Digital scan back
Encyclopedia
A scanning back is a type of digital camera back
. Digital imaging
devices typically use a matrix of light-sensitive photosensors, such as CCD
or CMOS
technologies. These sensors can be arranged in different ways, like a Bayer filter
, where each row captures RGB components, or using one full-sized layer for each color, such as the Foveon X3 sensor
. The main difference between both approaches is in the way the final image is constructed. In a Bayer filter
, since each pixel can only record one color, the resulting pixel has to be constructed with information from its neighbouring pixels using demosaicing
algorithms. Using one layer per component instead measures all color components for each pixel, so the resulting pixel is the value measured in every layer for that pixel. This approach delivers better image quality
, because all information is actually measured, as opposed to being guessed. The downsides compared to a Bayer filter are that it is much more expensive, because N layers of sensors have to be used, and the resulting digital image is much smaller (by a factor of N). However, since the image is more accurate, more work can be done on it. An image from a Bayer filter is usually downsampled to improve quality, and when upsampling it shows artifacts much faster.
A digital scan back takes a similar approach to the second type of photosensor, but instead of using one matrix for each component, it uses one array per component. This translates to a 3xN sensor matrix, where N is typically a large number (between 5,000 for earlier models and 15,000 for newer models), which is then placed vertically in a holder. To take an image, the sensor travels the x axis, taking one exposure
per point.
film cameras could print to similar sizes. Scan backs also have the advantage of not being subject to light fall-off due to off-axis lens positions, so wide angle lenses and perspective shifts on the camera can be used without issue. A somewhat less obvious advantage lies in that scanning backs are typically created using trilinear CCD's. This means that for every pixel position a separate measurement is taken for red then green then blue. This results in a much higher effective resolution than a similar resolution image created by a mosaic sensor such as those on nearly all more typical digital cameras. (With the notable exception of foveon
)
Another downside is that most of these backs have to be used tethered to a computer. One reason is that there would be no other way to know when critical focus has been achieved, and the second reason is the huge file sizes, measured in hundreds of megabyte
s or even gigabyte
s. High end 4 GB
Compact Flash cards can only store a handful of images.
Assume each individual pixel has a width and height of 6 μm per 6 μm. The array will be 6 μm wide and 6 cm high. Now we place this array in a holder 6 cm high per 6 cm wide. That means we can evenly divide the x axis in 10,000 points, so the array has to take 10,000 exposures. To capture an image the sensor would start at x=0, take one exposure at the selected shutter speed, move to x=1, take one exposure at the selected shutter speed, and so on until x=9,999.
Now instead of doing it with one array, we do it with three arrays at the same time (one for each component in RGB), where the first array would be making an exposure at x, the second at x-1, and the third at x-2. The resulting image would be 10,000 x 10,000, or 100 million pixels, with full color information for each pixel.
or almost three and a half minutes. A real sensor would have to take into account an accurate movement to the next point, stopping and waiting to reduce vibration, take the exposure, and so on. This overhead is required, and can double or triple this time.
The newest sensors can achieve even bigger file sizes, up to several gigabytes.
Digital camera back
A digital camera back is a device that attaches to the back of a camera in place of a film holder and contains an electronic image sensor. This lets cameras that were designed to use film take digital photographs...
. Digital imaging
Digital imaging
Digital imaging or digital image acquisition is the creation of digital images, typically from a physical scene. The term is often assumed to imply or include the processing, compression, storage, printing, and display of such images...
devices typically use a matrix of light-sensitive photosensors, such as CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
or CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
technologies. These sensors can be arranged in different ways, like a Bayer filter
Bayer filter
A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...
, where each row captures RGB components, or using one full-sized layer for each color, such as the Foveon X3 sensor
Foveon X3 sensor
The Foveon X3 sensor is a CMOSimage sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductorand Dongbu Electronics....
. The main difference between both approaches is in the way the final image is constructed. In a Bayer filter
Bayer filter
A Bayer filter mosaic is a color filter array for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camcorders, and scanners to create a color image...
, since each pixel can only record one color, the resulting pixel has to be constructed with information from its neighbouring pixels using demosaicing
Demosaicing
A demosaicing algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array...
algorithms. Using one layer per component instead measures all color components for each pixel, so the resulting pixel is the value measured in every layer for that pixel. This approach delivers better image quality
Image Quality
Image quality is a characteristic of an image that measures the perceived image degradation . Imaging systems may introduce some amounts of distortion or artifacts in the signal, so the quality assessment is an important problem.-In photographic imaging:In digital or film-based photography, an...
, because all information is actually measured, as opposed to being guessed. The downsides compared to a Bayer filter are that it is much more expensive, because N layers of sensors have to be used, and the resulting digital image is much smaller (by a factor of N). However, since the image is more accurate, more work can be done on it. An image from a Bayer filter is usually downsampled to improve quality, and when upsampling it shows artifacts much faster.
A digital scan back takes a similar approach to the second type of photosensor, but instead of using one matrix for each component, it uses one array per component. This translates to a 3xN sensor matrix, where N is typically a large number (between 5,000 for earlier models and 15,000 for newer models), which is then placed vertically in a holder. To take an image, the sensor travels the x axis, taking one exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
per point.
Advantages
The main advantages of this technology are the extremely high image quality and the huge resulting files. This translates to very accurate color reproduction, because every pixel is measured individually, allowing printing in very large sizes without loss of detail. Previously only large formatLarge format
Large format refers to any imaging format of 4×5 inches or larger. Large format is larger than "medium format", the 6×6 cm or 6×9 cm size of Hasselblad, Rollei, Kowa, Pentax etc cameras , and much larger than the 24×36 mm frame of 35 mm format.The main advantage...
film cameras could print to similar sizes. Scan backs also have the advantage of not being subject to light fall-off due to off-axis lens positions, so wide angle lenses and perspective shifts on the camera can be used without issue. A somewhat less obvious advantage lies in that scanning backs are typically created using trilinear CCD's. This means that for every pixel position a separate measurement is taken for red then green then blue. This results in a much higher effective resolution than a similar resolution image created by a mosaic sensor such as those on nearly all more typical digital cameras. (With the notable exception of foveon
Foveon
Foveon, Inc., is the company that makes the Foveon X3 sensor, which captures images in digital single-lens reflex cameras such as the Sigma Corporation SD9, SD10, SD14 and SD15 as well as in the compacts DP1, DP2 and Polaroid X530....
)
Disadvantages
The downside of capturing images this way is the amount of time it takes. Even at the fastest speed, the time taken to make a complete exposure is measured in seconds or minutes, because even though the shutter speed could be 1/1000 s, it has to be taken literally thousands of times. This makes it very inappropriate for moving subjects, such as sports, nature, or city life, and is practically restricted to still life, art reproduction, and landscapes.Another downside is that most of these backs have to be used tethered to a computer. One reason is that there would be no other way to know when critical focus has been achieved, and the second reason is the huge file sizes, measured in hundreds of megabyte
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
s or even gigabyte
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
s. High end 4 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...
Compact Flash cards can only store a handful of images.
Example
Let's say we have a 10,000 pixel array, and we want to take an image with a shutter speed of 1/50s, and 48 bit per pixel (16 bit per component) to achieve maximum quality.Assume each individual pixel has a width and height of 6 μm per 6 μm. The array will be 6 μm wide and 6 cm high. Now we place this array in a holder 6 cm high per 6 cm wide. That means we can evenly divide the x axis in 10,000 points, so the array has to take 10,000 exposures. To capture an image the sensor would start at x=0, take one exposure at the selected shutter speed, move to x=1, take one exposure at the selected shutter speed, and so on until x=9,999.
Now instead of doing it with one array, we do it with three arrays at the same time (one for each component in RGB), where the first array would be making an exposure at x, the second at x-1, and the third at x-2. The resulting image would be 10,000 x 10,000, or 100 million pixels, with full color information for each pixel.
- The total exposure time will be at least:
or almost three and a half minutes. A real sensor would have to take into account an accurate movement to the next point, stopping and waiting to reduce vibration, take the exposure, and so on. This overhead is required, and can double or triple this time.
- The final image size would be:
The newest sensors can achieve even bigger file sizes, up to several gigabytes.
Manufacturers
- Better LightBetter LightBetter Light, Inc. is a company in San Carlos, California, U.S.A. that manufactures high-end digital scanning backs for view cameras, which are used in many photographic applications including; landscape, commercial advertising, fine art reproduction and architecture.Better Light scan backs...
, specializes in digital scan backs. - Phase One, has the PowerPhase FX+ in their lineup.
- Anagramm.
- Kigamo.
- PentaconPentaconPentacon is the company name of a once-important camera manufacturer in Dresden, Germany.The name Pentacon is derived from the brand Contax of Zeiss Ikon Kamerawerke in Dresden and Pentagon, as a Pentaprism for Single-Lens Reflex cameras was for the first time developed in Dresden...
, German company has a range of scanning cameras including the Scan 3000 & 5000 series.