Why is there a mechanical shutter in DLSRs?

denisv · 02-19-2009, 01:03 PM

I always thought it had to do with the sensor heating up or something, but with the release of the 5DII and D90 that shoot video by keeping the shutter open and sampling, I've come up with the question why do DLSRs still have mechanical shutters?

Why not just flip up the mirror and sample the sensor electronically on exposure? It'd allow lighter, smaller, quieter, more reliable cameras and faster "shutter speeds" (1/250000 anyone?).

Ben_Edict · 02-19-2009, 01:13 PM

There were only very few DSLRs with an electronic shutter (Nikon D70, I think), simply because the mechanical shutter has some real-world advantages. An electronic shutter needs loads of space on the sensor.*The old interline transfer chips are the prototypical example, where each pixel row had a secondary transfer row, which transports the electrical charge out of the pixel to the electronics. This read-out process can be used as the shutter. But nevertheless, even if the "shutter is closed", i.e. during read out, light falls onto the sensor (without the mechanical shutter) and this can lead to blooming,colour and brightness shifts.

A mechnical shutter completely blocks light during the read-out phase of the sensor and thus prevents any detrimental effects by strong light on the already captured image.

Ben

asdf · 02-19-2009, 01:13 PM

Originally Posted by denisv

I always thought it had to do with the sensor heating up or something, but with the release of the 5DII and D90 that shoot video by keeping the shutter open and sampling, I've come up with the question why do DLSRs still have mechanical shutters?

Why not just flip up the mirror and sample the sensor electronically on exposure? It'd allow lighter, smaller, quieter, more reliable cameras and faster "shutter speeds" (1/250000 anyone?).

D90 has a 5 min limit for each video clip, because of the sensor heating up.

*Blue* · 02-19-2009, 01:16 PM

And all this time I thought my K2 and SuperProgram had electronic shutters.

Edit: My k200d and K20d also for that matter.

**Peter Zack** · 02-19-2009, 01:35 PM

Ben E's explaination is correct. Just to add to that interline transfer chips (as used in P&S cameras) uses part of each pixel to hold the charge to turn the pixel on and off. So not all of the pixel is light sensitive. This reduces the effective ISO that the chip can operate at and adds increased noise. The chips in DSLR's do not need these extra electronics and thus have a greater light sensitive surface area, lower ISO ability and lower noise floor.

*Blue* · 02-19-2009, 01:37 PM

Originally Posted by Peter Zack

Ben E's explaination is correct. Just to add to that interline transfer chips (as used in P&S cameras) uses part of each pixel to hold the charge to turn the pixel on and off. So not all of the pixel is light sensitive. This reduces the effective ISO that the chip can operate at and adds increased noise. The chips in DSLR's do not need these extra electronics and thus have a greater light sensitive surface area, lower ISO ability and lower noise floor.

Why are some pentax bodies designated in the specs to have electronic vertical run shutters such as the k2, SuperProgram, K200d and K20d?

Edit: The K1000 and KX has mechanical horizontal run shutters for example.

Edit: Edit: Then there's the LX with the hybrid electronic-mechanical horizontal run shutter . . .

**Peter Zack** · 02-19-2009, 01:48 PM

My understanding is the vertical electronic shutters have a shorter travel (the aspect ratio is wider than it is tall) and thus the shutter had a shorter 'travel' and can allow a higher sync speed. Horizontal mechanical shutters are slower with lower sync speeds and lower overall shutter speeds as well. IE a K1000 went to 1/1000th and a K200 or K20 can get 1/4000. PZ-1p could get 1/8000.

denisv · 02-19-2009, 02:20 PM

Originally Posted by Ben_Edict

But nevertheless, even if the "shutter is closed", i.e. during read out, light falls onto the sensor (without the mechanical shutter) and this can lead to blooming,colour and brightness shifts.

Is there somewhere I can read more about this?

Originally Posted by asdf

D90 has a 5 min limit for each video clip, because of the sensor heating up.

The 5DII records up to 24 minutes. I assume that's a arbitrary limitation since if it doesn't overheat in 24 minutes, it probably wouldn't in ten hours either. Besides, with photos, the sensor would be protected by the mirror when not in use, the only difference being that the mirror isn't precisely timed.

Originally Posted by Peter Zack

Ben E's explaination is correct. Just to add to that interline transfer chips (as used in P&S cameras) uses part of each pixel to hold the charge to turn the pixel on and off. So not all of the pixel is light sensitive. This reduces the effective ISO that the chip can operate at and adds increased noise. The chips in DSLR's do not need these extra electronics and thus have a greater light sensitive surface area, lower ISO ability and lower noise floor.

Don't DSLRs with video capture already have this? What about the K20D with 21fps burst mode? Or, uhm, any DSLR with live-view?

**Peter Zack** · 02-19-2009, 02:32 PM

Not being an engineer, I can't answer that but I assume that each pixel is somehow switchable to hold the charge needed to switch on and off. I'm making that assumption because the resolution drops to 1.6 Mp from the 14.3 Mp in the normal mode. So there is a big drop in the effeciency of the sensor.

Live view is different and the camera is just taking a 'feed' off the sensor to display the image. Any video frame rate off the chip needed for the LCD refresh could be done after the image leaves the sensor in a video buffer.

rfortson · 02-19-2009, 02:38 PM

I've wondered this as well. Why a shutter at all? Just capture the signal for the required interval and then stop capture. I guess it goes back to sensor heating, though I think you could work around that.

Ben_Edict · 02-19-2009, 02:43 PM

Originally Posted by denisv

Is there somewhere I can read more about this?
The 5DII records up to 24 minutes. I assume that's a arbitrary limitation since if it doesn't overheat in 24 minutes, it probably wouldn't in ten hours either. Besides, with photos, the sensor would be protected by the mirror when not in use, the only difference being that the mirror isn't precisely timed.

Don't DSLRs with video capture already have this? What about the K20D with 21fps burst mode? Or, uhm, any DSLR with live-view?

The noise increases drastically with an all electronic (on chip) shutter.*This is not really important for moving images, as the noise is statistically distributed and also the resolution is low, compared to still images. The burst mode of the K20 is very low res - and thus serves as a good example of the compromises, that need to be afforded.

Live-view: some cameras, the K20 is an example, don't expose during live-vioew, but use the shutter mechanism instead! YOu will see, when using the K20 in live-view mode, that for the exposure the mirror first goes down, the shutter releases and then the mirror goes up again, to make way for the sensor to record the live-view again.

I do not say, that using an on-chip shutter, is not possible.*It obviously is. But it affords quality compromises, which many DSLR users simply wouldn't want to make.

Using the mirror instead of an additional electronic shutter is simply a question of mechanics. The mirror has a really long travel and very high acceleration, when it starts and stops its movement - and then reverses its action. If you want to use it as a shutter, the acceleration simply gets to high. That would either mean a completely oversized mirror mechanics and also very heavy mirror slap or the mirror will break apart...

The mirror-as-a-shutter question is, by the way, not a purely DSLR-related question, the same question could arise with film SLRs.

Ben

Ben_Edict · 02-19-2009, 02:46 PM

Originally Posted by Blue

Why are some pentax bodies designated in the specs to have electronic vertical run shutters such as the k2, SuperProgram, K200d and K20d?

Edit: The K1000 and KX has mechanical horizontal run shutters for example.

Edit: Edit: Then there's the LX with the hybrid electronic-mechanical horizontal run shutter . . .

We must be careful not to confuse the mechanical shutter in form of a moving curtain or metall assembly, which might be CONTROLED electronically, with an electronic shutter on-board a sensor. The latter has no moving mechanical parts, no curtain or whatever, but is simply defined by the sampling time, allowed for the photo sites on the chip.

Ben

Ben_Edict · 02-19-2009, 02:47 PM

Originally Posted by rfortson

I've wondered this as well. Why a shutter at all? Just capture the signal for the required interval and then stop capture. I guess it goes back to sensor heating, though I think you could work around that.

Please read my post above, there are some more reasons.

Ben

*Blue* · 02-19-2009, 02:48 PM

Originally Posted by Ben_Edict

We must be careful not to confuse the mechanical shutter in form of a moving curtain or metall assembly, which might be CONTROLED electronically, with an electronic shutter on-board a sensor. The latter has no moving mechanical parts, no curtain or whatever, but is simply defined by the sampling time, allowed for the photo sites on the chip.

Ben

Thanks Ben. This is why a brought it up.

**Peter Zack** · 02-19-2009, 02:49 PM

Originally Posted by rfortson

I've wondered this as well. Why a shutter at all? Just capture the signal for the required interval and then stop capture. I guess it goes back to sensor heating, though I think you could work around that.

Guess you missed Ben's explanation?