Why a shutter curtain?

[CanineCandidsByL]

I was answering a question about mirrorless systems on another thread and it reminded me of a question I have had for a long time.

Why do we still use shutter curtains?

You would think with digital cameras we just flip the mirror (or not in mirrorless systems), tell the sensor to start recording, and then tell the sensor to stop recording 1/30th of second later (or whatever your shutter speed is).
What advantage does the curtain still offer us? Is it a leftover from the film days with no real purpose?

Done this way, we could even change the way we do exposures. If you want HDR, you could hit the sensor equivilent of a lap timer at each apropriate time. So a single exposure captures your darker, "normal", and brighter exposures. No need to high shutter sync. You can use your flash at virtually any speed without problems and without the flash having to "strobe" using up valuable batteries and recycle time. All kinds of things get better.

So, why?

 

[iris chrome]

hmmm interesting question. If it's true then I guess that kinda makes the shutter the equivalent of the human appendix LOL. I hope someone here knows the answer to this one.

 

[gferdinandsen]

Perhaps to further protect the sensor from environmental particulate.

 

[AJ]

I think originally it was meant to prevent sensor blooming in bright light. I'm not sure if this is still an issue today.

Another advantage of a shutterless camera: flash sych speed = max "shutter" (acquisition) speed. i.e. 1/4000 or 1/8000

 

[-zero-]

hmmm interesting question. If it's true then I guess that kinda makes the shutter the equivalent of the human appendix LOL. I hope someone here knows the answer to this one.

dont know about the shutter curtains but the appendix still has a role in the immune system of children

 

[Loswr]

The short version while you can 'turn on' a sensor to start a capture, when you turn it off you have to read the photons out as electrons. That process takes a finite amount of time, and if photons are still hitting the sensor during that time, your exposure is not what you think it's going to be. So, an electronic first curtain is possible, but shutterless is technically challenging. In fact, the higher-end dSLRs with a silent shooting mode use an electronic first curtain. The time it takes to read out the sensor also impacts the max shutter speed - it's faster to shut off the light with a second curtain than read out and clear the photosites over the same area. P&S cameras have a shutter that is analogous to the second curtain of a dSLR shutter, so the sensor can read out in darkness - and that's with the faster readout time of a CCD sensor (which are read row by row, instead of pixel by pixel for CMOS). So, bottom line, if you like your fast shutter speeds like 1/4000 or 1/8000 s, you need at least one curtain.

 

[NormanBates]

my wild, uninformed guess is that, among other problems, you'd get jello, as we do in video mode (because the top of the sensor starts recording before the bottom, and then stops also earlier)

given that, you use the curtain to make the sensor record pitch black in the transition times, while just a part of the sensor is recording, then open it for image aquisition, and close it again before the top of the sensor stops gathering light

 

[gmrza]

The short version while you can 'turn on' a sensor to start a capture, when you turn it off you have to read the photons out as electrons. That process takes a finite amount of time, and if photons are still hitting the sensor during that time, your exposure is not what you think it's going to be. So, an electronic first curtain is possible, but shutterless is technically challenging. In fact, the higher-end dSLRs with a silent shooting mode use an electronic first curtain. The time it takes to read out the sensor also impacts the max shutter speed - it's faster to shut off the light with a second curtain than read out and clear the photosites over the same area. P&S cameras have a shutter that is analogous to the second curtain of a dSLR shutter, so the sensor can read out in darkness - and that's with the faster readout time of a CCD sensor (which are read row by row, instead of pixel by pixel for CMOS). So, bottom line, if you like your fast shutter speeds like 1/4000 or 1/8000 s, you need at least one curtain.

The 1D (mark I) did also have an electronic shutter, but that still required a physical shutter (2nd curtain) in order to leave the sensor in darkness for readout. the 1D did also have a CCD sensor, unlike the 1DmkII which had a CMOS sensor.

Another concern is that the electronics for an electronic shutter take up real estate on the sensor, using up space which could otherwise be used for photosites - i.e. higher noise.

I suspect we may eventually see fully electronic shutters on a CMOS sensor, but that is a long way off, and a lot of development work would be required.

 

[Mt Spokane Photography]

I'm pretty sure that they would use electronic 2nd curtain if it were possible. The shutter mechanism in a camera is complex and expensive, as well as having a much lower reliability than electronics.

Once they are able to control and address every pixel at the same time, it will happen. Thats not happening soon either.

 

[aldvan]

In fact, the higher-end dSLRs with a silent shooting mode use an electronic first curtain.

A slightly off topic question, sorry... Speaking about silent shooting on 1Ds cameras, very short exposition time apart, is silent shooting affecting in any mode IQ?

 

[CanineCandidsByL]

Hopefully this message won't prevent aldvan's question from being seen and responded to, but I wanted to thank everyone for their answers. It didn't matter if they were guessed or known, especially since everyone made it clear which type of answer they were offering. From the info I was able to find the relevent info on sensor tech and the way they work. As always, great stuff guys!

 

[Loswr]

In fact, the higher-end dSLRs with a silent shooting mode use an electronic first curtain.

A slightly off topic question, sorry... Speaking about silent shooting on 1Ds cameras, very short exposition time apart, is silent shooting affecting in any mode IQ?

No direct effect on IQ - mechanical vs. electronic first curtain (EFC) makes no difference from the standpoint of the sensor capturing the image. But, the EFC has an indirect effect - it eliminates the vibration resulting from the mechanical first curtain. Charles Krebs demonstrated the improvement in macro photography with EFC - a standard picture (mirror slap and mechanical first curtain) has the most vibration, LiveView with mechanical first curtain is better (no mirror slap, but two shutter movements before the image capture), mirror lock up is even better, and Live View with EFC (silent shooting mode) is best.

BTW, while it's a selectable C.Fn on higher end bodies (and the 1-series have more than one silent mode), on Rebel/xxxD bodies the EFC is actually on by default with Live View.

 

[awinphoto]

I'm not going to pretend I know the intimate makings of these modern DSLRs... but just some food for thought... Lets say hypothetically with the lens cap off, the mirror within the camera blocks (lets say for example) 95% of the light the lens sends through and 5 percent somehow wraps around/bounces/or otherwise somehow still seeps through behind the mirror in which the shutter blocks the remaining light from getting to the sensor itself while not in use. What chance do you suppose if they get rid of the shutter completely and go fully electronic, that the light that does get past the mirror would eventually weaken/degrade the sensor if not immediately but over time or even worse give a burn in effect like we see with computer monitors? So lets say the 1d cameras which have a shutter/sensor rating of 300k exposures, well by 150k the sensor starts to downgrade or become less sensitive, would it be worth it or am I over-thinking this? Even worse the mirrorless camera that has been talked about, what affect that would have with light constantly hitting the sensor?

 

[Loswr]

What chance do you suppose if they get rid of the shutter completely and go fully electronic, that the light that does get past the mirror would eventually weaken/degrade the sensor if not immediately but over time or even worse give a burn in effect like we see with computer monitors? ...Even worse the mirrorless camera that has been talked about, what affect that would have with light constantly hitting the sensor?

It's a known issue. In the mid-60's, Canon launched the Pellix which featured a fixed pellicle mirror instead of a moving reflex mirror. With that release, they also changed the material used for the shutter curtains from cloth to metal due the potential damage to the cloth from constant light exposure, especially through a fast lens (FWIW, almost all modern shutters are new either aluminum or titatium).

Perhaps a more significant issue would be the increased amount of dust that would likely collect on the sensor in a shutterless camera...

 

[bycostello]

the mirror is still there so we buy the mk3 or mk4 when they release the electronic shutter versions... (Nikon used to make one like this)... perhaps cynical but the manufactures need a range of new features to make us part with our cash for each new version...

 

[Kim]

I know from digital medium format that it is a necessity with a curtain. This is due to the sensor keeps reading and "flushing" the information while not exposing.

If the sensor did not have the curtain you would get corrupted images with a lot of strange colors when reading your images from the sensor.

 

[photosites]

Electronics shutters are used in almost all Point and Shoot cameras, including the Powershot G12 and S95. As we are all aware, the G12 can hit a shutter speed of 1/4000, so there should be no limitations in terms of the speed of sensor read out.

So why not dSLR?

This could answer your question:
http://www.steves-digicams.com/knowledge-center/why-digital-cameras-have-mechanical-shutters.html

"Cameras, typically smaller point-and-shoot cameras, that use no mechanical shutters typically use an interline transfer sensor. An interline transfer sensor dedicates a portion of each pixel to store the charge for that pixel...
....Interline transfer sensor's typically have higher noise levels and lower sensitivity than the full frame sensor's used in high end digital SLR's."


I am truly impressed with the amount of technology that goes into our point and shoot really... Despite all that pixel cramming into a 1/2.5" sensor, you've got to deal with interline transfer... blah blah"

 

[awinphoto]

the mirror is still there so we buy the mk3 or mk4 when they release the electronic shutter versions... (Nikon used to make one like this)... perhaps cynical but the manufactures need a range of new features to make us part with our cash for each new version...

Sooo.... you're saying if IF they release a DSLR with an electronic shutter vs a real one, you would be more inclined to buy it? Really? REALLY? How does that improve the IQ? How does that improve DR or noise performance or the AF? What benefit does an electronic shutter have over a real (and if you think that the price will be cheaper you are fooling yourself). If not anything, as I mentioned before and neuro acknowledged, the light hitting the sensor consistently could degrade the sensor, give burn in, and shorten the lifespan of the camera, let alone the nightmare of dust... P&S does not change lenses so this isn't an issue with them, but would be horrible with DSLR's. I see ABSOLUTELY no up side in an electronic shutter...

I know from digital medium format that it is a necessity with a curtain. This is due to the sensor keeps reading and "flushing" the information while not exposing.

If the sensor did not have the curtain you would get corrupted images with a lot of strange colors when reading your images from the sensor.

exactly... Plus medium format also utilized leaf shutters and 3 shot studio capture... You're not going to take these out in the field to get dust or anything else... Also... considering you can buy a decent car for the price of a leaf digital back... and they rely of lenses shutters for their capture or if they thought it would be more beneficial to do that, they would have put pressure on camera manufacturers to make new lenses... hence they have not...