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Author Topic: Why a shutter?  (Read 4710 times)

PeteC

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Why a shutter?
« on: January 25, 2007, 09:57:11 pm »

Silly question?

Why do digital SLRs need a mechanical shutter?

Why can't the moment and duration of exposure be controlled purely electronically by gating the sensor, A/D, etc?

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Morgan_Moore

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Why a shutter?
« Reply #1 on: January 26, 2007, 01:10:57 am »

Quote
Silly question?

Why do digital SLRs need a mechanical shutter?

Why can't the moment and duration of exposure be controlled purely electronically by gating the sensor, A/D, etc?

Pete   
[a href=\"index.php?act=findpost&pid=97579\"][{POST_SNAPBACK}][/a]

The current generation of chips seem to need a black reference shot which is used to cancel out noise some how

Therefore a black image is needed and therefore at least a darkened chip at the end of the exposure is required

Ther could I imagine also be an issue with burning the chip say with a 600 pointed at the sun for a while

I agree that reliance on a mechanical device such as this seems stupid as it pushes up the cost of the devices

Shutterless cameras would surely be lighter cheaper etc

I have done experiments with using my eyelike back and no shutter - it doesnt work !

I think this is fundamentally a failure of the manufactureers to 'think out of the box' and realise the advantages of a combination of no shutter and live data coming from the back

I dont know how live view compacts handle this - but compact cameras seem to be way ahead of DSLRs - just look at 'face regognition auto focus' for example

WHy do you ask??

SMM
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Sam Morgan Moore Bristol UK

BJL

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Why a shutter?
« Reply #2 on: January 26, 2007, 05:34:43 pm »

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Why do digital SLRs need a mechanical shutter?
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Some of the sensors used in SLR's can do electronic shuttering, some cannot. The sensors I know of that can are all interline CCD's, with one sign being flash X-sync at up to 1/500s, faster than any mechanical shutter can offer. Cameras with that include the Nikon D1, D1H, D1X, D70, D70s, D50, D40, and the original Canon 1D (which had a 4MP Matsushita/Panasonic CCD). And I believe that most or all compact digital cameras use electronic shuttering. (Strangely, the Nikon D40 has the highest flash sync speed of any current SLR, except a few other lower end Nikon models about to be discontinued!)

But none of the newer SLR sensors of over 6MP seem to offer this, and none of the CMOS sensors, and the limitation to 1/500s X-sync suggests that electronic shuttering cannot handle the maximum shutter speeds desired on SLR's (flash at higher shutter speeds than 1/500s is desirable and is offered by other methods, but those methods are inferior in that they reduce available flash illumination).

So it seems that for now, higher shutter speeds and CMOS sensor require mechanical shutters.


Addendum
I retract the bit about a 1/500s shutter speed limit: I have just read that it is possible to trick the D70 into taking flash photos at 1/8000s. My guess is that 1/500s is just to ensure that the flash delivers most or all of its output within the right time interval.
Electronic shutters do apparently have some image quality problems, like "vertical smear" and a greater risk of blooming.
« Last Edit: January 31, 2007, 11:50:39 pm by BJL »
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PeteC

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Why a shutter?
« Reply #3 on: January 26, 2007, 09:34:32 pm »

Why do I ask? Just because I've been wondering for a fair while why a complex, fragile, mechanical device with limited performance (the shutter) is still required when just about every other mechanism in cameras has been replaced by an electronic solution. And because it seems to me that electronically controlling the "shutter" could give big advantages, e.g. reduced cost, silent operation, extreme accuracy, very high burst rates and potential special effects. Plus the possibilty of live view via a semi-silvered mirror, as in the old EOS RT, where the mirror doesn't have to move. These would seem to be huge benefits, so what's stopping it?

As you say, it's done in fixed lens digital cameras, so why not SLRs?

Is it to do with protecting the sensor from dust while changing lenses? Surely a simple cover which flipped out of the way with the mirror would do the job?

Protection from the sun? Well, point and shoots are exposed, as are video camcorders.

As for lack of imagination on the part of camera makers, that can't be it, surely. Olympus, especially, are well known for thinking sideways, and Minolta had some great innovations.

OTOH, if any manufacturer wants to contact me to discuss royalties on my idea, feel free, gentlemen.  

No, there must be a more fundamental reason, I assume, or someone would have done it. Wouldn't they?

Pete
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Jonathan Wienke

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Why a shutter?
« Reply #4 on: January 27, 2007, 03:11:39 am »

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I think this is fundamentally a failure of the manufactureers to 'think out of the box' and realise the advantages of a combination of no shutter and live data coming from the back

No, it's a deliberate tradeoff on the part of the manufacturers to eliminate the extra sensor circuitry needed for electronic shuttering and live video to free up sensor chip real estate so that the actual photodiodes can be as large as possible, giving them greater well capacity, more dynamic range, and less noise. It's part of the reason DSLRs have better per-pixel image quality than digicams.

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And because it seems to me that electronically controlling the "shutter" could give big advantages, e.g. reduced cost, silent operation, extreme accuracy, very high burst rates and potential special effects.

The shutter isn't the bottleneck limiting burst rate; motion picture cameras have mechanical shutters, and have no problems with constant frame rates well in excess of 24FPS. The bottleneck is getting the analog voltage levels at each pixel read off the chip and converted do digital values quickly. A Canon 1D-MkII has to read and digitize, and process ~72MB/second worth of image data (12MB per frame * 8 frames). Making the components capable of handling that kind of data flow fit into a camera body is a pretty good trick.
« Last Edit: January 27, 2007, 03:14:45 am by Jonathan Wienke »
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Morgan_Moore

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Why a shutter?
« Reply #5 on: January 27, 2007, 07:55:53 am »

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No, there must be a more fundamental reason, I assume, or someone would have done it. Wouldn't they?


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I think the fundamental reason is black reference files which are used to cancel out noise and other real world considerations like cotrolling all that data

I agree absolutley that shuterless cameras should be chaeper sturdier smaller etc

My initial interests was in using my eyelike back and nikkor 14mm 2.8 lense via a cheap bit in between

Incidentally the exposure length of the eyelike at least is controlled by the electronics - I am sure this is the norm

Imagine a mirrorless shutterless camera with a 39mp back a coule of rise and fall  non retrofocal lenses and an iphone size screen with live view and focus alert in the same manner as blown highlight alert - talk about the ultimate P+S...

SMM
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Sam Morgan Moore Bristol UK

jani

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Why a shutter?
« Reply #6 on: January 28, 2007, 09:13:54 am »

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The shutter isn't the bottleneck limiting burst rate; motion picture cameras have mechanical shutters, and have no problems with constant frame rates well in excess of 24FPS. The bottleneck is getting the analog voltage levels at each pixel read off the chip and converted do digital values quickly. A Canon 1D-MkII has to read and digitize, and process ~72MB/second worth of image data (12MB per frame * 8 frames). Making the components capable of handling that kind of data flow fit into a camera body is a pretty good trick.
Oh, no, that is "easy".

The tough part is for it to run cool and use little power. Modern memory circuitry and processors can move data far quicker, but also use far more power and therefore dissipate far more heat.
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Jonathan Wienke

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Why a shutter?
« Reply #7 on: January 28, 2007, 05:01:13 pm »

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Oh, no, that is "easy".

Not when you figure in the larger batteries to power them, the cooling fans, heat sinks, and ventilation, etc.
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Peter McLennan

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Why a shutter?
« Reply #8 on: January 28, 2007, 11:47:26 pm »

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motion picture cameras have mechanical shutters, and have no problems with constant frame rates well in excess of 24FPS.
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Most motion picture cameras have continuously-rotating mirror shutters that are far simpler than the complex array of electronically controlled cloth or metal curtains and flip-up mirrors in DSLRs.
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jani

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Why a shutter?
« Reply #9 on: January 28, 2007, 11:59:58 pm »

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Not when you figure in the larger batteries to power them, the cooling fans, heat sinks, and ventilation, etc.
You know, it is permissible to read the second part of what I posted, too.
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BJL

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Why a shutter?
« Reply #10 on: January 29, 2007, 01:11:56 pm »

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No, it's a deliberate tradeoff on the part of the manufacturers to eliminate the extra sensor circuitry needed for electronic shuttering and live video
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I have become skeptical about this idea that SLR sensors omit the circuitry needed for electronic shuttering and live video, because many SLR sensors seem to have some video out capability even if it is often not used in SLRs using those sensors. It seems that with CMOS and interline CCD (including Fuji SuperCCD), electronic shuttering and video out capability probably come "for free", and even so are often not used.

The CMOS sensor of the Canon 20D and 30D is capable of video output (and thus electronic shuttering), even though the capability was only ever used in the short-lived special purpose astro-photography camera, the 20Da.

Interline type CCD's as used in most Nikon and all Pentax and Konica-Minolta/Sony DSLR's are naturally capable of electronic shuttering, and the Sony 6MP interline CCD certainly has electronic shuttering, yet a number of cameras using the 6MP sensor with this capability do not use electronic shuttering at all, and even the ones that do use it do so only for the special case of 1/500s flash sync.

The newest Kodak sensor on 4/3" format (the KAI-10100) is capable of electronic shuttering, so let us see if Olympus uses that capability in any of this year's E system models. The E-400 apparently uses this sensor yet does not make use of electronic shuttering.

So it seems that many SLR sensors are capable of electronic shuttering, but other difficulties make it un-attractive in SLRs. Maybe the electronic shuttering is only suitable for the "rolling" shutter operation used for video? [Update: ignore the next sentence, I have learnt 1/8000s electronic shuttering is possible on the Nikon D70!] Maybe it is limited to about 1/500s shutter speed, so the mechanical shutter would be needed anyway?


P. S. Jonathan is clearly right that the shutter speed limit of high end DSLR's is mostly due to sensor read-out rate rather than the focal plane shutters. This is shown by the relatively low frame rates of many high end Canon and Nikon DSLR's compared to the 8fps and up of high end 35mm film SLRs from the same makers. The 1Ds MkII is limited to 4fps and the D2X to 5fps in its full resolution mode by read-out limitations, not shutters. The D2X has an 8fps mode, but only at 7MP, in order to to work within a read-out speed limit of about 60 MP/sec.
« Last Edit: January 31, 2007, 11:52:50 pm by BJL »
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