Why do digital cameras use shutters?

[alextor]

I hope I am posting this in the correct section.

My question is: why do dSLR cameras use mechanical shutters?

Wouldn't it be easier and cheaper to have a sensor that can turn itself off to finish the exposure, instead of using a shutter? If my knowledge is correct, video cameras don't use shutters, so why must dSLRs?

It seems like the shutter is one of the more problematic elements in dSLRs since they break quickly. Why have them at all?

Thanks,
Alex

[alextor]

Actually, I just searched the forum to find that a similar thread was posted before. Sean replied that this has a lot to do with processing time of the sensor, and that the sensor must not be receiving light when it is being processed, or something along those lines.

In that case, is the main difference between high-speed video cameras and dSLRs the processing power of the unit?

[Colin Southern]

In that case, is the main difference between high-speed video cameras and dSLRs the processing power of the unit?

Not really - more to do with the fact that high-speed video cameras have far fewer pixels to process.

For a bit of an analogy, consider the case of a meter-reader who has to drive around and read typically 8,000,000 meters - AND - he must finish the last one before the first one changes. Pretty big job. Now - if all meters stopped at the same time (and didn't start again until you'd finished) it would give you a LOT more time to get around them all.

Not a great analogy, but best I can do this time of night!

By the way, did you know that the original Canon 1D had a Panasonic CCD sensor with an "electronic shutter"?

[alextor]

I didn't know that about the 1D. I suspect "electronic shutter" refers to the sensor having the ability of 'turning off' in place of a shutter.

If this is the case, I must admit that I'm still confused about why cameras need shutters... the electronic method seems like a simpler solution than physical shutters.

I am also curious about the Live View on my xsi's sensor... does that also use a type of electronic shutter?


p.s. this thread is quite similar to the previous thread... I will take no offense if the moderators feel they should be merged.

[dendrophile]

a sensor that can turn itself off

Hey Alex,

I don't have an answer for you, and this is not a subject I have thought much about before. I guess I'd challenge with what do you mean by "turn off"? I know you mean stop being sensitive to light. My limited understanding tells me that the pixels or photosites on a CCD either charge up, or decrease their charge - I forget which - when photons strike them. There must be a way to either dump the charge on the photosites, or charge them up to full, prior to the exposure being taken. I imagine to "turn them off" you'd need to build into each photosite some electronics to stop the charge leaking away when photons hit them, and that adds complexity to probably the most complicated part of the camera from a manufacturing point of view. Sensors have large amounts of photosites and each of them have to work perfectly (well, guess the occasional hot or cold pixel is acceptable.) Complicating the sensor will reduce the manufacturing yield and push up the price. Given that mechanical shutters are well understood, and conceivably could be replaced if they wear out or break, U suspect that sticking with mechanical shutters is more cost effective.

Just my random musings based on a previous career designing integrated circuits (ASICs for graphics acceleration to be precise.)

HTH,
Graham

[Colin Southern]

If this is the case, I must admit that I'm still confused about why cameras need shutters... the electronic method seems like a simpler solution than physical shutters.

Don't forget though that the sensor on the 1D was a CCD type sensor - Canon have long since moved to CMOS sensors.

You might also like to keep in mind that shutters are no big deal - most are rated to around 100,000 to 300,000 actuation, and are relatively inexpensive to have replaced. I doubt that many photographers would ever have to replace one, and as others have mentioned, it also protects the sensor from dust & dirt and bright lights. Keep in mind also that the opening and closing of the shutter is only one of the things going on - even with an electronic shutter the mirror would still have to be moved out the way when the shot was taken.

[Raycer]

but with electronic shutter, the camera can provide faster x-sync speed. I always wonder why Nikon dump the electronic shutter after their 6 mp sensor and went with mechanical shutter only.

[SteveF]

Ok, for those interested here goes ... a CCD chip is either full-frame, frame transfer or interline.

Full frame means all the chip is live and needs a shutter as we all know and hear. The shutter is the same as we have grown to love to hear in film cameras, and yes has a finite life which maybe is far more than has been stated previously.

Frame transfer is a method whereby half the silicon wafer (the chip) is covered by an aluminium strip which takes the transfer from the uncovered pixels whilst the covered (or vice-versa) pixels take in more info. The covered pixels process the info as the uncovered recieve the info, etc, etc.

Interline processing is an advance on Frame Transfer in that every other pixel is covered and as such is the video mans favorite tool. Frame rate is much higher and video quality is much better due to low smear rates ( a video users terminology - he's my neighbour).

CMOS, Complimentry Metal Oxide Semiconductors have a slightly different but much better future using a lower power level which is why my 1D uses 5 batts a day against my 1d2, N and Ds using 1 batt per day.

CCD has a better light gathering capacity but CMOS is the new kid on the block and is making serious waves in the quality dept.

Getting back to the 1D, mine has a vertical plane shutter electronically controlled and has only 30K on the clock and gives me perfect images everytime.

[Colin Southern]

Good stuff - thanks Steve.

Perhaps a good translation into everyday words might be "If they could, they would" - but they haven't ... so I'm guessing it's because it's not cost-effective.

[alextor]

Thanks all,

your comments have been enlightening, since I knew very little coming into this topic.

Steve talked about the types of CCD sensor and how it may apply a type of "shutter". But it seemed that all of them somehow blocked light from entering certain pixels in order to read the voltage values.

My final question is, how does my live view work? My xsi has a mechanical shutter but yet it is still able to show 'video' via live view without using its mechanical shutter. Does the live view have its own method of blocking light from entering the sensor in order to read the voltage values? If so, isn't the mechanical shutter superfluous? Is there some fundamental difference between the xsi's CMOS and the CCD that Steve explained that I'm not understanding?

[SteveF]

Good stuff - thanks Steve.

Perhaps a good translation into everyday words might be "If they could, they would" - but they haven't ... so I'm guessing it's because it's not cost-effective.

I think you've hit the nail on the head there, regarding cost, Colin.

[SteveF]

Thanks all,

your comments have been enlightening, since I knew very little coming into this topic.

Steve talked about the types of CCD sensor and how it may apply a type of "shutter". But it seemed that all of them somehow blocked light from entering certain pixels in order to read the voltage values.

My final question is, how does my live view work? My xsi has a mechanical shutter but yet it is still able to show 'video' via live view without using its mechanical shutter. Does the live view have its own method of blocking light from entering the sensor in order to read the voltage values? If so, isn't the mechanical shutter superfluous? Is there some fundamental difference between the xsi's CMOS and the CCD that Steve explained that I'm not understanding?

I've not had any interest in Live View before my Daughter got her 40D (which I borrow because it's so quiet). It seems live view or rather live preview is using the CMOS sensor. With the lens off I tested live view looking into the body and watched the mirror rise and the sensor exposed. Then triggering the shutter the blades moved in front of the sensor and the image captured. I initially thought there would be another, low res, sensor fitted which would be employed for live view because I can still hear the shutter as I do without live view on.

So it would seem that the CMOS sensor has the ability to switch off some of it's micro-lenses otherwise the sensor would get hot as it does on long exposures. On the other hand, CMOS takes very little power compared to CCD chips as such would possibly not get anywhere near as hot as a CCD chip.

Some manufacturers have employed a separate chip to provide live preview in the past such as Olympus and/or have split the incoming light 80/20% - the 20% for the preview in bridge cameras.

Not really sure of the answer with the Canons and not having had an interest in the Live View function I haven't studied it in detail.

Live View, of course, on the 40D is not the same as the video capability on the 5DII. But that's another case to solve for someone with better qualifications than me! Canon isn't about to release their secrets either.

[Colin Southern]

So it would seem that the CMOS sensor has the ability to switch off some of it's micro-lenses otherwise the sensor would get hot as it does on long exposures. On the other hand, CMOS takes very little power compared to CCD chips as such would possibly not get anywhere near as hot as a CCD chip.

Canon mention in the manual for the 1Ds3 that the sensor may get quite hot if LiveView is used for extended periods - they also mention that it may cause higher noise levels, but other than that it's harmless. Off memory, I think it also has a protection mechanism that shuts it off to protect other internal components if things start to get too warm. Can't say I've ever noticed any issues with mine - I use it for composition quite regularly (the old eye sight isn't what it was - and LiveView works better with my reading glasses!).

Not really sure of the answer with the Canons and not having had an interest in the Live View function I haven't studied it in detail.

Live View, of course, on the 40D is not the same as the video capability on the 5DII. But that's another case to solve for someone with better qualifications than me! Canon isn't about to release their secrets either.

Canon did go so far as to say that adding video capability "wasn't a big deal", so I'm guessing mostly just firmware over and above Liveview.