The Digital Sensor: A Guide to Understanding Digital Cameras
by Wesley Fink on April 21, 2008 1:00 AM EST- Posted in
- Digital Camera
CCD and CMOS
At the dawn of the digital SLR era, almost every sensor was a CCD (Charge Coupled Device). CCDs were relatively easy to make, but they were and remain basically a single function sensor. CCD sensors create very high image quality and low noise, but they require support circuitry for almost every function provided by the CCD. The CMOS (Complementary Metal Oxide Semiconductor) sensor was always a possible alternative in sensor design, but manufacturing CMOS sensors was very difficult with the technology available at that time. CMOS sensors are inherently lower image quality and higher noise than CCD, which led some experts to predict that a production CMOS sensor for DSLR imaging would never be made.
There are many reasons manufacturers would have preferred the alternative of the CMOS sensor. While difficult to manufacture they are much cheaper to make in volume than CCD sensors. Power consumption for CMOS is inherently lower than the CCD sensor. In addition, the CMOS sensor lends itself to integrating other electronics, such as the analog to digital conversion and noise reduction electronics into the sensor itself - something not really possible with the CCD design.
Those who did not think a commercial CMOS sensor was possible were silenced with the introduction of the $3000 Canon D30 in the fall of 2000. Canon has championed the CMOS sensor, almost exclusively, since that time. However, in the past year other sensor manufacturers have been able to produce their own CMOS sensors. As a result, almost every new sensor introduction in recent months has been CMOS.
In fact, Sony was the first to market with a 12MP+ APS C size CMOS sensor, used in the Nikon D300 and Sony A700. This was followed in about 6 months by the introduction of the Samsung 14.6MP APS-C CMOS sensor in the Pentax K20D. It appears other sensor makers who previously trailed Canon in CMOS development are now the ones pushing the envelope in CMOS sensor development. Sony has also announced a 24.81 effective megapixel CMOS sensor that will be used in a full-frame Sony later this year that will likely be called the A900. The Sony and Samsung thrust into CMOS sensors makes more sense when you realize that Sony and Samsung jointly own several recent patents in CMOS technology. Panasonic is also producing CMOS sensors as seen in their LiveMOS sensors used in the Olympus E-3 and E-501/410/420.
LEFT: Conventional Image CMOS Image Sensor Circuit Structure
RIGHT: Column-Parallel A/D Converter CMOS Image Sensor Circuit Structure
Sony has also taken advantage of another CMOS capability by combining analog to digital conversion and image noise reduction on the CMOS sensor itself in the 12.2MP A700. Some photo review sites had apoplectic fits when they realized Sony was doing noise reduction on the A700 chip, but this is an inevitable development. This move to larger integration of electronics into the CMOS sensor module is just starting, and it could eventually lead to a much larger scale integration of digital image processing functions into the sensor chip - or even a camera on a chip.
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Johnmcl7 - Monday, April 21, 2008 - link
On page 4:"The Foveon sensor falls between 4/4 and the Canon 1.6 in size and has a 1.7X lens multiplier."
I assume this should read '4/3'
Obvious question is what about Fuji? While I realise they re-use Nikon bodies and lenses, the discussion about Bater and Foveon makes no mention of Fuji's sensor. While it is more conventional than the Foveon, it's not just a bog standard bayer sensor and while Fuji are a minor player, so are Sigma.
For those who are not familiar with Fuji's design, they use two photodiodes at every point one larger and one smaller with the two combined to produce a single pixel in the output image. The idea is that these pair of pixels can capture more extended dynamic range than a standard bayer sensor of the same size. The latest version of this sensor is in the Fuji S5 Pro however it's limited to just 6MP although I can't remember if they still produce 12MP files from this. The S5 itself is basically a Fuji version of the very good Nikon D200 body although Nikon have of course moved on with the very impressive D300.
John
Wesley Fink - Monday, April 21, 2008 - link
Yes, it should have read 4/3 and the reference is corrected. The Fuji S3 was one of my favorite Nikon bodies and the dynamic range was certainly impressive in the studio.We did not mean to slight Fuji, but as one Nikon-mount body with a sensor that hasn't been updated in several years (the S3 and S5 sensors are the same as I understand it) we decided not to include the Fuji since there have been no sensor updates in quite a while.
Johnmcl7 - Monday, April 21, 2008 - link
"We did not mean to slight Fuji, but as one Nikon-mount body with a sensor that hasn't been updated in several years (the S3 and S5 sensors are the same as I understand it) we decided not to include the Fuji since there have been no sensor updates in quite a while. "I can't say I really agree there, given the article more focuses on concept in parts I think the S5 sensor is still relevent as it's something slightly different to the bayer sensor. The article refers to only Bayer and Foveon which implies there is nothing else, I think for completeness even if you don't go into any detail it's still worth mentioning Fuji are doing something else.
Also the S3 and S5 sensor are not the same, while they have the same amount of pixels it appears there's been some slight improvements although clearly not much. To be far to Fuji though, the Foveon sensor hasn't really come on much either - it's gone up very slightly in resolution with some small other changes but that's it pretty much. If the Foveon sensor had been just mentioned in passing I could definitely understand leaving Fuji with a similar mention but generally Fuji and Sigma are considered in the same boat as doing something a bit different although arguably with the Nikon body and mount Fuji have had more success.
John
melgross - Tuesday, April 22, 2008 - link
I never saw an advantage to their designs. I can't see what purpose having a smaller photo site on the sensor would do. It just has more noise, and less dynamic range than the larger sensor. I've read their papers on the subject, and they don't seem to have made a good case for it. Somehow, I think they understand that now.Johnmcl7 - Thursday, April 24, 2008 - link
"I never saw an advantage to their designs. I can't see what purpose having a smaller photo site on the sensor would do. It just has more noise, and less dynamic range than the larger sensor. I've read their papers on the subject, and they don't seem to have made a good case for it. Somehow, I think they understand that now."Are you referring to Fuji? If so, your information is incorrect - the last measurement I saw put the S5's sensor at slightly more dynamic range than the 35mm sensor in the Nikon D3. Their real problem at the moment seems to be resolution as well as having the older D200 based body.
John
strikeback03 - Tuesday, April 22, 2008 - link
I think Sigma (and Foveon) would be better off if Sigma could license a major player's mount, like Kodak did with the SLR/n and SLR/c. There are plenty of people who would like to have the sensor for the situations where it excels, but have no interest in a whole Sigma SA mount setup.pinto4402 - Monday, April 21, 2008 - link
I've been reading Anandtech for over 8 years now. I was a bit skeptical about your doing articles on digital cameras; however, this article put my reservations to rest. Very nicely done.I see why it makes sense for Anandtech to write about digital cameras. The nexus between computer tech and camera tech are very obvious if you've been following the trends. I'm a professional portrait photographer. In the last few years, it has become virtually impossible to remain in business unless you have a firm grasp on the latest camera tech as well as computer tech. I spend as much time on my computer as I do behind the camera. The camera has become a computer accessory (or vice versa). Many old timers who are hanging on to film are slowly being forced out. I'm somewhat of an old timer myself because I learned about photography when it was cool to have a darkroom, but I embraced digital equipment early.
Your graphs make it very easy to explain to people why their P&S (piece of s***) cameras are simply not adequate for serious portraiture. The MP count is marketing crap. As you demonstrated, it's the physical size of the sensor that matters.
Looking forward to part II of the series. Also, do you have any solid info on the introduction of 5D Mk II?
bjacobson - Tuesday, April 22, 2008 - link
"The MP count is marketing crap."Mostly. If you're willing to do some post processing yourself, the higher MP will enable you to decrease the IQ gap between the higher end DSLR and the ho-hum consumer camera. For this reason, since I wanted something compact and didn't really need a lot of optical zoom, I chose the Canon SD1100IS. 8MP, and while you begin to get noise at ISO400, more at ISO800, and tons at ISO1600, using a non-linear digital filter should correct most of that without blurring the image much (if at all).
strikeback03 - Wednesday, April 23, 2008 - link
Problem is that the camera has already blurred away lots of your detail at ISO 400 and up.http://www.dpreview.com/reviews/canonsd1100is/page...">http://www.dpreview.com/reviews/canonsd1100is/page...
Too bad there is no option to reduce/turn off the in-camera NR, for those of us who own a better program for it already.
Wesley Fink - Monday, April 21, 2008 - link
I wish I did have definitive info about the 5D Mark II launch, but the best info I have is this fall at Photokina. Rumors pop up every month that the new 5D will be here in a few weeks - and the last rumor was a definite April 22, which is tomorrow. Rumors are just that - rumors.