The Digital Sensor: A Guide to Understanding Digital Cameras
by Wesley Fink on April 21, 2008 1:00 AM EST- Posted in
- Digital Camera
Bayer vs. Foveon
This title is something of a misstatement, as it would more accurately be Foveon/Sigma versus everyone else in the world. All DSLR imaging sensors except the Foveon use Bayer technology. Digital color images are created by capturing red, blue, and green pixels and then combining them to create a full color image.
All current sensors except one use Bayer technology. The sensor consists of X megapixels, or light gathering cavities. The sensor is covered with a Bayer array, which is a series of microlenses that allow only certain colors to fall in certain cavities (or pixels). The microlenses are arranged in a defined grid that consists of alternating red-green and green-blue filters.
If you noticed there are twice as many green as red or blue areas in the above image, that is by design in the Bayer array. The human eye is more sensitive to green light than red or blue and the Bayer array uses this fact to produce images that appear to have finer detail and less noise. The doubling of the green receptors is corrected in the image processing.
There is no demosaicing or interpolation step with the Foveon sensor. Where before there was one pixel of color information, the Foveon is now capturing red, green, and blue data pixels in the same pixel location. On the surface, this certainly appears a superior way of capturing color images, but things are not always as they appear. It is fair to ask: if this approach is so good then why is Sigma, a minor player in the digital camera market, the only company to embrace the Foveon sensor? A closer look at Foveon specs helps to understand some of the issues.
The top current Foveon sensor is the 14.1MP Foveon used in the Sigma SD14 digital camera. Foveon gets to the 14.1MP total by counting all pixels used to create the finished image, which is 2688x1768 pixels or 4.7MP. Many would argue this is fair since each pixel in a competing 14.1MP is only collecting one piece of color information. However, the practical reality is that a 14.1MP Foveon is reported to be about equivalent to an 8MP Bayer DSLR when shooting JPEGs and about as good a 10MP camera when shooting in the preferred and native RAW mode. The apparent resolution then is somewhere between the 4.7MP image size and the 14.1MP that are used to create that image.
Several other issues have also held back Foveon. Where the best Bayer cameras can now capture useful images at ISO 1600, 3200, or even higher, the Foveon sensor is best at lower ISOs. It is usable to about ISO 400 and then noise climbs rapidly as the ISO increases. The latest SD14 Foveon is reported to be better in ISO sensitivity, but it still falls far short of the extended ISO performance of its competition.
The other major issue with the Foveon sensor is that the separation into 3 distinct colors for each pixel site is not nearly as straightforward as Foveon describes it. The captured image still requires a lot of image processing to extract the 3 colors from each pixel site and reconstruct the finished image. In fact some critics claim the image processing required by the Foveon sensor is even more extensive than Bayer Array demosaicing. The purer the data in any extraction process the more faithful it is to the original capture, so the Foveon sensor may offer fewer "post-processing" advantages than it first appears. When conditions and lighting are correct the Foveon can deliver stunning images, but things in photography are rarely perfect.
For the purpose of our discussion the Foveon is more a novelty than a sensor you will likely use today. Despite its limited availability, however, the concept of the Foveon sensor is as compelling as ever, and you can actually buy a production camera, the Sigma SD14, that uses this sensor. As you can see from the specs, the sensor size is 20.7x13.8mm, which places the Foveon between the Canon small APS C and the Olympus 4/3 sensor. The lens multiplier is 1.7X.
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bjacobson - Tuesday, April 22, 2008 - link
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aliasfox - Monday, April 21, 2008 - link
When everybody was still on 35mm film, it was impossible to tell whether an SLR shot an image or a good point and shoot/rangefinder style camera. Image noise was a function of the film, and within a certain range, SLRs and point and shoots could reproduce the same sharpness and depth of field. All of that is pretty much impossible these days with the split in sensor sizes.My biggest question is: if camera companies could make a pocketable 35mm/APS sized camera 10 years ago, why can't they make the same pocketable size with a digital sensor? An APS-C sized sensor in a Canon G9-style body shouldn't be impossible, and while it would be expensive, it would have image quality worth the price.
I have a feeling that the companies producing SLRs are just trying to protect their high-margin lens business - a company that's not vested in the SLR side of things could carve out a very solid niche by making a point and shoot with an SLR sensor.
And yes, I know the Sigma DP1 exists, but without a zoom lens and max aperture of F4.0, $800 is a lot to ask...
idealego - Wednesday, April 23, 2008 - link
A lot of it has to do with consumers expecting zoom lenses with cameras. The larger the sensor the larger the lens has to be, especially with a zoom lens. I don't think it's possible to have a decent zoom lens on a pocketable camera with a large sensor (with current technology). This is why the DP1 has these limitations. This is also why a camera such as the Sony R1 has such a large lens compared to other cameras with smaller sensors.However, I'd be quite happy with a fast prime lens on a well-designed, pocketable, large-sensor camera, even if it did have a bunch of downsides to it.
If the DP1 was more refined and had a much faster lens, it would be an awesome camera. Unfortunately, it's rather crude compared to cameras from other leading brands.
Johnmcl7 - Monday, April 21, 2008 - link
"My biggest question is: if camera companies could make a pocketable 35mm/APS sized camera 10 years ago, why can't they make the same pocketable size with a digital sensor? An APS-C sized sensor in a Canon G9-style body shouldn't be impossible, and while it would be expensive, it would have image quality worth the price.I have a feeling that the companies producing SLRs are just trying to protect their high-margin lens business - a company that's not vested in the SLR side of things could carve out a very solid niche by making a point and shoot with an SLR sensor. "
I don't think that's the case at all, film and digital cameras have to be a lot more different than you perhaps appreciate. The Sigma DP1 has had a prolonged and difficult development, while I think that's partially because Sigma are less experienced it's not entirely the story. One of the main problems I believe is the way light hits the sensor, with a digital sensor to work optimally you need to have the light hitting the sensor as straight as possible. Film was more flexible in this sense which made it easer to produce small cameras using comparatively large pieces of film. I believe this was one of the difficulties in producing the DP1 as the lens is very close to the sensor which meant the light is hitting the sensor at very high angles. This is easier on an SLR as the sensor is a reasonable distance back from the rear of the lens (Olympus particularly have made a goal of telecentric lenses) although even the 35mm full frame cameras can suffer from poor corner sharpness.
Also with film you didn't have any low pass or IR filter in front of the film itself which is normally part of a digital camera, I believe the DP1 (as well as the Leica M8) don't actually have an AA filter at all which causes its own problems.
Hopefully other companies will have a go seeing the interest that the DP1 is generating, I think Panasonic who produce a 4/3 sensor and are having little success in the SLR market are in a prime condition to do so. However it's a more difficult task than it seems.
John
Heidfirst - Monday, April 21, 2008 - link
"I have a feeling that the companies producing SLRs are just trying to protect their high-margin lens business "when Kodak got out of doing DSLR bodies (even if they were basically Nikons with a different sensor) they said it was because the profits weren't in the bodies but in the lenses - which of course they didn't make ...
Pentax is supposed to be getting out of compacts because they're becoming a commodity item & hence price-sensitive/low margin.
I could imagine someone like Leica doing what you want though as they seem able to command a significant premium.
aliasfox - Monday, April 21, 2008 - link
Leica's currently tied to Panasonic sensors, and relatively speaking, Panasonic hasn't figured out how to make a sensor that has low noise above ISO 200 yet, regardless of size (otherwise I'd seriously contemplate the Lumix LX-2).Also, while I'm willing to pay a premium for a nice fixed lens camera (I'll consider up to $700, maybe even $1000 for a camera with the right handling, image quality, and features), the market for such an expensive "small" camera is probably pretty small (unless professionals are all clamoring for a pocketable camera as a backup in the field).
Lastly, the Leica name generally commands an even healthier premium than I'm willing (or financially able) to consider...
pinto4402 - Monday, April 21, 2008 - link
The sensor is only half the story. The lens is the other half. This is NOT marketing hype. The difference between consumer kit lenses and pro series glass is easily discernible (even to non pixel peepers). Does this justify the 8X difference in price? I don't know.However, I'm pretty sure that it's not possible to construct a high quality P&S with a great lens that is "pocketable," regardless of the sensor size. Good glass tends to be heavy. Even the old Leica and Contax rangefinders were not pocket cameras. They were compact, but I doubt anyone considers them pocket cameras (unless you're talking about cargo or jacket pockets, and you trust having a $6000 camera hanging out of your shirt pocket). When you add zoom functions to the equation (which is a must nowadays), you looking at some serious IQ trade-offs.
That said, I would be the first in line to purchase a good P&S camera that produces high quality professional grade images.
aliasfox - Monday, April 21, 2008 - link
Honestly, I'll be happy if someone can fit an under 2" thick camera with an adequate (say, 28-112mm equivalence, f2.8 - f4.0) lens and a large/larger sensor. Maybe not even an APS-C sensor - a 4/3" design is already significantly larger than a 1/1.8" sized unit. Olympus, are you listening?The Olympus E-420, even with its pancake 28mm equivalence lens, is too thick to put into a jacket pocket (or crammed into some looser fitting jeans).
I rarely ever take dedicated trips to take photos - so packing a backpack half full of photographic equipment to take a "nice" shot is something I'm loathe to do. But I do go cycling, hiking, and other normal touristy activities where I'd like to be able to bring a camera along...
strikeback03 - Tuesday, April 22, 2008 - link
There was the Sony R1 - APS-C sensor and 24-120 Zeiss-branded lens. Not exactly pocketable, even not having to deal with the registration distances required by an SLR with the swinging mirror. Maybe if you were willing to do something with folded optics you could fit a zoom in a smaller body, but who knows how much of an image compromise that would be.My XT with 28 1.8 does fit in the cargo pocket of one pair of pants I own, but I would rather just have it in a small shoulder bag.