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Imatest Documentation Imatest Instructions -- Sharpness modules Using SFR Part 1 Setting up and photographing SFR targets The slanted-edge test image Imatest SFR measures the spatial frequency response, also known as the Modulation Transfer Function (MTF), of digital cameras and digitized film images. ISO 12233 test chart with Imatest SFR cropping indicated by the red rectangle. It uses a simple slanted-edge (black-to-white) target described in Sharpness: What is it and how is it measured? Although it doesn't provide the as strong a visual indication of MTF as the Log Frequency test chart, it provides a more accurate quantitative measurement. Imatest SFR also measures Chromatic aberration, noise and Shannon information capacity— a useful but unfamiliar indicator of image quality. Imatest SFR uses a standardized sharpening algorithm to compensate for the different amounts of sharpening in different cameras, allowing them to be fairly compared. These instructions also apply to the Slanted-edge SFR module of Rescharts, which is performs identical calculations to Imatest SFR, but has a more interactive interface. Origins of Imatest SFR The algorithms for calculating MTF/SFR were adapted from a Matlab program, sfrmat, written by Peter Burns ( ) to implement the ISO 12233 standard. Imatest SFR incorporates numerous improvements, including improved edge detection, better handling of lens distortion, and far more detailed output. A description can be found here. The original Matlab code is available on the I3A ISO tools download page by clicking on ISO 12233 Slant Edge Analysis Tool sfrmat 2.0. In comparing sfrmat 2.0 results with Imatest, keep the following in mind: If an OECF (tonal response curve) file is not entered into sfrmat, it assumes that there is no tonal response curve, i.e., gamma = 1. In Imatest, the default gamma is 0.5, which is typical of digital cameras. To obtain good agreement with sfrmat, you must set gamma to 1. 118 of 451
Imatest Documentation A horizontal slanted-edge for measuring vertical MTF performance is shown below; a vertical slanted-edge for measuring horizontal MTF performance is shown on the right. These edges were cropped from an image of the PIMA/ISO 12233 test chart taken by the Canon EOS-10D digital SLR, shown above. A crop area is indicated by a red rectangle. Imatest Master can analyze edges of nearly any angle. For best results, exact vertical, horizontal, and 45° edges should be avoided. Printing, assembling, and photographing the target Summary Obtain a test chart Print the chart or Display a pattern on an LCD screen. Assemble the printed charts into a target. Photograph the target. Tips: Lighting | Distance | Exposure | Photographing the target Test chart print quality You can run Imatest SFR with any clean, sharp, straight black-to-white or dark gray-to-white edge. The solid areas need to be smooth and uniform: white on one side and black or dark gray on the other. Since Imatest does not yet sell a printed chart, the quickest way to obtain a chart is to print one on a high quality inkjet printer. Obtain a test chart. Several alternatives are available for obtaining test charts. Download one of the test images below and print it on a high quality inkjet printer (details below) or send it to a lab for printing, for example, one of the labs listed by Dry Creek Photo. Create and print a test image with the Imatest Test Charts module. A variety of contrast ratios and highlight colors are available. Several Structured vector graphics (SVG) charts are available in Imatest Master. Some of the slanted square on an m x n grid patterns (example on right) are exceptionally well suited for printing large and for automated testing, and have numerous advantages over the ISO 12233 chart. Purchase a chart such as the ISO 12233 (QA-72) or QA-77 (available from Applied Image) or as a less expensive alternative from Danes-Picta in the Czech Republic (the DCR3 chart on their Digital Imaging page). The Applied Image charts are printed with much higher resolution than an inkjet printer can achieve, hence they can be use at much closer distances. Their disadvantage is that they have very high contrasts (>=40:1), which often results in reduced accuracy, and they have a great deal of wasted real estate: usable edges may not be found in regions of greatest interest such as the four corners. A printable vector-graphics version of the ISO 12233 chart is available courtesy of Stephen H. Westin of the Cornell University Computer Graphics Department. It should be printed as large as possible (17x22 inches (A2) minimum; 24x26 inches (A1) if possible for 8+ megapixel cameras) so measured sharpness is limited by the camera and lens, not the printer. Because of the limitations of the ISO chart (described above), the Imatest SVG charts are generally preferable. You can download printable test charts by right-clicking on the thumbnails below. The bitmap chart, which is included in the samples folder of the Imatest installation, has pixel levels 47 and 255, has a contrast ratio of (255/47) 2.2 = 41 when printed at gamma = 2.2 (the normal setting). This is close to the minimum recommended by the ISO 12233 standard (but, as we've learned recently, higher than optimum). The two SVG charts have contrast ratios of 20 and 2. Instructions for their use can be found in Test Charts. The recommended print size is letter or A4, but these charts print at maximum quality for any size. SVG charts — can be viewed, edited, and printed from Inkscape. Right-click on the links to save the file. Download size: 304 KB (SVG); 500 KB (PDF). 119 of 451
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Imatest Documentation Using SFRplus
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