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Imatest Documentation Target alignment Make sure the corners, as well as the center, are in focus. Illumination Should be as even as possible. Exposure Take care not to clip shadows or highlights. This will reduce accuracy of the results. Raw conversion and settings The choice of RAW converter (in or out of the camera) and settings, particularly Sharpening, can make a huge difference. Contrast and White balance are also important. Settings that affect contrast and transfer curve can also have a strong effect. If possible a "Linear" setting (meaning a straight gamma curve with no additional tonal response adjustments) should be used. Gamma SFR sharpness results are moderately sensitive to the Gamma setting: A 10% gamma error changes MTF50 by 2.5%. For best results gamma should be measured by running Colorcheck or Stepchart. Ideally a Q-14 target (similar to the Q-13, but larger) should be mounted close to the slanted edge images. Cleanliness and filters File formats Lens settings Lens surfaces should be clean. You should note whether you have a protective (UV or Skylight) filter. It can make a difference— more likely reduced contrast than reduced sharpness. With Imatest you can find out. Use RAW, TIFF, or the highest JPEG quality. Never use less than the maximum resolution or JPEG quality unless you are specifically testing the effects of these settings. Lens performance is a strong function of the aperture (f-stop) and focal length (for zooms). Be sure to record these settings (easy because they're saved with the EXIF data) and include them in your writeup. The optimum (sharpest) aperture is of particular interest. Lens performance is also somewhat affected by the distance to the target. White balance Should be close as possible to neutral, particularly in Colorcheck. This may seem like a lot of fuss, but the technique you develop in testing cameras and lenses will spill over to your daily photography. Alfred Stieglitz tested film and developers extensively when he discovered photography as a student in Berlin. Ansel Adams performed extensive tests in the development of his zone system. Although nobody would claim that testing is responsible for their unique vision, it certainly contributed to the skill that transformed their vision into prints of transcendent beauty. 94 of 451
Imatest Documentation RAW files RAW files The unprocessed digital output of an image sensor is called a RAW file. In this document we sometimes refer to it as Camera RAW to distinguish it from Bayer RAW files, which are standard image files that contain undemosaiced (Bayer) data. New in Imatest 3.5.2: Generalized Raw read capability had been added. You can specify a file extension and read properties for files with this extension. The file will be interpreted as a Bayer RAW file. New in Imatest 3.5: the dcraw front-end interface has been updated to include an auto white level checkbox (should be unchecked for sensitivity measurements) and a gamma setting (the old defaults, 0.45 for bit depth = 8 and 1.0 or bit depth = 16 no longer apply.) The pixels in most digital image sensors are covered with a Bayer Color Filter Array (CFA), which makes each pixel sensitive to a single primary color, Red, Green, or Blue. The order of the colors is RGRGRG..., GBGBGB..., etc. There are twice as many greens as reds or blues because the eye is most sensitive to green. The four possible pixel arrangements (Red locations) are shown in the diagram below, where pixel locations start with Row 1 Column 1 (R1C1) in the upper left corner. There is no universal standard for mapping colors (R, G, or B) to pixel position (RmCn). The arrangement can be determined by examining a Bayer RAW image file with the Rawview utility, which can be called from a button on the right side of the Imatest main window. R1C1 R1C2 R1C3 R1C4 R2C1 R2C2 R2C3 R2C4 R3C1 R3C3 R3C3 R3C4 R4C1 R4C2 R4C3 R4C4 Red in R1C1 R1C1 R1C2 R1C3 R1C4 R2C1 R2C2 R2C3 R2C4 R3C1 R3C3 R3C3 R3C4 R4C1 R4C2 R4C3 R4C4 Red in R2C1 R1C1 R1C2 R1C3 R1C4 R2C1 R2C2 R2C3 R2C4 R3C1 R3C3 R3C3 R3C4 R4C1 R4C2 R4C3 R4C4 Red in R1C2 R1C1 R1C2 R1C3 R1C4 R2C1 R2C2 R2C3 R2C4 R3C1 R3C3 R3C3 R3C4 R4C1 R4C2 R4C3 R4C4 Red in R2C2 Most camera RAW formats are proprietary to the camera manufacturer (e.g., CR2 for Canon and NEF for Nikon) and must be converted into standard file formats (JPEG, TIFF, PNG, etc.) for most applications. This usually involves demosaicing, which is a form of interpolation that fills in the two missing colors in each pixel. Demosaicing is the primary function of RAW converter programs (which may be either built into the camera or installed on a separate computer). In demosaicing, detail in the missing colors is inferred from detail in other, neighboring colors. Algorithms can be very mathematically sophisticated. Demosaicing (RAW conversion) programs usually incorporate a number of secondary image processing steps, including 1. 2. 3. 4. applying a gamma curve, typically with a tonal response curve often applied on top of it, white balance and color adjustment, sharpening, usually in the neighborhood of contrasty features, and software noise reduction, typically in the absence of contrasty features. All these steps can affect Imatest results. To minimize their effects, Imatest uses dcraw, a freely available program for converting RAW images to useful file formats. Dcraw applies simple gamma curves (with no additional tonal response curves) and does not perform sharpening or noise reduction. RAW files in Imatest 95 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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Imatest Documentation Image Quality
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