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Imatest Documentation Imatest Instructions -- Tone, color, and spatial modules Using Stepchart Stepchart measures the tonal response, noise, dynamic range, and ISO sensitivity of digital cameras and scanners using reflection step charts such as the Kodak Q-13 and Q-14 Gray Scales, the Jessops Colour & Mono Separation Guide (UK), several from Danes-Picta (Czech republic), charts created by Test Charts (which can be printed any size), or transmission step charts from Stouffer, Applied Image, Kodak, DSC Labs, and Danes-Picta. Transmission charts are required for measuring dynamic range. Charts from Applied Image and additional ISO charts, shown below. (Imatest Master only) Stepchart can also measure veiling glare (lens flare). New in Imatest 3.5 ISO sensitivity is calculated when the incident lux level is entered. New in Imatest 3.1 Highly distorted images can be analyzed if automatic patch location is turned off. Several images can be combined (averaged) to facilitate fixed pattern noise measurement. New in Imatest 2.7 A new Dynamic Range postprocessor allows dynamic range to be measured from several exposures of a reflective step chart, which can be much more convenient than using a transmission chart. The Kodak Q-13 Gray Scale is an 8 inch long chart consisting of 20 zones, labeled 0-19, which have optical densities from 0.05 to 1.95 in steps of 0.1 (reflectances from 0.891 to 0.011). The Kodak Q-14 is identical, except that it's 14 inches long. The chart is printed on a (semigloss) surface. The Jessops chart, from the UK, is similar to the Q-13, except that it has only 18 zones. This image of the Q-13 Gray Scale was photographed slightly out of focus to minimize noise. The Q-13 has finer steps and a higher maximum density than the GretagMacbeth ColorChecker (Dmax = 1.95 vs. 1.5), mostly because of its glossier surface. It considerably less expensive. The longer Q-14 is well suited for photographing near ColorCheckers or slanted-edge SFR test targets. Iliah Borg has analyzed the Q-13 Gray Scale. He says, "I'm pretty sure it's screen-printed, most likely with automotive enamel. The spectral response is flat from 420 to 730nm, similar to titanium dioxide mixed with carbon in different proportions to achieve different reflectivities. The layer is pretty thick, to isolate from the substrate." Applied Image and ISO targets in Imatest Master Several monochrome targets, shown below, can be analyzed with Imatest Master. To run them, open the image file, then select the approximate region of interest for the key features in the target, following the instructions in Stepchart with Applied Image/ISO targets. Additional dialog boxes allow you to specify the target type and refine the crop. 266 of 451
Imatest Documentation QA-61 ISO-16067-1 Scanner QA-62 20-patch OECF targets ST-51 EIA Grayscale ITE Grayscale ST-52 ISO-14524 12-patch OECF target SFRplus 20-patch stepchart ISO-15739 Noise target Stepchart also analyzes transmission (i.e., film) charts, which have higher maximum densities than reflective charts, making them suitable for measuring the dynamic range of digital cameras and scanners. Photographing the chart Light. Be sure the light is even and free of glare. Lighting uniformity can be measured with an illuminance (Lux) meter. A variation of no more than ±5% is recommended. As little as possible should come from behind the camera— it can cause glare and flatten image tones. At least two lamps with an incident angle of about 20-40° are recommended. Lighting recommendations can be found in The Imatest Test Lab. Be especially careful to avoid glare in the dark zones on the right of the Q-13/Q-14 charts; it can hard to avoid with wide angle lenses. The light should not emphasize the texture of the chart, which would cause erroreous noise measurements. If possible, the surroundings of the Step chart should be gray or black to minimize flare light (unless you are measuring flare light). An 18% gray surround is recommended for cameras with auto-exposure. Focus. The chart may be photographed slightly out of focus to minimize noise measurement errors due to texture in the patches. Slightly is emphasized— the boundaries between the patches must remain distinct. Distance. Distance is not critical. The noise analysis will be most accurate if the total cropped chart length is at least 1000 pixels (about 50 pixels per zone), about 1/3 the width of a 6 megapixel image. Increasing the size improves the accuracy of the noise measurement up to a point; there is little improvement over 2000 pixels wide. Useful (though less accurate) noise analysis results may be obtained with total cropped length as small as 400 pixels. The tonal response can be measured with very small images, as long as the zones are distinctly visible. There is no need to fill the frame with the chart image. Filling the frame may reduce accuracy if there is significant vignetting (light falloff due to the lens system, which is often quite large in wide-angle lenses). Photograph the chart. Save the image as a RAW file, TIFF, or high quality JPEG, then load it on your computer. Raw images can be converted using dcraw. If you are using another RAW converter, convert to JPEG (maximum quality), TIFF (without LZW compression), or PNG. If you are using film, develop and scan it. Running Stepchart Run Imatest. Click on the Stepchart button in the Imatest main window. Open the input file using the dialog box. Imatest remembers the directory name of the last input file opened (for each module, individually). Multiple file selection Several files can be selected in Imatest Master using standard Windows techniques (shift-click or control-click). Depending on your response to the multi-image dialog box you can combine (average) several files or run them sequentially (batch mode). Combined (averaged) files are useful for measuring fixed-pattern noise (at least 8 identical images captured at low ISO speed are recommended). The combined file can be saved. Its name will be the same as the first selected file with _comb_n appended, where n is the number of files combined. Batch mode allows several files to be analyzed in sequence. There are three requirements. The files should (1) be in the same folder, (2) have the same pixel size, and (3) be framed identically. 267 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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