutter effect when capturing fast moving objects, but I don't understand why using a mechanical shutter can prevent this!

Is the sensor read out in a different way (not line by line) when using a mechanical shutter? Why does it make a difference to quickly shut off any further light by closing the curtains? Do the sensor pixels itself have a kind of "memory" which can then be read out in a slow way, not having the risk of data is being changed afterwards (since everything is dark?)

Like ... does the sensor act like a collection of many tiny "batteries" which are being charged by incoming light? And when the incoming light stops, they keep their charge?

I checked out some explanation videos on YouTube, but many of them only seem to compare the pros and cons, without explaining the "why" behind it! I would love if someone could explain it to me. Thanks in advance!

 

[Vale46]

In fact the opposite is the case.
With mech. shutter there‘s no rolling shutter efect. With electr. shutter and slower sensor readout like the S5 fast moving subjects suffer from rolling shutter distortion. The OM System OM1 has hardly rolling shutter distortion due to faster sensor readout speed.
Sadly I don‘t know the techn. background, too.

 

[pdk42]

Hey folks,

I hope it's okay to ask a question that is not specific to the L-Mount.

Today I had some spare time to think about the technical properties of modern cameras and it became clear to me that I am not fully understanding the concept of the mechanical shutter in cameras which use a digital sensor instead of film.

I understand that when using the electronic shutter, short readout times do reduce the unwanted rolling shutter effect when capturing fast moving objects, but I don't understand why using a mechanical shutter can prevent this!

They don’t “prevent it” since you can get rolling shutter from mechanical shutters too - but because they scan more quickly than most electronic shutters, the effect is smaller. The typical time for a focal plane shutter to traverse the frame when in “slit mode” (shutter time shorter than the flash sync speed), is around 1/150 to 1/250s. This is quite fast compared to most electronic shutters, but it’s still possible for the subject to move in this time so you will get distorted shapes (i.e. rolling shutter).

Some of the very latest digital cameras do offer very fast readout times which match focal plane shutters - e.g. the Nikon Z8 and Z9 which are around 1/200s, or the OM1 at 1/120s. For this reason, the Z8/9 don’t have a mechanical shutter.

Is the sensor read out in a different way (not line by line) when using a mechanical shutter? Why does it make a difference to quickly shut off any further light by closing the curtains? Do the sensor pixels itself have a kind of "memory" which can then be read out in a slow way, not having the risk of data is being changed afterwards (since everything is dark?)

No, it’s read the same way, but when it’s being read it’s not being exposed to light. The sequence is roughly this:
- Clear down sensel wells (from live view mode)
- Open shutter
- Close shutter
- Read sensel wells
- Re-enable live view

Like ... does the sensor act like a collection of many tiny "batteries" which are being charged by incoming light? And when the incoming light stops, they keep their charge?

Yes, sensors are big arrays of charge wells that accumulate electrons. The electrons are liberated by light as it hits the silicon. The accumulated electrons create a charge which results in a small voltage that‘s then turned into a digital value using an Analogue to Digital converter (ADC). From then on, the image is an array of numbers.

I checked out some explanation videos on YouTube, but many of them only seem to compare the pros and cons, without explaining the "why" behind it! I would love if someone could explain it to me. Thanks in advance!

 

[robin0112358]

For photography I stick with the mechanical shutter... but that might just be habit!

 

[Vale46]

I use the elctr. shutter most of the time. I switch to the mech. shutter only when I use flash or have faster moving subjects.

 

[pdk42]

Another use case where mechanical shutter is needed is in AC artificial light - the 50/60Hz frequency means you’ll get banding as the light source slightly increases and decreases in intensity as the voltage oscillates.

 

[AlanC]

No, it’s read the same way, but when it’s being read it’s not being exposed to light. The sequence is roughly this:
- Clear down sensel wells (from live view mode)
- Open shutter
- Close shutter
- Read sensel wells
- Re-enable live view

So in electronic shutter mode the shutter speed would correspond to the time between the sensel well being cleared and being read?

Does clearing the sensel wells happen at the same speed as the sensor readout, or is it faster?

It's something that's been puzzling me for a while: the Sony A7Riv and A7Rv take 0.1 seconds to read the sensor. If you're using electronic first curtain, and it takes the same sort of time to clear the sensor as it does to read it then, assuming the mechanical shutter closes in around 1/250 second, wouldn't there be a significant difference in the exposure across the sensor? The top would have been cleared 0.1 seconds before the bottom.

On the other hand, if all the sensel wells can be cleared simultaneously, wouldn't a similar problem arise for the electronic shutter? The bottom of the sensor would be read out 0.1 seconds after the top and so would receive that much longer of an exposure?

 

[Michael_too]

The process to read every pixel, convert it to a 14 bit number and sending it out over a data bus to memory is quite slow. I believe the S5 needs about 50ms to do an entire frame. As it can only do one horizontal line at once these lines are scanned one after another. This takes so much time that when something is changing in frame because of movements or changes in light intensity, one frame captures a distorted image or uneven brightness. The mechanical shutter moves much faster across the image plane, avoiding these artifacts. While the shutter is closed, the sensor can continue chugging through its rows of pixels without these pixels being affected by changes in the scene.
The readout speed for video can be increased by either reducing the bit depth or skipping lines or a combination of both. I suspect they can also to some extent speed up the sampling process, leading to a sloppier sampling as electronics need a bit of time to stabilize as electrons are being shuffled around.

 

[Michael_too]

Does clearing the sensel wells happen at the same speed as the sensor readout, or is it faster?

In electronic shutter mode, clearing must be done at the same speed and in the same sequence as the read-out to ensure all pixels get the same exosure. This is easy to do as both are controlled by the digital clock.
In EFC mode, the manufacturer must know exactly at what speed the shutter is moving at each height of the image. I.e. the closing shutter must first get up to speed, then travels for some time across the frame and in the end possibly slows down to dampen vibrations. The clearing must mimic this pattern. Due to mechanical variances and wear and tear, a certain error margin is required. Therefore you can't have very fast shutter speeds in EFC modes as that would require the clearing to super accurately precede the dropping of the shutter.
In full mechanical the clearing doesn't matter much, just ensure each row gets cleared after the shutter has covered that row.

 

[AlanC]

In electronic shutter mode, clearing must be done at the same speed and in the same sequence as the read-out to ensure all pixels get the same exosure. This is easy to do as both are controlled by the digital clock.
In EFC mode, the manufacturer must know exactly at what speed the shutter is moving at each height of the image. I.e. the closing shutter must first get up to speed, then travels for some time across the frame and in the end possibly slows down to dampen vibrations. The clearing must mimic this pattern. Due to mechanical variances and wear and tear, a certain error margin is required. Therefore you can't have very fast shutter speeds in EFC modes as that would require the clearing to super accurately precede the dropping of the shutter.
In full mechanical the clearing doesn't matter much, just ensure each row gets cleared after the shutter has covered that row.

Thank you, Michael.

So the slow bit during the read out of the image is the process of putting the values through the ADC and storing the results; actually scanning the cells of the sensor can be done at a speed to match the shutter.

 

[John Vickers]

Another use case where mechanical shutter is needed is in AC artificial light - the 50/60Hz frequency means you’ll get banding as the light source slightly increases and decreases in intensity as the voltage oscillates.

This is seriously mistaken.

Have a think about how it's possible to shoot 29.97P video without flicker or banding under lighting which has 50Hz AC-mains related flicker - on any modern camera.

As it happens, due to some undocumented Panasonic S-series features, electronic shutter is the best choice when 50Hz or 60Hz AC-mains related flicker or banding is more of a problem than rolling shutter would be.

If folk here want correct explanations of how to avoid flicker, banding, or inconsistent exposures under lighting with 50Hz or 60Hz AC-mains related flicker, post a question on the
"Beginners Questions" forum at DPRevived: https://dprevived.com/c/beginners-questions/46/

Such questions have previously been discussed on DPReview.com .

I'll post about the finer points of Mechanical vs. EFCS vs. Electronic shutter on S-series at DPRevived in a few days. There's a subtlety about EFCS exposure times that I want to run past other technically-minded folk first.

I'm disinclined to post explanations here. If I posted an answer, it would not be subject to review, criticism, addition, by other technically minded folk.

 

[John Vickers]

In electronic shutter mode, clearing must be done at the same speed and in the same sequence as the read-out to ensure all pixels get the same exposure. This is easy to do as both are controlled by the digital clock.
In EFC mode, the manufacturer must know exactly at what speed the shutter is moving at each height of the image. I.e. the closing shutter must first get up to speed, then travels for some time across the frame and in the end possibly slows down to dampen vibrations. The clearing must mimic this pattern. Due to mechanical variances and wear and tear, a certain error margin is required. Therefore you can't have very fast shutter speeds in EFC modes as that would require the clearing to super accurately precede the dropping of the shutter.
In full mechanical the clearing doesn't matter much, just ensure each row gets cleared after the shutter has covered that row.

That's mostly OK as far as it goes. But, with EFCS, exposing every line on the sensor for a nearly equal time is more complicated than that. As exposure time decreases, it becomes increasingly important for the EFCS controller to know about the lens - in particular, the location of the exit pupil.

The EFCS "speed limit" is set by bokeh truncation. It's lens and camera dependent. Tolerances in the mechanical shutter aren't the problem.

 

[John Vickers]

For photography I stick with the mechanical shutter... but that might just be habit!

Just set shutter type to AUTO. There's no penalty.

 

[pdk42]

This is seriously mistaken.

Have a think about how it's possible to shoot 29.97P video without flicker or banding under lighting which has 50Hz AC-mains related flicker - on any modern camera.

As it happens, due to some undocumented Panasonic S-series features, electronic shutter is the best choice when 50Hz or 60Hz AC-mains related flicker or banding is more of a problem than rolling shutter would be.

Maybe you're talking about video, of which I have almost no knowledge, but what I can say is that in stills photography I see banding when using the electronic shutter, but not with the mechanical shutter. Can you explain therefore why I'm mistaken (seriously or otherwise)?

 

[John Vickers]

Maybe you're talking about video, of which I have almost no knowledge, but what I can say is that in stills photography I see banding when using the electronic shutter, but not with the mechanical shutter. Can you explain therefore why I'm mistaken (seriously or otherwise)?

Paul -

Had you been paying more attention on DPReview.com, you would understand the issues.

As I said above, I am not prepared to explain the issues to you here.
If you wish detailed explanations, ask at DPRevived.com. At DPRevived.com, there are experts who understand this material, who will not hesitate to correct me (or yourself) if I or you talk nonsense.

You are not a "details" person. You should avoid attempting to explain technical matters, and certainly in the absence of peer review.

If your comment above about banding refers to mains-frequency-related lighting flicker, it is incorrect and misleading, when referring to S1 or S5. The use of electronic shutter on S1 or S5 can reliably eliminate mains-related lighting problems under most realistic conditions, which mechanical shutter cannot. This is not because of some special property of electronic shutter, it is because of undocumented Panasonic design choices.

 

[pdk42]

Paul -

Had you been paying more attention on DPReview.com, you would understand the issues.

As I said above, I am not prepared to explain the issues to you here.
If you wish detailed explanations, ask at DPRevived.com. At DPRevived.com, there are experts who understand this material, who will not hesitate to correct me (or yourself) if I or you talk nonsense.

You are not a "details" person. You should avoid attempting to explain technical matters, and certainly in the absence of peer review.

If your comment above about banding refers to mains-frequency-related lighting flicker, it is incorrect and misleading, when referring to S1 or S5. The use of electronic shutter on S1 or S5 can reliably eliminate mains-related lighting problems under most realistic conditions, which mechanical shutter cannot. This is not because of some special property of electronic shutter, it is because of undocumented Panasonic design choices.

John, this is a very unhelpful answer that contains a number of personal slights directed at me that I feel are unnecessary.

Putting that aside though, I really see no reason why you can’t explain your point here (at least in summary) rather than re-directing me to some sub forum on another site.

 

[pdk42]

If your comment above about banding refers to mains-frequency-related lighting flicker, it is incorrect and misleading, when referring to S1 or S5. The use of electronic shutter on S1 or S5 can reliably eliminate mains-related lighting problems under most realistic conditions, which mechanical shutter cannot. This is not because of some special property of electronic shutter, it is because of undocumented Panasonic design choices.

And perhaps you might like to explain why there's banding on one of these shots but not the other? (hint - one is with electronic shutter, the other mechanical). The illumination is LED lighting at 50Hz. Both with the S5, 1/400s at f2.

Electronic shutter

• Panasonic - DC-S5
• 35mm F2 DG DN | Contemporary 020
• 35.0 mm
• ƒ/2
• 1/400 sec
• Pattern
• Auto exposure
• ISO 1600

Mechanical shutter

• Panasonic - DC-S5
• 35mm F2 DG DN | Contemporary 020
• 35.0 mm
• ƒ/2
• 1/400 sec
• Pattern
• Auto exposure
• ISO 1600

 

[Pete_W]

If you wish detailed explanations, ask at DPRevived.com. At DPRevived.com, there are experts who understand this material, who will not hesitate to correct me (or yourself) if I or you talk nonsense.

Hi John,

To be fair, the so-called experts at DPR who now inhabit DPRevived argue amongst themselves so much that it's difficult for the rest of us to determine the truth. Witness the 600+ message threads at DPRevived arguing about exposure. It would be a brave person to follow all of that and come away with any sense that they'd learned something.

 

[CharlesH]

Had you been paying more attention on DPReview.com, you would understand the issues.

As I said above, I am not prepared to explain the issues to you here.
If you wish detailed explanations, ask at DPRevived.com. At DPRevived.com, there are experts who understand this material, who will not hesitate to correct me (or yourself) if I or you talk nonsense.

John, please feel free to post the explanation here. I will understand what you post and would not hesitate politely correct it if appropriate. For reference I am a physicist with substantial optical system experience and holder of the Royal Photographic Society Progress Medal. Chuck

 

[Michael_too]

And perhaps you might like to explain why there's banding on one of these shots but not the other?

This is actually how I figured out that the rolling shutter is 50ms, by counting the bands and knowing that the light on a 50Hz grid flickers at 100Hz.

Anyway, the only way to avoid flicker using Electronic shutter is to capture the image at a shutter speed which is a multiple of 1/100 (or 1/120 if the powergrid runs at 60Hz), so 2/100, 3/100, 4/100 are all fine as you are always capturing a full cycle of the light.

The undocumented special sauce John Vickers is referring to is probably flash band compensation: https://eww.pavc.panasonic.co.jp/hdw/oi/AG-X2/html/AG-X2-X20_DVQP2773_eng/0099.html. It is a feature usually only implemented on camcorders and eliminates the uneven exposure you may get when flash photographers are shooting away in your presence or other abrupt bright flashes of light. This has nothing to do with the regular light cycle of AC powered lights. From the side effects you can deduce it simply repeats partial frames, taking the darker part of two different frames and stitching them together. Only applies to video capture though.

Note that on Panasonic cameras there is a feature which enables you to set up the camera in such a way that when you shoot video while the camera is set to a photo mode, it will automatically override your set shutter speed and switch to a multiple of 1/100 or 1/120, depending on what system frquency you have set (PAL or NTSC). Again, this only applies to video recording.