I understand a digital camera is just like a normal camera (more or less) except it records the focused image on a bunch of photo sensors, instead of film. If these sensors are electronic, why is a shutter needed in digital cameras? Can’t the sensors just be turned on and off electronically, instead of having a shutter do the work?
There are two reasons that I can think of on why digital cameras have a shutter. One would be that the sensors are sensitive and can be damaged by light easily. Another thing would be some sort of limitation of the sensors reaction time, if they couldn’t be controlled to the exactness that a shutter can.
I’ve never seen a digital camera with a shutter. What make and model are we talking about here?
A digital camera uses a chip called a CCD (Charge Coupled Device), which is basically an array of photodiodes and tiny capacitors on a silicon wafer. It’s all electronically switched and doesn’t need a shutter to control the exposure.
Actually most digicams do have a mechanical shutter. It’s possible to make a digicam without a mechanical shutter but the only cams I’ve seen without them are things like webcams and ultra low end still cameras. I’ll ask around on some specialized digicam boards but I’m pretty sure it’s to get more precise exposures and it may even make very high shutter speeds possible. Also some cameras will take a sensor reading while the shutter is briefly closed before the shot to establish a black level which can be used to reduce the noise in the image. The camera I use, a Minolta Dimage 7, actually takes this black “reference frame” after the exposure for long shutter times when noise is a particular problem.
Well, I assumed they have a shutter. My camera, a Fujifilm 3800, has a window that opens and shuts and looks exactly like a normal shutter on a normal camera. Is this called something differently on a digital camera?
Well, I assumed they have a shutter. My camera, a Fujifilm 3800, has a window that opens and shuts and looks exactly like a normal shutter on a normal camera. Is this called something differently on a digital camera?
Because it takes time to read out the CCD, and during the readout process, the charge is transferred across the sensor. A CCD is a 2-dimentional array, but it only has one readout amplifier at the corner. The whole thing acts like a conveyer belt, and the collected charge is transferred slowly to the readout amp where it is read. If light is shining on the sensor during the readout process, it will create a false signal. It’s called “smearing.”
There are ways to eliminate the shutter. If you have a small number of pixels (say, less than a million), and if the exposure time is sufficiently long (say, longer than 1/100 s) then exposure time becomes significantly longer than readout time, and smearing is minimal. Most video cameras work that way, I believe. Or you can mask every other line on the sensor and move the charge quickly onto those lines, and read them out slowly. But obviously you lose half the light collecting area. In practice, it’s often easier to just use a mechanical shutter.
I’m not too familiar with CMOS sensors but I don’t think they usually need a shutter. Those have readout transistors on each pixel.
Hey Padeye, is your camera noisy? I have a Dimage 7i, and the images I get appear to be noisy (or grainy), even when I use the lowest ISO speed, put the camera on a tripod, and crank the aperture down to about f/8.
OK if most DC’s have a shutter then how come you can activate the lcd display to aim it, doesn’t the shutter have to be open or non-existant for you to actually see anything on the display?
I think there might be an aesthetic quality to having a shutter. People who are used to that click-click when they take a picture might feel something’s missing when they don’t hear it.
Actually the aesthetic quality can be - and often is - simulated by a speaker that makes the “click” sound. I’ve seen cameras that allow you to select between a beep and a realistic “ca-chunk” sound, or turn it off.
I believe the LCD “viewfinder” is achieved by keeping the shutter and operating the CCD in a shutterless mode. I’m not an expert here, but I think it uses on-chip binning or some other technique to speed up the readout. When you press the button the shutter closes, the CCD goes into exposure mode, the shutter opens and closes for exposure, and then the CCD is read out. My old Nikon obviously worked this way; there was smearing on the viewfinder but not on the actual photo.
Now that I think about it, the tiny shutters on digital cameras don’t make much noise anyway, and I think some cameras with shutters still use speakers to make the noise. On a film camera, besides the shutter there’s the film advance motor that makes a satisfying noise and vibration.
One more datapoint:
My Ixus digital (Which I believe was sold as ‘Canon Powershot 100’ in the states) has a tiny shutter, and it makes a small noise.
In ‘viewfinder’ mode the shutter is open, and I believe that it uses on-chip binning, or some other technique, as scr4 hints at. (Turning the viewfinder off, the shutter visibly shuts.)
However, it looks like the shutter only shuts. As in, I believe that it remains open, until the photo is taken, and then immediately closes, to enable the read-out.
Does that sound feasible?
And yes, on this particular specimen it is indeed audible. Not highly so, but if there’s no other noise, I can hear it.
(Thank heavens for digital cameras - otherwise I would sit here with a roll of pictures of my eye and nose.)
This seems odd. I’ve worked with a lot of optical chips that work perfectly well with light shining on them. I’ve never heard of “smearing” before, but I’ve only worked with specialized items, not commercial digital cameras.
Now that I look more closely, the ‘reverse shutter’ theory is what might be happening. In my camera, when you focus on something, it’s hard to see but some type of shutter closes for a split second, then opens again. Then when a picture is taken, the shutter mechanism closes again for a split second. In other words, the shutter appears to close when a picture is taken, sort of the exact opposite of a normal 35mm camera.
On my camera it’s impossible to turn the electronic viewfinder off without turning the camera off, and the shutter seems to be always open when the camera is on. I think Popup and the others have the right explanation.
DigiCams that use a shutter and LCD keep the Shutter open most of the time, for the viewfinder. When you trigger the action, the shutter closes, the CCD is flushed to a blank state, and then the shutter opens and closes. While the CCD is closed, the CCD is read, and the photo transferred. Then the shutter reopens. This is why there is often Lag (delay between shutter press and actual shot) on digital cameras (along with the AutoFocus/AutoExposure system). It also limits the speed of sequential shooting.
My 2 year old Olympus 2100UZ can shoot 1.5 frames (up to 5 frames in a 16Mb buffer) per second on a 2.1Mp camera, using a conventional shutter type arrangement.
Some cameras do rely on electronic shuttering - the Olympus E100RS achieved up to 25 frames per second using a special CCD that was optimised for electronic shuttering (only 1.3m pixels tho) and a 32Mb buffer. It had a 10x optical zoom, image stabiliser and was optimised for sport and nature shooting (it would capture 5 frames per second when you halfpressed the shutter, so you had 5 images from before you pressed the shutter - eats memory, but you never missed the shot you wanted cause you were a bit too slow on the button).
No-one seems to want to pursue that sort of innovation in the current Digital Camera marketplace - the manufacturers put more pixels into cameras (that few people really need) and don’t look at the features people will use when they have them in the real world (real [greater than 3x] zoom because you are never close enough, low light cause it’s always too dark and the tiny flash is useless, pre-capture and fast sequence shooting because your kids don’t stand still and my finger is too slow). Since I can get great 8x10s from a 2.1Mp camera, I wouldn’t upgrade until someone gave me the features I love in my camera (10x optical zoom) with something really compelling to address the deficiencies (like using more pixels for better low-light photography, as does the Fuji 6900).
That sounds plausible. The problem is the readout process, not the start of the exposure.
I should have put a disclaimer that I don’t know much about commercial digital camera design. I’m extrapolating from scientific CCD detectors which I do know about. With scientific observations it’s vital to control the exposure time accurately, so you use a mechanical shutter to start and end the exposure.
CalMeacham, what kind of detectors were they? Perhaps they were smaller detectors with fast readout, or possibly CMOS detectors. There are surprisingly few explanations and examples on the web, but here’s one: http://www.ccd-sensor.de/english/html/hauptteil_smear.html
Is everyone in here that says they have a shutter, sure that is what it is, and not a lens cover? I’ve seen plenty of digital cameras with lens covers (powered and unpowered) but none with a shutter.