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Imatest Documentation When the Stepchart calculations are complete, the Save Stepchart Results? dialog box appears. It allows you to select figures to save and choose where to save them. The default is subdirectory Results in the data file directory. You can change to another existing directory, but new results directories must be created outside of Imatest— using a utility such as Windows Explorer. (This is a limitation of this version of Matlab.) The selections are saved between runs. You can examine the output figures before you check or uncheck the boxes. Figures, CSV, and XML data are saved in files whose names consist of a root file name with a suffix and extension. The root file name defaults to the image file name, but can be changed using the Results root file name box. Be sure to press enter. Checking Close figures after save is recommended for preventing a buildup of figures (which slows down most systems) in batch runs. After you click on Yes or No , the Imatest main window reappears. Dynamic range of cameras and scanners Save dialog box The Dynamic Range module calculates dynamic range from several reflective stepchart images, which are easier to work with than transmission step charts. Dynamic range (DR) is the range of tones over which a camera responds. It is usually measured in f-stops, or equivalently, zones or EV. (It can also be measured in density units, where one density unit = 3.322 f-stops.) It can be specified in two ways: The total range. Stepchart is extremely sensitive at detecting a camera's total dynamic range, even when dark areas are extremely noisy or boundaries between chart zones become indistinct. A range of tones over which the RMS noise, measured in f-stops (the inverse of the signal-to-noise ratio, SNR), remains under a specified maximum value. The lower this value (the higher the minimum SNR), the better the image quality but the smaller the dynamic range. Noise tends to be worst in the darkest regions. Imatest calculates the dynamic range for several maximum RMS noise levels, from 0.1 f-stop (high image quality; SNR = 10) to 1 f-stop (low quality; SNR = 1). The dynamic range corresponding SNR = 1 (1 f-stop of noise) corresponds to the intent of the definition of ISO Dynamic range in section 6.3 of the ISO noise measurement standard: ISO 15739: Photography — Electronic still-picture imaging — Noise measurements. The Imatest measurement differs in several details from ISO 15739; hence the results cannot be expected to be identical. Imatest may well produce more accurate results because it measures DR directly from a transmission chart, rather than extrapolating results for a reflective chart with maximum density = 2.0. Change in dynamic range definition (Imatest 1.5.5, November 24, 2005) The definition of total dynamic range now includes indistinct zones (dark zones that the original Stepchart algorithm had difficulty detecting). This may cause some short-term confusion because Figure 2 will change: total DR will sometimes increase. But it better represents true camera performance. 274 of 451
Imatest Documentation A camera's (or scanner's) dynamic range can be accurately measured using a transmission step chart illuinated from behind by a lightbox. Reflection step charts such as the Kodak Q-13 or Q-14 are inadequate because their density range of around 1.9 is equivalent to 1.9 * 3.32 = 6.3 f-stops, well below that of digital cameras. Transmission step chart The table below lists several transmission step charts, all of which have a density range of at least 3 (10 f-stops). Kodak Photographic Step Tablets can be purchased calibrated or uncalibrated. Uncalibrated is usually sufficient. The Stouffer charts are attractively priced. Product Steps Density increment Dmax Size Kodak Photographic Step Tablet No. 2 or 3 21 0.15 (1/2 f-stop) 3.05 1x5.5" (#2) larger (#3) Stouffer Transmission Step Wedge T2115 21 0.15 (1/2 f-stop) 3.05 0.5x5" Stouffer Transmission Step Wedge T3110 31 0.10 (1/3 f-stop) 3.05 3/4x8" Stouffer Transmission Step Wedge T4110 41 0.10 (1/3 f-stop) 4.05 1x9" Danes-Picta TS28D (on their Digital Imaging page) 28 0.15 (1/2 f-stop) 4.2 10x230 mm (0.4x9") DSC Labs 72-dB 13-step Greyscale 13 0.30 ( 1 f-stop) 3.7 (large) Esser Test Charts TE 241 20 from table 4.1 (large) The Stouffer T4110 (13.3 f-stops range), Danes-Picta TS28D (13.6 f-stops range), or TE 241 (13.6 f-stops range) are recommended for digital SLRs, which can have dynamic ranges over 10 f-stops. The charts with Dmax = 3.05 = 10 f-stops are probably adequate for compact digital cameras with small pixel sizes (under 3 microns). Lightbox You'll need a lightbox that can evenly illuminate the transmission step chart. 8x10 inches is large enough. Avoid thin or "mini" models, which may not have even enough illumination. The Logan Tru-View 810/920 is inexpensive and quite adequate. Light boxes are widely available, though they may be hard to find on dealers' websites. On Adorama, click on Filing/Storage | Mounting & Viewing Equipment | Lightboxes & Loupes. On B&H, click on Projection & Viewing | General Presentation Equipment | Lightboxes, Loupes & Slide Viewers | Lightboxes & Slide Viewers | Lightboxes & Light Tables. T he Kyoritsu calibrated light sources, especially the pattern light boxes, available in the North America from C.R.I.S,, are worth checking out. Image Engineering/E sser Test Charts makes an excellent integrating sphere, ideal for use with the TE 241. To measure dynamic range, Prepare a fixture for mounting the the step chart. It should be large enough to keep stray light out of the camera. Stray light can reduce the measured dynamic range; it should be avoided at all costs. I made a fixture out of scrap mat board held together with Scotch magic tape and Elmer's glue. (That old mainstay, duct tape, wasn't quite right for the job.) Place the fixture and step chart on top of the lightbox— or any other source of uniform diffuse light. Photograph the chart in a darkened room. No stray light should reach the front of the target; it will distort the results. The sourroundings of the chart should be kept as dark as possible to minimize flare light. An example is shown above. The density difference between the darker zones is not very visible, but it shows up clearly in the measurements. I used a Kodak step wedge (density steps of 0.15) that I purchased in 1969! Use your camera's histogram to determine the minimum exposure that saturates the lightest region of the chart. Overexposure (or underexposure) reduces the number of useful zones. The lightest region should have a relative pixel level of at least 0.98 (pixel level 250 or 255); otherwise the full dynamic range of the camera will not be detected. If the lightest zone is below this level, and a transmission chart is selected, a Dynamic range warning is issued. For flatbed scanners with transparency units (TPUs, i.e., light sources for transparencies), you can simply lay the step chart down on the glass. Stray light shouldn't be an issue, though there is no harm in keeping it to a minimum. 35mm film scanners may be difficult to test since most can only scan 36mm 275 of 451
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