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Imatest Documentation relatively small (up to 13x19 inches) with several mounted on large foam boards (32x40 inches or larger). The larger grids (3x5 and up) are optimized for printing on wide format printers, and are available in Imatest Master only. The squares are tilted as shown to facilitate the measurement of Lateral Chromatic Aberration, which is done best using tangential (or near-tangential) edges near the boundaries of the image. SVG charts have a number of significant advantages over the standard ISO 12233 chart, shown on the right. Far less wasted area. The ISO chart was designed by committee. (I know some members who joke about it.) Less that 10% of its total area is useful for SFR measurements; 90% is wasted. Slanted-edges may not be available in important regions, such as the corners. Larger ROI location tolerance. Imatest SFR requires that ISO 12233 chart. ROIs (regions of interest) be selected prior to the run. On the ISO chart, very limited areas are usable for ROIs. If the ROI selection is made carelessly, interfering patterns may spoil the measurement. The relatively large squares in the SVG charts allow for considerable tolerance. The illustration below shows how the ROI can be shifted much further (vertically) for the SVG chart. The square in the ISO chart is very small: the ROI needs to be as large as possible to ensure good results. ISO chart ROI selection: Low (vertical) position tolerance. SVG chart ROI selection: High position tolerance. Better suited to automated testing. The spacing of the slanted squares, the absence of interfering patterns, and the large ROI selection tolerance in the SVG charts is highly compatible with automated ROI refinement, available with Imatest Master 2.6 (released January 2008). This feature allows regions to be automatically refined (shifted) in repeated runs, where it may not be possible to maintain exact chart alignment. Better contrast control: less clipping. The standard ISO chart is specified to have a minimum contrast ratio of 40:1, and is typically printed closer to 80:1. This often results in clipping in the shadows or highlights (image too close to levels 0 or 255; sometimes both) in contrasty digital cameras . It can also reduce the accuracy of MTF measurements because many cameras have complex tonal response curves (pictorially-pleasing "S" curves), which are hard to linearize. This problem has been recognized by the ISO committee, which is designing charts with much lower contrast for the revised standard (which will include several charts rather than trying to fit all measurements into one chart). A 10:1 contrast ratio is a good choice for a (relatively) high-contrast chart. "Previous target designs had several resolution features that, while visually and instinctively appealing, offered little to enable automated and robust camera evaluation. The predominance of high contrast features, lack of in situ neutral gray patches, absence of machine-vision registration features, and unnecessary complexity are, in retrospect, seen as design flaws." Applying and Extending ISO/TC42 Digital Camera Resolution Standards to Mobile Imaging Products by Don Williams and Peter D. Burns (both members of the original ISO 12233 committee) Lateral Chromatic Aberration (LCA). The vertical edges near the left and right borders are oriented tangentially to facilitate LCA measurement. Nonlinear image processing. Many (perhaps most) digital cameras employ some type of nonlinear signal processing to enhance image appearance. In practice this means that sharpness is increased (MTF is boosted at high spatial frequencies) in the proximity of contrasty edges and noise is reduced (MTF is cut at high spatial frequencies) if there are no contrasty edges nearby. Hence, low and high contrast edges often have very difference MTF response. The SVG charts can have two contrasts (primary and secondary), as illustrated below: primary contrast is 20:1; secondary contrast (in two sections) is 2:1. Tones have been inverted to illustrate one of the many options. 380 of 451
Imatest Documentation SVG SFR chart: 4 x7 grid, inverted tones, 20:1 primary (2:1 secondary) contrast, with optional step pattern (0.1 density increment). Imatest can supply custom SVG test charts in a variety of sizes. Contact for details. SVG Squares & Wedges (for printing letter size, A4, etc.) This pattern consists of tilted squares, "hyperbolic" wedges, bars of varying spatial frequencies, and a replica of the Kodak Q-13 step chart with density steps of 0.1. It is optimized for printing A4 or letter size on a high quality inkjet printer and mounted on a 3x3 grid, as shown on the right— though it can be printed any size without loss of quality. This arrangement it will have a field width of around 32 inches (80 cm), sufficient for measuring the performance of the best modern DSLRs. If you require a chart printed on a single sheet or if you plan to do automated testing, one of the m x n slanted square on grid charts described above (preferably 5x9) is recommended. The chart below was created using Contrast ratio = 20, Gamma = 2.2, Highlight color = White, and Chart lightness = Light. The highlight and dark pixel values are 243 and 62, respectively, resulting in a display contrast of (243/62) 3x3 chart arrangement (for letter/A4 prints) 2.2 = 20.2 at display gamma = 2.2. One or two of the corner charts could have very low contrast (2 or lower) to better characterize cameras with nonlinear signal processing. A contrast ratio of 20 is high enough to turn on the maximum sharpening in cameras with nonlinear signal processing (which sharpen only in the presence of edges), but not so high that it is likely to cause clipping (pixel levels 0 or 255). It is half the minimum value of 40 in the soon-to-be-replaced ISO 12233 standard, now understood to be too high for reliable results. We also recommend printing a low contrast target— with contrast = 2 or lower— to better characterize camera performance. Results at contrasts = 20 and 2 will differ— often strongly— in the presence of nonlinear signal processing, which is nearly universal in consumer digital cameras. 381 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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