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Imatest Documentation Log F-Contrast Analysis of charts that vary in log frequency and contrast Introduction Log F-Contrast (short for Log Frequency-Contrast; Imatest Master only), which uses the Rescharts interface, measures the image contrast of charts that vary in spatial frequency on one axis (log frequency increases with x) and in contrast on the other (contrast is proportional to ( y/h) 2 for image height h). When the image pattern is sinusoidal (rather than a bar pattern) the contrast is equivalent to SFR or MTF. This module can be used to measure fine, low-contrast detail lost to software noise reduction, which can result in a "plastic" appearance in areas where fine texture is missing. This type of nonlinear signal processing is especially common in cameras with tiny pixels (< 2 microns), such as cameraphones. The chart can be created by Test Charts and printed on a high quality inkjet printer. An image of a Log Frequency-Contrast chart, acquired with a Canon EOS-20D camera, is shown below, slightly reduced. Log F-Contrast Image, Canon EOS-20D camera, 24-70mm f/2.8L lens set at 42mm, f/5.6, ISO 100. Click on image to load full-size image. The chart is divided into twenty-five zones (numbered from top to bottom) for analyzing contrast (MTF), which can be displayed in several ways. Second-order interpolation and smoothing are used to increase the accuracy of the MTF calculations. Spatial frequency is derived from the pattern. Creating and printing the chart Since the Log Frequency-contrast chart is not (to our knowledge) commercially available, you will need to download it or create it using Imatest Test Charts and print it on a high quality inkjet printer. (Imatest LLC can supply high quality charts on request. Contact us for details.) 202 of 451
Imatest Documentation Downloadable charts Two Log F-Contrast sine charts are available as PNG files (around 0.8 MB) for download and printing on high quality inkjet printers. One is optimized for printing at 600 dpi on HP and Canon printers; the other is for 720 dpi on Epson printers. Both are designed to be printed 20 cm (7.874 inches) wide (20 cm high if printed in landscape mode) on letter size or A4 paper using high quality print settings. 600 dpi 20 cm chart | 720 dpi 20 cm chart (Right-click to download.) Both charts have a frequency ratio of 200:1 and a maximum frequency of 800 cycles/width. They should work well covering roughly 1/3 of the image width of typical digital cameras, as shown in the example below. Test Charts has a number of options. The dialog box and recommended settings are shown below. Box Recommended setting Pattern Log Frequency-contrast. Pixels pre inch (PPI) Height Depends on printer. For best results set to 720 (Epson) or 600 (HP, Canon) and print at the indicated Height. Print height (refers to landscape orientation). To get the indicated PPI— for best print quality— the image should be printed at this height. Highlight color Usually White, but can be set at any of R, G, B, C, M, or Y. Contrast ratio Only affects the bar region; not important for this pattern. Type (bar or sine) Gamma Determines pattern type. Use sine for MTF measurements. Bar may be of interest to some users. Sine (single) is used for the examples on this page. In Imatest Studio, Bar, sine (4x) should be used for creating the Log Frequency pattern. Should be same as the gamma used for printing or the color space, e.g., 2.2 if you print using sRGB or Adobe RGB (1998). Frequency ratio The ratio between the lowest spatial frequencies (on the left) and the highest. 200 usually works well. Chart lightness Only affects bar region; not important. Ink spread comp. Best left at 0. Max. frq cycles/width This number refers to m = the maximum cycles/width of the printed chart. 1000 is a good choice in most cases. If a photographic image of the chart occupies a fraction f of the image pixel width W, the maximum 203 of 451
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Imatest Documentation Using SFRplus
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