Editing Foveon X3 sensor (section)

==Comparison to Bayer-filter sensors==
The operation of the Foveon X3 sensor is different from that of the [[Bayer filter]] image sensor, which is more commonly used in [[digital cameras]]. In the Bayer sensor, each photosite in the array consists of a single light sensor (either CMOS or CCD) that, as a result of filtration, is exposed to only one of the three colors detectable on the sensor: red, green, or blue. Constructing a full-color image from a Bayer sensor requires [[demosaicing]], an [[Image scaling|interpolative process]] in which the output pixel associated with each photosite is assigned an [[RGB color model|RGB]] value based in part on the level of red, green, and blue reported by those photosites adjacent to it. However, the Foveon X3 sensor creates its RGB color output for each photosite by combining the outputs of each of the stacked photodiodes at each of its photosites. This operational difference results in several significant consequences.

===Color artifacts===
Because demosaicing is not required for the Foveon X3 sensor to produce a full-color image, the color artifacts ("colored [[jaggies]]") associated with the process are not seen. The separate anti-aliasing filter<ref>See, ''Optical anti-aliasing filter'' section of [[anti-aliasing filter]]</ref> commonly used<ref group="n">Though its use is almost universal with Bayer sensors in digital cameras, it is not absolutely necessary. Kodak once produced two digital cameras, the [[Kodak DCS Pro SLR/n|DCS Pro SLR/n]] and DCS Pro SLR/c (''Digital Photography Review,'' [http://www.dpreview.com/reviews/kodakslrc/page2.asp Kodak DCS Pro SLR/c Review], June 2004, Retrieved March 3, 2007) using Bayer sensors without such a filter. However, significant [[moiré pattern]]s [http://www.dpreview.com/reviews/kodakdcs14n/page22.asp were produced when photographing very fine detail]. Retrieved March 3, 2007.</ref> to mitigate those artifacts in a Bayer sensor is not required; this is because little [[aliasing]] occurs when the photodiodes for each color, with the assistance of the [[microlens]]es, integrate the optical image over a region almost as big as the spacing of sensors for that color.<ref group="n">Microlenses are commonly used in all types of image sensors in digital cameras; in Bayer-filter sensors, microlenses allow the area of the optical image being averaged (i.e., integrated) per sample to approach 25 percent for red and blue, and 50 percent for green, resulting in very little anti-aliasing. For Foveon X3 sensors, the area being averaged can approach 100 percent for each color, resulting in a significant anti-alias filter effect.</ref><ref>{{cite book|author=Brian W. Keelan|title=Handbook of Image Quality: Characterization and Prediction|publisher=Marcel–Dekker|year=2004|url= https://books.google.com/books?id=E45MTZn17gEC&pg=RA1-PA388|isbn=0-8247-0770-2|page=390}}</ref> On the other hand, the method of color separation by silicon penetration depth gives more cross-contamination between color layers, meaning more issues with color accuracy.

===Light gathering and low-light performance===
Theoretically, the Foveon X3 photosensor can detect more photons entering the camera lens than a mosaic sensor, because each of the color filters overlaying each photosite of a mosaic sensor passes only one of the primary colors and absorbs the other two. However, the individual layers in a Foveon sensor do not respond as sharply to the respective colors; thus color-indicating information in the sensor's raw data requires an "aggressive" matrix (i.e., the removal of common-mode signals) to produce color data in a standard [[color space]], which can increase color noise in low-light situations.<ref>{{cite web|url=http://www.auspiciousdragon.net/photowords/?p=880|title=Know raw? Part 2|date=2007-07-05|work=Photostream on auspiciousdragon.net|url-status=unfit|archive-url=https://web.archive.org/web/20070928113408/http://www.auspiciousdragon.net/photowords/?p=880|archive-date=2007-09-28}}</ref>

===Spatial resolution===
According to [[Sigma Corporation]], "there has been some controversy in how to specify the number of [[pixel]]s in Foveon sensors."<ref>{{cite web|url=http://www.cameratown.com/news/news.cfm/hurl/id%7C3599|title=Sigma SD14 White Papers|access-date=2007-04-29|archive-date=November 26, 2007|archive-url=https://web.archive.org/web/20071126090538/http://www.cameratown.com/news/news.cfm/hurl/id%7C3599|url-status=dead}}</ref> The argument has been over whether sellers should count the number of photosites or the total number of photodiodes, as a megapixel count, and whether either of those should be compared with the number of photodiodes in a [[Bayer filter]] sensor or camera as a measure of resolution.

For example, the dimensions of the photosite array in the sensor in the Sigma SD10 camera are 2268 × 1512, and the camera produces a native file size of those dimensions (times three color layers), which amounts to approximately 3.4 million three-color pixels. However, it has been advertised as a 10.2 MP camera by taking into account that each photosite contains stacked red, green, and blue color-sensing photodiodes, or [[active pixel sensor|pixel sensors]] (2268 × 1512 × 3). By comparison, the dimensions of the photosite array in the 10.2 MP Bayer sensor in the Nikon D200 camera are 3872 × 2592, but there is only one photodiode, or one-pixel sensor, at each site. The cameras have equal numbers of photodiodes and produce similar raw data file sizes, but the [[Bayer filter]] camera produces a larger native file size via [[demosaicing]].

The actual resolution produced by the Bayer sensor is more complicated than the count of its photosites, or its native file size might suggest; the demosaicing and the separate anti-aliasing filter are both commonly used to reduce the occurrence or severity of color [[moiré pattern]]s that the mosaic characteristic of the Bayer sensor produces. The effect of this filter blurs the image output of the sensor which produces a lower resolution than the photosite count would seem to imply. This filter is mostly unnecessary with the Foveon X3 sensor and is not used<!-- In general or for a specific camera? -->. The earliest camera with a Foveon X3 sensor, the [[Sigma SD9]], showed visible luminance moiré patterns without color moiré.<ref>
{{cite web|url=http://www.dpreview.com/reviews/SigmaSD9/page23.asp|title=Sigma SD9 Review|work=DPReview|author=Phil Askey|date=November 2002}}
</ref>

Subsequent X3-equipped cameras have less aliasing because they include micro-lenses, which provide an [[Anti-aliasing filter#Optical anti-aliasing filter|anti-aliasing filter]] by averaging the optical signal over an area commensurate with the sample density. This is not possible in any color channel of a Bayer-type sensor. Aliasing from the Foveon X3 sensor is "far less bothersome because it's monochrome," said Norman Koren.<ref>
{{cite web|url=http://www.imatest.com/docs/print_sharpness.html|title=Sharpness: What is it and how is it measured?|work=Imatest docs|author=Norman Koren|access-date=2007-12-16}}
</ref> In theory, it is possible for a Foveon X3 sensor with the same number of photodiodes as a Bayer sensor and no separate anti-aliasing filter to attain a higher spatial resolution than that Bayer sensor. Independent tests indicate that the "10.2 MP" array of the Foveon X3 sensor (in the Sigma SD10) has a resolution similar to a 5 MP<ref>
''Popular Photography & Imaging'', Vol. 69, No. 6 (June, 2005), (table on p. 47).
</ref> or 6 MP<ref>
[http://www.dpreview.com/reviews/sigmasd10/page17.asp Digital Photography Review: Sigma SD10 Review], March 2004, Retrieved March 3, 2007.
</ref>
Bayer sensor. At low [[film speed|ISO speed]], it is even similar to a 7.2 MP<ref>
c|net Reviews, [http://reviews.cnet.com/Sigma_SD10/4505-6501_7-30588638.html Sigma SD10] Retrieved March 6, 2007.
</ref> Bayer sensor.

With the introduction of the [[Sigma SD14]], the 14 MP (4.7 MP red + 4.7 MP green + 4.7 MP blue) Foveon X3 sensor resolution is compared favorably by reviewers to that of 10 MP Bayer sensors. For example, Mike Chaney of ddisoftware says "the SD14 produces better photos than a typical 10 MP DSLR because it is able to carry sharp detail all the way to the 'falloff' point at 1700 LPI, whereas contrast, color detail, and sharpness begin to degrade long before the 1700 LPI limit on a Bayer based 10 MP DSLR."<ref>
{{cite web | url = http://www.ddisoftware.com/sd14-5d/ | title = Sigma SD14 Resolution: 14 MP? 4.6 MP? How does the SD14 stack up against high-end cameras like the Canon EOD 5D? | author = Mike Chaney | work=ddisoftware | year = 2007}}
</ref>

Another article judges the Foveon X3 sensor as roughly equivalent to a 9 MP Bayer sensor.<ref>{{cite web|url=http://www.popphoto.com/howto/4276/foveon-x3-sensor-claims-put-to-the-test-color-resolution-vs-b-w-resolution-page2.html |title=Foveon X3 Sensor Claims Put to the Test |url-status=dead |archive-url=https://web.archive.org/web/20080205032245/http://www.popphoto.com/howto/4276/foveon-x3-sensor-claims-put-to-the-test-color-resolution-vs-b-w-resolution-page2.html |archive-date=February 5, 2008 }}
</ref>

A visual comparison between a 14 MP Foveon sensor and a 12.3 MP Bayer sensor shows Foveon has crisper details.<ref>
{{cite web|author=Carl Rytterfalk|date=February 23, 2010|url=http://www.rytterfalk.com/2010/02/23/a-tiny-sigma-sd14-vs-nikon-d90-comparison/|title=A tiny Sigma SD14 vs Nikon D90 comparison..|website=Carl Rytterfalk Photography|url-status=dead|archive-url=https://web.archive.org/web/20100715011940/http://www.rytterfalk.com/2010/02/23/a-tiny-sigma-sd14-vs-Nikon-d90-comparison/|archive-date=2010-07-15|access-date=May 20, 2010}}
</ref>

===Noise===
The Foveon X3 sensor, as used in the Sigma SD10 camera, has been characterized by two independent reviewers as noisier than the sensors in some other DSLRs using the Bayer sensor at higher [[film speed|ISO film speed equivalents]],<ref>
See, ''e.g.'', c|net Reviews, [http://reviews.cnet.com/Sigma_SD10/4505-6501_7-30588638.html Sigma SD10] Retrieved March 6, 2007 and Steve's Digicams [http://www.steves-digicams.com/2003_reviews/sigma_sd10_pg7.html Sigma SD10 review] (November 28, 2003) Retrieved March 6, 2007.
</ref> [[colorfulness|chroma]] noise in particular.<ref>Camera Labs: [http://www.cameralabs.com/reviews/Sigma_DP1/sample_images.shtml Sigma DP1 Gallery]
</ref><ref>
[http://reviews.photographyreview.com/blog/sigma-dp1-review/ Sigma DP1 Review], photographyreview.com
</ref> Another noted higher noise during long exposure times.<ref>Imaging Resource [http://www.imaging-resource.com/PRODS/SSD10/SD10A12.HTM Sigma SD10 review] (First posted 10-22-03.). Retrieved March 6, 2007.</ref><ref group="n">This observation is consistent with a comparison of the images, displayed in ''Digital Photography Review,'' taken by the Sigma SD10 ([http://www.dpreview.com/reviews/sigmasd10/page14.asp see here]) with those taken approximately contemporaneously of the same scene by the Bayer sensor-equipped Nikon D70 ([http://www.dpreview.com/reviews/nikond70 see here])/page15.asp. Both retrieved March 6, 2007.</ref> However, these reviewers offer no opinion as to whether this is an inherent property of the sensor or the camera's image-processing algorithms.

With regards to the Sigma SD14, which uses a more recent Foveon X3 sensor, one reviewer judged its noise levels as ranging from "very low" at ISO 100 to "moderate" at ISO 1600 when using the camera's [[Raw image format]].<ref>
{{cite web | url =http://www.popphoto.com/gear/2008/12/camera-test-sigma-sd14 | title = Camera Test: Sigma SD14 | author = Michael J. McNamara|date=December 2008 |access-date=June 6, 2013 }}
</ref>

===Sample images===
Sigma's [[Sigma SD14|SD14]] site has galleries of full-resolution images showing the color produced by the Foveon technology. The 14 MP Foveon chip produces 4.7 MP native-size RGB files; 14 MP [[Bayer filter]] cameras produce a 14 MP native file size by interpolation (i.e., demosaicing). Direct visual comparison of images from 12.7 MP Bayer sensors and 14.1 MP Foveon sensors show Bayer images are superior on fine monochrome detail, such as the lines between bricks on a distant building, but the Foveon images are superior in color resolution.<ref>{{cite web | title = Sigma SD14 Resolution: 14 MP? 4.6 MP? | author = Mike Chaney | date = March 16, 2007 | url = http://www.ddisoftware.com/sd14-5d/ | work = Digital Domain Inc.}}</ref>

{{Commons category multi|Taken with Sigma SD14}}