The Digital Sensor: A Guide to Understanding Digital Cameras
by Wesley Fink on April 21, 2008 1:00 AM EST- Posted in
- Digital Camera
Fast Forward
With digital SLR sales continuing to show record growth in a photographic market whose overall growth is much slower, it should be clear that a lot of photo buyers are selecting digital SLR cameras instead. The reasons many of these new buyers select a digital SLR is because they want better quality pictures than they can get with a point and shoot camera. They may also choose a DSLR for the flexibility and growth potential if they get hooked on the photo hobby.
These are exactly the same reasons buyers chose film SLRs instead of 110 point and shoots in the 70s, and 80s. Those reasons are just as valid in the DSLR market as they were in film, and maybe even more so. Digital sensors, like other electronics, are constantly evolving and improving, and whatever megapixel assumptions we talk about today will certainly become invalid and outdated in the near future. However, it is very clear with today's sensors that the tiny sensors in compact point-and-shoot cameras are reaching the point where higher resolutions are simply being traded for noise. Somewhere around 8-10MP we are finding that higher resolution also generally means higher noise and lower sensitivity.
No doubt this roadblock will be passed with advancements in sensor technology, but today more than 8MP of clean resolution and usable sensitivities greater than ISO 400 are rare indeed in the compact camera market. APS C sensors in digital SLRs, however, seem to be getting better and better at higher and higher sensitivities at ever-increasing resolutions. Pundits are already screaming we are going too far with14MP sensors, but they forget that the smallest 4/3 sensor is still more than nine times greater area than the average compact sensor. There is still a lot of room for growth in resolution.
The other complaint - that lenses are finally reaching resolving limits with higher sensor resolutions - is certainly true with the cheap lenses that were the wunderkind of the developing SLR market. It looks like time for quality optics again as the industry has been skating for far too long in the low demands of the developing digital SLR market.
It also appears that prosumers, the serious amateurs among us, will be facing a difficult decision today and even more so in the near future. The cost of larger and larger sensors has been dropping rapidly, and CMOS sensor development from all the majors is also a factor in lowering costs and increasing resolution. Like it or not Canon and Nikon have already begun segmenting their SLR line into full-frame and APS C sensors. Those who couldn't figure out why Sony was introducing mainly full-frame lenses will finally get their answer later this year with Sony's 24.6MP full-frame flagship model.
Despite the fact that full-frame will be aimed at the top of the DSLR market by Canon/Nikon/Sony, the APS C market does not appear to be in any danger. Developments and new models will definitely continue. Players like Pentax and Samsung seem positively locked into the APS C space with no full-frame peeking around the corner, and Olympus has fought too hard for credibility with 4/3 to start singing a full-frame song. Similarly Nikon, Canon, and Sony will adamantly define the full-frame as pro and the rest of their line as prosumer and entry-level. Nikon may also have struck the marketing chord that will develop with full-frame sensors being touted more for their incredible range of ISO sensitivity than for their higher megapixel resolutions.
The problem is that prosumers lust after pro gear and a prosumer today will have to ask another question in their buying decision for accessories now that full-frame looks like it will be "for real". That question is: "will it work on a full-frame". The current $2000 street price of the Canon 5D and the coming release of the Canon 5D Mark II are making that question an important one for many prosumer buyers. The final street price of the presumer Sony "A900" is also still a mystery, but if it is in line with the Canon 5D, as many expect, then this question in the back of the minds of prosumers will move up-front very quickly.
The purpose of this sensor guide was not to explore every facet of sensor design and performance considerations. Each topic discussed could have been an individual article in its own right with more in-depth discussion of the factors that drive design decisions. Instead, the hope was to provide a framework of basic sensor information to provide a better understanding of the evolution of digital sensors and the types of concerns and decisions that are being made in the market today. We sincerely hope you come away with a better understanding and appreciation of the current digital SLR market, and perhaps of your own digital SLR camera or one you might buy in the future.
Part 2 of this Sensor Series is in the works and many of the images are already in the can. It will take a closer look at the sensitivity range and noise of the most recent sensors in the 14MP, 12MP, and 10MP classes of sensors. A few more cameras are on the way, and as soon as they are prepped and tested we will be sharing more of our findings on the newest sensors in the higher resolution sensor classes.
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melgross - Monday, April 21, 2008 - link
The smaller process technology will have no positive effect on the sensors themselves, though it will for the associated electronics integrated on the die.The same problems he mentioned about smaller sensing sites will remain. The smaller the sensor, the poorer the performance viz a viz larger sensors.
He did mention that the photo division was the purchase, not the entire company (unless he changed the article after your post).
Wesley Fink - Monday, April 21, 2008 - link
I changed the wording on the Sony purchase a bit to better reflect that Sony bought the Minolta camera assets of KM and not the company. Thanks for pointing this out.finbarqs - Monday, April 21, 2008 - link
Canon's 1DS MK2 was a 16.7MP CMOS sensor also, and of course, the MK3 is a 21MP CMOS sensor...Where i'm lost is if CCD's are so much better (in IQ) why dont' they stick with CCD's? why the move to CMOS besides the lower cost and the battery life that it saves?
Why are professional level DSLR's (From canon and Nikon) are both CMOS when we know that CCD is the way to go for better IQ?
melgross - Monday, April 21, 2008 - link
It USED to be true that CCD's were better. Not so any longer. The best CMOS sensors are better than the CCD's they replace.The desire to go CMOS is obvious to the manufacturers of the sensors.
CCD sensor technology is a completely different manufacturing process from that of CMOS, which the entire industry uses for everything else (almost).
Moving to that allows CMOS sensors to not only be able to integrate other electronics on the sensor chip, resulting in simplicity, price advantages, and the ability to more favorably utilize their process lines, but that higher quality you're concerned about.
Putting functions on the same chip improves the quality of the signals.
And, by the way, an error in the article: Canon was not the first to make, or use, a CMOS sensor. They were the first to come out with a high quality sensor. I believe that it was Vivitar that used the first one, though I forgot the name of the manufacturer.
Anther omission is that there are trilinear sensors used in camera backs such as the Betterlight scanning backs. So there are three different major technologies in use.
And not all of the negatives of the Foveon chip was mentioned.
s12033722 - Monday, April 21, 2008 - link
No, CCD is definitely still the IQ king. CCD still has a far better SNR than any CMOS technology.As a digital camera design engineer, I deal with image sensors every day. The major reasons why CMOS sensors are attractive are all cost related. Not only are CMOS sensors themselves cheaper, but they lend themselves to integration with other electronics better and they are MUCH nicer to design with. A typical CMOS sensor will require ground, 3.3V, and maybe some other standard voltages (1.8V, 2.5V, etc.), whereas a typical CCD will require ground and anything from 8 to 12 other DC voltage rails. For instance, I am working on a camera that requires -15V, -9V, -6V, -4V, -1.5V, ground, 2V, 3.5V, 11 V, 15V, 24.5V, and has a clock signal that must run up to 40V. While making the voltages and driving clocks at them is fairly straightforward, it requires a lot more components than a CMOS sensor design would. More components directly equals higher cost. Also, as the article mentions, more functions can be integrated onto CMOS sensors than CCDs.
The other advantage of CMOS vs. CCD is in random-access readout. If you want to read a small region of interest on a CCD, you either have to read out the whole frame and digitally ignore the parts you don't care about (no increase in read speed) or the chip has to support charge dumping, where portions of the image can be dropped without reading them out. CMOS makes it much easier to read small portions of the image, and thus things like live view are simply done.
Lastly, I'd like to mention an issue with the Foveon sensor that the article didn't mention. While the foveon technology presents itself as having three discrete pixels stacked on top of each other, the reality is much more ambiguous. Foveon relies on the ability of different wavelengths of light (colors) to penetrate to different levels in silicon, however, far from being discrete, easily separated regions, the depth of capture of different wavelengths in silicon tends to be very blurry and ill defined. This results in significant color mixing in the foveon design. They manage to pull out the images they do through the use of extensive processing. That makes the technology pretty unappealing to design with, thus the dominance of Bayer sensors. Honestly, if I needed to do a camera with true RGB per pixel, I would use a 3-CCD design where a full sensor is dedicated to each color rather than using anything like a foveon. It would be more expensive, but far better quality.
Wesley Fink - Tuesday, April 22, 2008 - link
Thank you for clarifying several points from the Design Engineer perspective. I appreciate your insights into the CCD vs. CMOS issues.Sometimes it is difficult for people to wrap their heads around the idea that a technology (CMOS) is not the low noise champion, but that it is winning nonetheless because of other attributes such as lower cost, manufacturing efficiency, lower cost, integration advantages, lower cost, lower power consumption, and lower cost. Your comments put that reality into perspective.
melgross - Monday, April 21, 2008 - link
That's all very interesting, but unless you are designing a very high IQ, special purpose (read, very expensive) device, that's simply not an assumption that can be made..CCD's have numerous problems. High power requirements, which lead to higher temperatures, which leads to higher noise levels, requires cooling for the best results, and so on.
There is no inherent IQ advantage to CCD's. The longer development time led to an early start, and all the advantages accrued from that. But that lead shrunk.
Also, when talking about cost/performance, we must realize that it is very important to not lose sight of the fact that performance must be compared at reasonable cost levels. NASA can afford to spend a million for a sensor, which they do, but it's irrelevant to everyone else.
As for the Foveon chip, yes, that is one of the problems I was talking about, and the biggest one.
Some enthusiastic reviews and articles have taken Foveon's word that they undergo little processing compared to Bayer chips. That's only true in the de-matrixing area, and so they don't need a the aliasing filter. But the color mixing problems are just as serious, and I've found, in using the camera, that color quality is more variable than with my 5D. Often noticeably poorer as well.
7thSerapHim - Monday, April 21, 2008 - link
As stated on the Canon CMOS Technology page, it says that although CCD sensors achieve high IQ, they are have slow data-reading speed, which means that it wouldn't be capable to capture at a fast FPS mode.CMOS is capable of high data-reading speed, but due to crosstalk (between pixels) the IQ suffers as a result. However, due to developments in CMOS technology, we can assume that IQ has improved closer to CCD.
My opinion is that the cost, speed, battery life and potential for improvements is what compelled many to adopt CMOS instead of CCD.
Zak - Monday, April 21, 2008 - link
I've recently bought Canon 40D that uses CMOS (right?) and I'm a bit disappointed in the image quality over my Rebel XTi (dead). There is definitely much less noise and it's incredibly fast, but I just can't get images to be as sharp as with the XTi. In particular with 100mm Canon macro lens. I don't know if I got a defective body or the CMOS sensor is indeed softer than CCD in the XTi. I'm considering returning the 40D and fixing my XTi instead.Z.
strikeback03 - Tuesday, April 22, 2008 - link
Canon typically tunes the JPEGS from the entry-level bodies to be more "punchy" (more saturation, more sharpening, more contrast) than on higher-level bodies, to provide results closer to the average P&S output.If RAW, might just be processing variations between RAW profiles for different bodies. Have you tried some sharpening in photoshop or similar to compare images?
Also possible the body is missing focus, but before jumping to that conclusion I'd try the image adjustments first.