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Imatest Documentation the camera, which can cause glare. Check for glare and lighting uniformity before you expose. A detailed description of the recommended lighting setup, which uses six high quality (CRI > 98) 4700K (near-daylight) 50W SoLux quartz-halogen lamps, can be found in The Imatest Test Lab. SoLux Task Lamps may also be used. The BK Precision 615 Light meter (Lux meter) is an outstanding low-cost instrument (about $100 USD) for measuring the intensity and uniformity of illumination. Distance Distance and field of view The camera must be far enough from the chart so you are measuring the sharpness of your camera and lens, not the chart. But remember, It's the field of view, not the chart-to-camera distance, that counts. A rough rule of thumb: For an inkjet-printed chart the field of view should be at least 22 inches (60 cm) for a 6-megapixel camera; 35 inches (90 cm) for a 16 megapixel camera. Details below. For a high-quality photographically-printed chart (such as the charts from Applied Imaging) you can get quite a bit closer. Using a chart printed on Premium Luster paper on the Epson 2200 (a high quality pigment-based inkjet photo printer), the MTF of the 6.3 megapixel Canon EOS-10D showed no change if the image field was at least 22 inches (56 cm) wide-- twice the length of the chart. Performance falls off slowly for smaller widths. Choose a camera-to-target distance that gives at least this image field width. The actual distance depends on the sensor pixel count and the focal length of the lens. Cameras with more pixels, and hence higher potential resolution, should should have a larger image field width. Some guidelines for the minimum field width are, Image field width (in inches) > 8.8 * sqrt(megapixels) ( > means "greater than.") Image field width (in cm) > 22 * sqrt(megapixels) -- or -- The distance to the target should be at least 40X the focal length of the lens for digital SLRs. (25X is the absolute minimum; 40X leaves some margin.) For compact digital cameras, which have much smaller sensors, the distance should be at least 100X the actual focal length (not the 35mm-equivalent): the field of view is about the same as an SLR with comparable pixel count. The recommended distance is described in geeky detail in Chart quality and distance. Exposure The camera-to-target distance is not critical as long as it is greater than a reasonable minimum. More on distance can be found in SFR Instructions Part 1. 254 of 451
Imatest Documentation Proper exposure is important for accurate Imatest SFR results. Neither the black nor the white regions of the chart should clip-- have substantial areas that reach pixel levels 0 or 255. The best way to ensure proper exposure is to use the histogram in your digital camera. Blacks (the peaks on the left) should be above the minimum and whites (the peaks on the right) should be below the maximum. The histogram, taken from the Canon File Viewer Utility, indicates excellent exposure. Tips on photographing the chart Distance doesn't matter as long as the target far enough from the camera so sharpness is limited by the camera and lens, not by the target. For a target printed on the Epson 2200 printer, a distance that gives at least a 24 inch (horizontal) field of view seems to be sufficient. The target should be evenly lit and free of glare. White balance should be approximately neutral. Use a sturdy tripod and a cable release. If possible, use the mirror lock. You can use Imatest SFR to find the difference made by a good tripod or mirror lock. Imatest SFR can sharpen your technique, literally ( pun intended ). Be sure to expose the image so detail is maintained in both light and dark areas. Neither should be blocked (clipped). Use your camera's histogram. If more than 0.5% of the pixels are at levels 0 or 255, Imatest SFR will assume that clipping has taken place and issue a warning message. This has no effect on the calculations-- it's just a warning that accuracy may be compromised. Be sure the camera is correctly focused on the chart. Imatest SFR can test the accuracy of your camera's autofocus. Place slanted-edge images near the corners of the field as well as near the center. Save the image as a RAW file or maximum quality JPEG. If you are using a RAW converter, convert to JPEG (maximum quality), TIFF (without LZW compression, which isn't supported), or PNG. If you are using film, develop and scan it. The file name should be descriptive and should indicate the parameters you are testing. Use dashes and underscores ( - and _ ), but try to avoid spaces, which work with Imatest, but can cause trouble in DOS command lines and web pages. An example of a file name is Canon_EOS10D_70-200f4L_100mm_f8.jpg. If the folder contains meaningless camera-generated file names such as IMG_3734.jpg, IMG_3735.jpg, etc., you can change them to meaningful names that include focal length, aperture, etc., with the View/Rename Files utility, which takes advantage of EXIF data stored in each file. Run Imatest SFR or Rescharts Slanted-edge SFR These instructions are excerpted from Using Imatest SFR, Part 2 . Start Imatest by double-clicking the Imatest icon on the Desktop, the Windows Start menu, or in the Imatest directory (usually C:\Program files\Imatest in English installations). After several seconds, the Imatest main window opens. Then, either Click SFR: New File . The SFR: Current file button is grayed out (inoperative) for the first run because no file has yet been read. It can be used in succeeding runs to save time when analyzing different regions, though multiple ROI (region of interest) runs are more efficient. Click Rescharts , then either Click Read image file if Slanted-edge SFR is the selected Chart type, or Select Slanted-edge SFR from the Chart type: dropdown window. The Rescharts Slanted-edge SFR module performs the same calculations as SFR, but with more interactive interface, described on the Rescharts page. Select the image file or batches of files When you click SFR: New file , the window below appears, requesting the image file name(s). The folder saved from the previous run appears in the Look in: box on the top. You are free to change it. You can open a single file by double-clicking on it. 255 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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