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Imatest Documentation Dynamic Range Calculate Dynamic Range from several Stepchart images Introduction to Dynamic Range Thanks for Jonathan Sachs for suggesting this module. Dynamic Range is a postprocessor for Stepchart that calculates a camera's dynamic range— the range of exposure it can capture at a specified quality level— from up to four exposures of reflective step charts such as the Kodak Q-13 and Q-14. In most instances this approach is more convenient than using a transmission step chart, which requires an even light source and must be photographed in a totally darkened room. As with all Imatest modules, a great many display options are available. We start by describing the operation of Dynamic Range. Dynamic range is explained in detail below. Dynamic Range reads up to four CSV files created by Stepchart. The files should be the results for four stepchart images taken with the same camera and lens but with exposures separated by 2-4 f-stops. aligns the x-axis (log exposure) to represent the relative exposures. The resulting range is far larger than the density range of a single reflective chart— about 6.5 f-stops. calculates dynamic range based on f-stop noise used in Stepchart) or pixel noise (calculated in Stepchart but not used for dynamic range). Operation Example of Dynamic range output showing f-stop noise and corresponding dynamic ranges Capture several images of a reflective test chart, with each image differing in exposure by 1 or 2 f-stops (EV). The total range from the most to least exposed image should be at least 6 f-stops (7 or 8 doesn't hurt). The least exposed images will appear nearly black, but they should contain some detail. The lightest patch in the most exposed image should be saturated (R, G, and B = 255 in 24-bit color images or 65515 in 48-bit color images. Be sure the light is glare-free. The Imatest Test Lab page has a great many valuable recommendations. To determine a camera's ultimate potential, store the image files in RAW format for later conversion. You may also with to capture and store JPEG files for comparison— to see how much dynamic range is lost with in-camera JPEG conversion. +1 f-stop exposure (first patch saturated) -2 f-stop exposure -5 f-stop exposure (some detail is present, but barely visible) Images of Stepchart and GretagMacbeth Colorchecker at +1, -2, and -5 f-stop exposure. (Images were taken at 1 f-stop increments from +1 to -6.) 284 of 451
Imatest Documentation Results If RAW images are to be analyzed, convert the to 16-bit TIFF format for best results. You may use any RAW converter. Be sure to record the settings, which affect the tonal response curve. Noise reduction (one of the functions of RAW converters) also has an effect on the measured dynamic range. Analyze the images with Stepchart. Be sure to save the CSV output files. Open Dynamic Range by clicking on the button on the left of the Imatest main window. The following welcome screen appears. It may contain instructions that are more up-to-date than the image below. Previously-entered CSV files are read and stored. Dynamic Range opening screen Read the CSV results files generated by Stepchart into windows A, B, C, and/or D. (One or two of the windows can be empty. None can be entered if needs be.) The files should be for exposures separated by 2-4 f-stops (EV), with 3 about optimum. The number of detected patches is shown to the right of the window ( N 20 20 17 10 ). If results have not already appeared, click on Calculate . Several displays and display options are available. They are selected in the lower-right region of the Dynamic Range window. Display Pixel levels vs. exposure, where exposure = -log10(patch density). Log Pixel levels vs. exposure, where Log denotes log10. Local gamma: d(Log pixel level) / d(Log exposure), i.e., the slope of the Log pixel level plot. Gamma is contrast. f-stop noise: Noise referenced to the image. S/N (1/f-stop noise) SNR (1/f-stop noise) dB = 20 log10(1/f-stop noise). Pixel S/N (Pixel level/noise). Displayed optionally by Stepchart. Referenced to the file. Pixel SNR dB = 20 log10(Pixel level/noise) Exif data Channel (Imatest Master only) Selects color channel: Y (luminance), R, G, or B. X-axis scale 285 of 451
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