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Scanning Backs...I Would Like to Know More!
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Q: How does one determine the megapixel rating for a scanning back? “The number of pixels on the image sensor which receive input light through the optical lens, and which are effectively reflected in the final output data of the still image.” This simple statement establishes that a “pixel” must be physically present on the image sensor, it must be light-sensitive, and the information it provides must be relevant to (if not directly present in) the final image data. This last qualification allows instant-capture digital cameras to include so-called “ring” pixels surrounding the area that actually becomes part of the final image, since the “ring” pixels are used to determine some characteristics of the final image. The JCIA guideline also considers devices with multiple image sensors, as well as scanning camera designs: “For DSCs [digital still cameras] that use an image sensor ( including a linear sensor) that capture an image by spatial or optical movement, the number of effective pixels of the image sensor and the number of sampling positions should be noted. If the total number of sampled pixels (ring pixels can be included) is also noted, the description should clearly state that the noted number is the total number of effective pixels, to avoid consumer misunderstanding.” Just as instant-capture digital camera vendors can count all three colors of pixels physically present on their image sensors, Better Light can count all three colors of pixels physically present on the trilinear image sensors we use in our scanning backs. There are no “ring” pixels on our trilinear sensors, although there are additional pixels on the sensor that are not counted as “effective pixels”. To arrive at the “total number of effective pixels”, we multiply the number of “effective pixels” on the image sensor by the number of sampling positions.
Q: How many megapixels does each Better Light model have? All of our scanning back models have variable resolution, which will be the topic of another question. For each scanning back model, here are the maximum number of “effective pixels” on the image sensor, the maximum number of sampling positions, and the maximum “total number of effective pixels”. Note that 1 megapixel is equal to 1,000,000 pixels
Note that with the Super6K and Super8K models that provide “Enhanced Resolution”, the additional sampling positions, which do contribute to increased optical resolution, are counted, while the interpolated pixels along the CCD axis, which do not contribute to increased optical resolution, are not counted.
Q: How long does it take a scanning back to capture an image? Two factors determine the overall scanning time for any given image: the EXPOSURE TIME PER LINE, which is affected by the amount of light reaching the image sensor, and the NUMBER OF LINES in the image being captured. The EXPOSURE TIME PER LINE is called the “Line Time” setting, expressed in traditional photographic fractions of a second. The Line Time setting, in combination with the ISO sensitivity setting, determine the response of the scanning back to light reaching the sensor, which in turn is controlled by the amount of light on the scene, and the lens aperture setting. The Line Time setting is equivalent to the shutter speed setting in an instant-capture camera, but in a scanning system this exposure gets repeated for each line being recorded in the final image. The NUMBER OF LINES in the image is a function of the Resolution setting for a particular scanning back model, and of any cropping in the scan direction. Seamless panoramic scans made with Better Light’s Pano/WideView adapter can have up to 65,000 lines! Better Light scanning backs do not stop and start during each scan, as other systems typically must do to control their data flow, so the overall scan time for any Better Light device is always just the product of the exposure time per line multiplied by the number of lines in the overall scan. Better Light’s newest “HS” backs can capture images quickly when there is enough light to allow a short exposure time per line, but things slow down considerably at lower light levels, since the longer exposure time per line is multiplied by thousands of lines.
Q: How do I determine the exposure settings for a scanning back? Better Light’s ViewFinder control software uses traditional photographic terminology for controlling exposure, so it’s possible to use traditional light metering techniques to determine initial exposure settings. However, because ViewFinder software displays the digital (prescan) image data values in several easy-to-understand ways (in addition to a full-color picture), and also interactively displays what happens to these digital values when the exposure settings are adjusted, most users quickly learn to “guess” at their initial exposure settings, and then simply dial in the exact exposure after an initial prescan by observing the image data values. With a scanning back, the shutter in the lens is not used (or required) – the image sensor’s exposure is set via software and controlled electronically for each line of the image being recorded. This exposure time per line is called the “Line Time” setting, expressed in traditional photographic fractions of a second, at 1/3 f-stop intervals. The Line Time setting is equivalent to the shutter speed setting of an instant-capture camera, but in a scanning system this exposure gets repeated for each line being recorded in the final image.Better Light scanning backs also feature continuously-adjustable ISO sensitivity that can add up to four f-stops of exposure sensitivity boost at any Resolution setting. ISO sensitivity is selected in 1/10 or 1/3 f-stop increments, and can also be manually adjusted with 0.1CC precision. Color balance and contrast (Tone curve) adjustments may also have some affect on the overall exposure. Color balance is often pre-selected for a given situation, but can be quickly readjusted whenever necessary. Likewise, the Tone curve is often pre-selected for a given situation, but can quickly be modified or replaced entirely with a different Tone curve, as desired. Like the basic exposure adjustments of Line Time and ISO sensitivity, changes to color balance and Tone curve can be based on visual evaluation of the prescan image, and on numerical evaluation of the digital image data values. The Line Time setting, in combination with the ISO sensitivity setting, determine the response of the scanning back to light reaching the sensor, which in turn is controlled by the amount of light on the scene, and the lens aperture setting. Adjustments to Line Time and/or ISO sensitivity (and/or color balance and/or contrast) are immediately reflected in the existing prescan image data, but changes to the lens aperture setting, or in the amount of light on the scene, will require a fresh prescan to update the prescan image data. Once the prescan image data is properly exposed, the final image is captured using the same exposure settings.
Q: How can I compare scanning back and film exposure settings? A scanning back’s Line Time setting is equivalent to the shutter speed setting for a film exposure, and each recording medium also has an ISO sensitivity rating (adjustable with the scanning back). A scanning back’s range of Line Time and ISO sensitivity combinations can be compared to traditional film exposures using the following relationship: (scanning back’s ISO setting x Line Time setting) = (film ISO rating x shutter speed setting) Scanning backs have a somewhat limited range of Line Time settings – the LONGEST Line Time setting for all models is 1/8 second per line, while the SHORTEST Line Time setting for most models is 1/240 second per line, or 1/160 second per line for the Super8K-HS. However, scanning backs also have a fairly broad range of ISO sensitivity settings that helps to extend their usability – the two “E” models have an ISO range from 100 up to 1600, the 6000-HS and Super6K-HS have an ISO range from 200 up to 3200, and the Super8K-HS ranges from ISO 120 up to 2000. For example, a 6000-HS scanning back being operated at its lowest ISO (200) and fastest Line Time (1/240 sec.) would be equivalent to exposing ISO 100 film for 1/120 (typically 1/125) sec. This same scanning back could continue to operate at its fastest Line Time setting as its ISO sensitivity is increased to as high as ISO 3200, which would be equivalent to exposing ISO 100 film for 1/8 sec. Extending the scanning back’s Line Time setting to its maximum (1/8 sec.) while keeping ISO sensitivity at its maximum (3200) would be equivalent to exposing ISO 100 film for 4 seconds (not including any film reciprocity correction).
Q: Can I continue to use my existing large-format lenses? YES! The image area of a Better Light scanning back is slightly smaller than 4x5 inch film – it’s 72 x 96 mm (about 3 x 4 inches) for all models. This smaller image area produces a modest “lens factor” multiplier of 1.25, so (for example) a 120mm lens used with the scanning back would have the same coverage as a 150mm lens used with 4x5 film. Unlike most other digital cameras, the trilinear image sensors used in Better Light scanning backs do not have any particular problem with off-axis light rays, even at very steep angles, so any combination of lens focal length and view camera movements that would work with film can be used with the scanning back. A 47mm lens used with the scanning back would provide coverage equivalent to a 58mm lens used with 4x5 film, but the 47mm lens could still be used, right to the edge of its image circle. However, the increased clarity (freedom from film grain) of any Better Light scanning back model, coupled with the ease with which digital images can be magnified and closely scrutinized, may reveal some shortcomings of your lenses that have always been there, but have remained undetected until now. In particular, the common chromatic aberrations of axial color (failure to focus all colors to the same plane) and lateral color (failure to maintain the same image size for all colors) may be more detectable in all but the finest lenses. Image degradation towards the edges of the image circle may also be more apparent. Furthermore, the very high resolution of any Better Light scanning back model, coupled with the ease with which digital images can be magnified and closely scrutinized, may reveal some shortcomings in your technique that have always been there, but have remained undetected until now. In particular, using lens apertures smaller (higher f-number) than f-22 may start to show the effects of diffraction (loss of contrast in fine details), regardless of the lens being used. Likewise, minor errors in view camera setup and/or focusing that result in minor errors in focal plane position may be more apparent in a high resolution digital image. It is important to remember that one is typically able to view a digital image at much higher magnification than its eventual print size, and that if a particular lens and/or technique produces outstanding prints of a given size when using 4x5 film, the same lens and/or technique should produce outstanding prints of at least that size when using a scanning back, even if close inspection of the digital image data reveals a few imperfections.
Q: What do I need to use a scanning back in the field? Any of Better Light’s new “HS” scanning backs can readily be used in the field with the addition of a rechargeable battery. A laptop computer running a recent version of Windows XP or Mac OS-X, with a Hi-Speed USB2 port, is also necessary. A sturdy view camera and tripod will help reduce camera vibration in windy areas. In addition to your 4x5 view camera, lenses, and tripod, you’ll be carrying about eight pounds of Better Light equipment, plus your laptop computer. Your focusing cloth can double as a shade for viewing your laptop screen outdoors, or several types of collapsible laptop hoods are available for this purpose. An “HS” scanning back will operate for about five hours on a fully-charged Better Light battery – certain third-party batteries offer longer operating time and/or less weight, at somewhat higher cost. Solar panels can be used to recharge everything during extended outings. Additional image data storage capacity might also be necessary in some situations.
Q: What do I need to use a scanning back for fine-art reproduction? A Better Light scanning back is among the finest ways to capture fine art originals for reproduction. Ideally, the resolution capabilities of the scanning back should be sufficient to record the desired number of pixels per inch (not to be confused with printer dots per inch) across the largest originals you want to scan. However, even the smallest Better Light scanning back can deliver image quality superior to film or other digital capture devices, and this very clean image data can readily be interpolated to larger print sizes with outstanding results. Better Light scanning backs are upgradeable – any model can be upgraded to any higher-performance model at any time – so your digital capture capabilities can grow with your business while preserving your initial investment. In addition to the scanning back, appropriate lighting is an essential part of a fine-art scanning setup. Scanning backs require continuous illumination, which includes inexpensive (but inefficient) tungsten lighting, efficient (and increasingly inexpensive) fluorescent lighting, and state-of-the-art high-intensity discharge (HID) lighting, among others. Larger subjects will require proportionally larger light sources, and shiny subjects often require cross-polarization to control specular reflections, an effective but inefficient technique that requires significantly more light to begin with. High-quality lighting with a modest Better Light scanning back can often yield better results than a larger scanning back used with marginal lighting. A sturdy 4x5 inch camera on an equally sturdy support (tripod, camera stand, or copy stand), with a high-quality large-format lens, will also be necessary. The array of controls on a large-format view camera can be somewhat intimidating at first, but most of these will not be used (“zeroed out”) when scanning flat subjects. An accurate tool for aligning the camera parallel to the subject, such as a Zig-Align, is also an absolute must for this application. Because of the requirement for precise alignment of camera and subject, we recommend using a geared tripod head, rather than a pan/tilt head or ball head, to mount the camera to its support. Like the Zig-Align, the additional expense of a geared tripod head will quickly pay for itself in reduced setup time and consistently accurate results. Finally, most fine-art scanners learn to use third-party ICC profiling software with an appropriate color reference chart to make specific color profiles for the scanning back and lighting combination they are using. In terms of precise color accuracy, a properly-exposed image from a Better Light scanning back can be better than 90% “perfect” right out of the device, and a properly-made ICC profile might improve this color rendition to better than 95%, but an improperly-made profile can easily do more damage than good. Fortunately, digital camera profiling software has greatly improved in both quality and ease of use, and we can help you learn how to achieve optimum results from your entire image capture workflow.
Q: What kinds of lighting can be used with a scanning back? Scanning backs expose the image one set of lines at a time, so the subject must be continuously illuminated during the entire scan. This rules out the use of strobe or flash lighting, but nearly any kind of continuous light source can be used effectively with a Better Light system. Here are the most popular: Inexpensive tungten “hot” lights, available in a wide variety of shapes and sizes, work well with a scanning back. Tungsten lights consume a lot of AC power, and generate a lot of heat, but only need to be turned on during each prescan or scan, since these lights reach full output within a few seconds. Most tungsten lights are subject to changes in intensity if the AC mains voltage changes, and changes in subject lighting during a scan will appear as darker or lighter bands in the image. Accurate color rendition can be more of a challenge when using tungsten lighting, usually because of the high level of near-infrared from these lights being reflected from certain materials, particularly darker colors of dyed synthetics. Nonetheless, these sources are quite popular and effective. Fluorescent lighting is also available in a variety of shapes and sizes, although it’s hard to find a “focusing” (highly directional) fluorescent light. High-frequency “flicker-free” electronic ballasts are preferred, since these generally produce more light output for similar power consumption, in addition to allowing the use of faster Line Time settings without encountering “flicker” artifacts in images. Modern fluorescent lamps pack more power into smaller form factors, for increased brightness. Even inexpensive household fluorescent lights becoming commonly available as tungsten lamp replacements can be used successfully with a scanning back. The electronic ballasts in most fluorescent lights are regulated against modest changes in the AC mains voltage, so they will maintain a constant intensity even if the AC voltage changes. Color rendition of fluorescent sources can vary widely, but the better fluorescent lamps can provide very good color reproduction. High-intensity discharge (HID) lighting is a relatively new source, even more efficient and color-accurate than fluorescent lighting. Like fluorescent lighting, HID fixtures will require up to ten minutes of warm-up time after being turned on before they reach full, stable output. This usually means that these lights are left on for the duration of a photography session, but since they are about five times more efficient than tungsten lighting, this is often a very acceptable trade-off. HID fixtures should also have high-frequency electronic ballasts for the best regulation against AC voltage changes. Since HID lamps are small-area sources like tungsten lamps, they can be used in many of the same kinds of lighting fixtures, including focusing fixtures, for a wide variety of shapes and qualities of light. HID lamps feature long life (5,000 hours) and modest replacement cost. Copyright 2004-2008 Better Light, Inc. All Rights Reserved. |
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