Instead of megapixels-worth of light sensors, a
Instead of megapixels-worth of light sensors, a new experimental camera uses a series of mirrors to focus all the light on just one sensor. Somewhat related question that I’ve been wondering about for awhile: why do digital cameras need shutters? Why can’t you just turn the sensors on and off electronically? Seems like you could then use many more arbitrary “shutter” speeds, like 5 seconds or 1/50000 of a second.
Reader comments
VLOct 05, 2006 at 4:45PM
But what of that satisfying click-snap-and-whine? :)
These folks have pondered this question, and suggest space for additional circuitry and a limit on electronic shuttering (really!) as possible culprits. More here.
Brian DonovanOct 05, 2006 at 4:47PM
I'd guess that the shutters might be at least partly in place to protect lenses from scratching? Everyone I know, myself included, tends to drop digicams into bags, jacket pockets, etc. Without a shutter, there'd be increase risk of damage to the lens.
aleshOct 05, 2006 at 4:49PM
The D70, for example, does have an electronic shutter, and in fact it does allow very fast shutter speeds (up to 1/5000 i believe, which is probably faster then anything you'd need in practical situations), as well as a fast (1/500) flash sync speed. It's a valid question, though, why replacing the electronic shutter with a mechanical shutter on the D80 (and dropping the sync speed to 1/250) would save money.
The 'click' you hear is mirror swinging up, which is incidental to the shutter in any case.
MargaretOct 05, 2006 at 4:53PM
A somewhat naive guess is that the time it takes for the CCD's semiconductors (the kind in typical digital cameras at least) to load their initial charge is nontrivial... So you might be able to rig it to act like an electronic shutter, but the preparation time for each shot might be longer than you'd prefer. There are constant read-out arrays with no mechanical shutters that you can tell "start collecting" and "stop collecting" to get a particular exposure/integration time, but my impression is that it's more expensive in general, both the detector itself and the implementation.
(Or it's something completely different.)
kwcOct 05, 2006 at 4:54PM
I believe it is similar to issues behind hot pixels (pixels overly bright on long exposures): an SLR digital sensor has improved noise handling if it can be read/kept in darkness.
Jason WallOct 05, 2006 at 5:20PM
Most if not all digital cameras use an electronic shutter, i.e. turning the sensor on and off. You still need the physical shutter mechanism to provide aperature control.
Graham ParksOct 05, 2006 at 5:23PM
All but high-end D-SLRs do have electronic shutters. They work by resetting the pixels, waiting a while for charge to build up, then streaming the charge off line-by-line to the side of the sensor where it is read by a voltmeter and yadda yadda. This causes several problems, mainly related to there not being any way to block more light and therefore more charge hitting the sensor during the shifting process. This is why very cheap cameras have horizontal banding in bright light, because while each line is being shifted across, it picks up more charge.
You'll also note that the last lines to be read off will have been exposed slightly longer than the first. Similarly, "resetting" at the start of the exposure uses exactly the same process, they just read off whatever charge is there, which is much less sudden then a real shutter.
Finally, you can't turn a CCD "on and off". Charge builds up in the cells whether it's switched on or not for as long as it's exposed to light.
Charles DarkeOct 05, 2006 at 5:26PM
Some CCD based sensors do have electronic shutters. Essentially, these hold the reading at zero which can help since you allow time for the mechanical shutter to open. But most will have a shutter so that it stops light going to the CCD.
CMOS sensors (as used in some Canon dSLRs) do not have electronic shutters. As far as I'm aware there's no (practical) way to do this with a CMOS sensor
nexOct 05, 2006 at 5:40PM
cameras with purely electronic shutters are extremely popular. not that i really know about these things, but i suspect every cheap&tiny one, like those in mobile phones, lacks a mechanical shutter. what you can observe in images from them is that not all parts of the image have been recorded at the same time. rather, the pixels are read out sequentially, like in any other digicam, but without the shutter making sure that every pixel is exposed for the same time period. therefore, anything that moves is distorted; especially with cheap&tiny devices, which naturally don't use the fastest electronics. the mirror cam has the same issue, but it might operate substantially faster.
alesh, i don't think a mechanical shutter would save money, as it's precisely the cheapest cameras that have electronic shutters. one advantage of having a purely mechanical shutter is that you need less electronics in the sensor, and a larger percentage of the sensor area can actually sense light; this makes the CCD more sensitive. so for a DSLR, this makes perfect sense and is a genuine upgrade, not a cost savings measure. but on the downside, without an electronic shutter you can't have a live preview on an LCD, which makes purely mechanical shutters a showstopper for consumer gear.
AFAIK it's not uncommon for a camera to have both an electronic and a mechanical shutter, but as i said, i don't really know about these things, so i'm hoping someone who does will follow up with a more enlightning comment.
brian, you need to read up on what a shutter is, you're thinking of something else :-)
jkottkeOct 05, 2006 at 5:53PM
Huh, I thought the D70 had a mechanical shutter, but it's just the mirror flopping out of the way. Thanks for the information, folks.
Graham ParksOct 05, 2006 at 5:59PM
(to clarify: An electronic shutter works by resetting the image sensor and a few moments later reading off the charges. CCD and CMOS cameras work by resetting the image sensor and a few moments later reading off the charges. You simply can't have a digital camera without an electronic shutter. A mechanical shutter is purely a supplement to this, opening slightly later and closing slightly earlier)
nexOct 05, 2006 at 6:38PM
"CCD and CMOS cameras work by resetting the image sensor and a few moments later reading off the charges."
yes, but with a CMOS, this doesn't happen in a way that is suitable for shuttering, thus it can hardly be called an 'electronic shutter'.
i thought of something else that i failed to include in my first comment: some cameras have a noise reduction function that takes a second exposure with the (mechanical) shutter closed; this can identify noise and then remove it from the image. that way, 'hot pixels' can be corrected to a great degree. when i do this on my old coolpix, it takes very long until it's ready for the next exposure, but i guess modern DSLRs have better methods for using a mechanical shutter for effective noise management.
Graham ParksOct 05, 2006 at 7:26PM
yes, but with a CMOS, this doesn't happen in a way that is suitable for shuttering, thus it can hardly be called an 'electronic shutter'.
The $10 keyring camera next to me is definitely CMOS and definitely has no mechanical shutter. Are you talking about a specific kind of CMOS?
Joshua SchachterOct 05, 2006 at 7:49PM
More importantly, one could use different shutter speeds in different parts of the same image.
RyanOct 05, 2006 at 8:01PM
most digital cameras don't have shutters. They have whats called a "virtual shutter" software does almost exactly what jay is talking about. DSLRs however have "physical shutters" which are just like shutters in film cameras. A spring loaded plate that inturupts the light.
the virtual shutter is why your average digital camera has shutter lag (ie you push the button but the pic doesn't take for a second or so).
Some of you guys are talking about electronic/mechanical shutters. Both of these are physical shutters. one is run by springs and gears (mechanical) and one is triggered by electronic motors.
The reason DSLRs have pysical shutters is: no shutter lag, better quality photos, wider range of shutter speeds. On the whole its a lot better way to do things, but these issues don't actually matter to most users. People whould rather have cheaper smaller camera with a virtual shutter.
ChrisOct 05, 2006 at 9:53PM
Don't everybody forget that you need the mirror to refelct light onto the light meter so you can figure out your exposure (in both manual and auto modes). The light metering sensors etc. are in the prism/viewfinder so if you get rid of the mirror then you can't see what the CCD is seeing and you won't be able to meter the exposure.
Also, if they designed digital cameras in a way to optimize the technology, the camera itself would look so drastically different than what people are used to using and we all know how most people are with technology they're not familirat with. So for the time being, we have digital cameras that look like the film cameras we've been used to seeing for the last 40+ years. It's kind of like why they don't do a completely new redesign of a car, it will freak people out because they won't be ready for it, so they change the design incrementally over a few years. (and yes i'm a photographer.)
LennyOct 05, 2006 at 9:55PM
I just wanted to clarify a previous comment: the aperture is in the lens, not the shutter. Cameras without shutters can still have aperture control.
I actually wondered about digicam shutters, myself, so thanks for the answers.
nexOct 05, 2006 at 10:14PM
graham, i've essentially parrotted what charles had said, i.e. the conventional wisdom that you absolutely need a physical shutter with a full-frame sensor (one that uses 100% of the sensor area for capturing light, thus cannot contain the circuitry of, say, a frame-transfer or interline CCD) and other overgeneralisations. turns out i was completely wrong there, sorry.
semi-coherent rant on what threw me off the track: when photographers talk about full-frame sensors, they usually refer to the _size_ of the sensor. for such large sensors, using CCD technology is way expensive, therefore CMOS is more widely used, and they just happen to be used in DSLRs (which have mechanical shutters) pretty much exclusively. con-/prosumer point&shoots, OTOH, generally use CCDs and don't need mechanical shutters. the conclusion that there must be a logical connection because of some intrinsic quality of CCD/CMOS technology was purely my own idiocy, please ignore that everyone :-/ well, at least _i_ learned something new, if no one else ... and i found an interesting article on the subject.
so, there _are_ sensors that can't by themselves provide shuttering capability that's good enough for photographic imaging, but they can be CCD just as well as CMOS, and there _are_ CMOS sensors with virtual shutters.
ryan, it's a good idea to use the terms "virtual shutter" and "physical shutter" to avoid ambiguity, but i don't think "electronic shutter" generally means "electronically controlled physical shutter". for example, this (old) sony press release calls a virtual shutter an "electronic shutter", just like pretty much everyone here.
aleshOct 05, 2006 at 10:32PM
I direct your attention to this page, down to the D80/D70s comparison chart. The 7s gets the green light for "Combined mechanical and CCD electronic shutter".
As far as I can tell, Charles Darke's comment jibes with this very well: the electronic shutter keeps the CCD off while the mechanical shutter opens, effectively providing the "front wall," while the mechanical shutter clamps down at the percise moment, providing the "rear wall." Some crafty-assed precision engineering, that, especially when we're talking about a total time of 1/8000th of a second (not 1/5000, as I incorrectly said before).
What Ryan is saying makes perfect sense in light of that (ie dSLR's work very differently from digital P&S's.). On the other hand, Chris, I'm not sure cameras can be "optimized" any better then they are. Olympus has this "innovative" 4:3 system going, and god bless them for trying, but in practical terms they're lagging behind Nikon's cameras on the performance/price ratio, as far as I know.
nexOct 05, 2006 at 10:43PM
one of olympus' 4/3 cameras has live preview on a tilting LCD; even a mode in which you can have the through-the-lens image on the LCD _and_ in the viewfinder. i say, make the LCD tilt&twist in the next iteration, maybe give it more pixels and less latency too, and that's a killer feature. does nikon offer anything similar? (would be nice if they already had that now, because then i would probably be able to afford it in 3 years, and who would buy an olympus if he/she could get a nikon :-)
Jason WallOct 06, 2006 at 12:19AM
Lenny,
Thanks for the correction. I mistook the aperature iris for the shutter mechanism. I went and did some research to get a clearer picture and in doing so discovered that according to the Nikon product specifications, all of their pro DSLR models use a physical shutter, described as an electronically controlled vertical-travel focal-plane shutter. Canon appears to be the same for all their prosumer cameras and if i read the specs right, the 1D II and 1Ds II use a purely electronic shutter.
easOct 06, 2006 at 1:04AM
Fascinating thread. Takes me back to my usenet days.
I'll add my two bits to the pot of understanding.
1. Even if electronic shutters account for part of the shutter lag in (some) digital cameras, much more is the result of the autofocus system.
2. The aperatures on a lot of digital cameras are pretty simplistic. I think my PowerShot S400 just has a plate with two different sized holes in it. I doubt that most phone cams have an adjustable iris at all.
easOct 06, 2006 at 1:05AM
BTW, interesting link about the micromirror camera. Kind of the opposite of a DLP display.
Charles DarkeOct 06, 2006 at 5:02AM
The D70 does have a mechanical shutter. In doubt there is a dSLR that doesn't have a mechanical shutter. Just lift up the mirror and you'll see it. If you're not sure, set an exposure time of 2 seconds and press the shutter release while holding up the mirror. You'll see the shutter open and close.
Charles DarkeOct 06, 2006 at 5:09AM
BTW, some may be confusing terminology as electronically controlled mechanical shutters are sometimes referred to as 'electronic shutters'.
Frank GrouseOct 06, 2006 at 9:07AM
My little, older Canon PowerShot S230 has a mechanical shutter, as does the fancy new 10 megapixel Casio Exilim I just got my fiancee.
Interesting, from http://www.usa.canon.com/EOS-1D/faqs.html :
....
Q: Were the maximum 1/16,000-second shutter speed and 1/500-second X-sync made possible by a mechanical shutter?
A: No, they were realized by an electronic shutter that changes the duration of time that electric signals are stored in the CCD sensor. The mechanical shutter employed in the EOS-1D works only when controlling the bulb exposure length, and also functions as a cover that protects the CCD sensor.
....
nexOct 06, 2006 at 10:52AM
"Even if electronic shutters account for part of the shutter lag in (some) digital cameras, much more is the result of the autofocus system."
that is correct for snap-shots, but not when you've trained the camera on a subject and just wait for the right moment to shoot.
it sounds plausible to me that in most consumer cams the default setting is to only start focusing when you start to press the trigger, in order to conserve power, but usually you can also set the autofocus to "continuous" (aloways focussing), and either way, you can always press the trigger half way and get the camera to lock on to something. once that's done, you can fully press the trigger at any time, and there can't be any lag from focussing.
"I doubt that most phone cams have an adjustable iris at all."
i doubt there are any phone cams that have more than one or two fixed focus lenses (and a fixed aperture) in front of the imager.
"The aperatures on a lot of digital cameras are pretty simplistic. I think my PowerShot S400 just has a plate with two different sized holes in it."
that sounds so odd, but you're on to something here. when you browse flickr for images taken with that model, the aperture really seems to be either maximum or minimum. manually seraching is tedious, however, as only images taken with the same focal length are comparable and i didn't find a way to filter by that criterium in the search. is there a flickr API hacker among us? :-)
my old coolpix gives me 10 steps in 1/3 EV increments, using a 7-blade iris diaphragm, but back when that thing was new, it was a prosumer model. hmm ...
RyanOct 06, 2006 at 1:30PM
nex: i'm using that terminology because its the jargon we used back when i used to sell cameras. its the same language most of the documentation, and sales reps from big companies used as well.
besides calling a virtual shutter an electronic shutter is rather confusing. battery operated film SLRs out there are billed as having alternatly a motor driven shutter or electronic shutter meaning the same thing.
Frank Grouse: i'm sad to see you picked up an exilim. continueing on the "i used to sell cameras" schtick we used to sell a lot of those. Most people returned them within a few weeks. those that didn't almost always brought them back for tremendously expensive repairs after a month or so. watch the battery door, they tend to jam open or break off. and those huge screens that stand as the camera's major selling point tend to just stop working. now that casio stopped using pentax lenses i suspect the lens barrell will tend to jam as well.
we did have a few customers who loved the camera, and didn't have problems with breakage. these people typically ended up buy one or two extra batteries because the casio cams suck power like you wouldn't believe, and the batteries tend to wear out or develop severe memories very quickly
Niels OlsonOct 06, 2006 at 11:42PM
Regardless of theories, the mechanical shutter is an awesome dust guard. See the Copper Hill tutorial
nexOct 06, 2006 at 11:55PM
wait a moment, niels, there are cameras with exchangeable lenses that don't have a mechanical shutter?
ryanOct 07, 2006 at 12:35PM
not that i remember. but i seem to remember a sony prosumer level camera with exchanagable lenses. It wasn't technically a DSLR, but you could take the lense off, and i'm not sure if they ever sold alternate lenses for it. that one probably had a virtual shutter.
if course my tired mind could be making this camera up.
nexOct 07, 2006 at 2:28PM
sony make fine "SLR-like" cameras with large lenses, large sensors (APS size), and live preview instead of an SLR system. all i could find are all-in-one, though. you'll certainly find ones with exchangeable lenses and without a mechanical shutter, though, if you expand your search to video cameras :-) and yup, the dust-on-the-sensor problem is a bitch with them.
This thread is closed to new comments. Thanks to everyone who responded.