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Imatest Documentation For film and scanner users More film/scanner users than I expected are using Imatest. And film users prefer results in Line Pairs per mm (LP/mm). You can select cycles (line pairs) per inch or millimeter in the Plot box. Scanned images tend to be cropped by the time they are entered into Imatest because complete images are huge. (Try running the numbers for a 4,000 dpi scanner.) In this case, you must manually enter the picture height in pixels to get LW/PH or LP/PH. The best way is to use the equation, Picture height (pixels) = picture height (mm) * pixels/mm. For example, a 4,000 dpi scanner has 4000/25.4 = 157.5 pixels/mm. For 35mm film, the picture height is 24 mm = 3780 pixels. With medium format cameras, the picture height is somewhat smaller than the nominal size. It is 41.5 mm for the 645 format and 56mm for the 6x6 and 6x7 (cm) formats. Picture Width is optional, but it's a good idea to enter it. It is calculated in the same way. Crop location is the position of the center of the selected ROI (Region of interest), expressed as the percentage the distance from the image center to the corner. The orientation of the ROI with respect to the image center is included. It is blank if the input image has not been cropped. Crop location can be entered manually. This is only recommended when that image has been cropped outside of Imatest SFR. Additional parameters (all optional) for Excel .CSV output contains a detailed description of the camera, lens, and test conditions. EXIF data is entered, if available, but can be overridden by manual settings. Description & settings is particularly useful for annotating the test system (it is displayed in MTF Compare).These settings are optional but can be useful when several tests are run for different lenses, focal lengths, apertures, or other settings. The settings are displayed next to the MTF plots. They are saved and reused in subsequent runs for files with the same pixel dimensions. If EXIF data is available (currently, only in JPEG files) it overrides the saved settings. The Reset button clears all entries. ISO standard SFR If this checkbox is checked, SFR calculations are performed according to the ISO 12233 standard, and the y-axis is labeled SFR (MTF) (ISO standard). This method is slightly less accurate than the normal Imatest calculation, which incorporates a number of refinements, including a better edge detection algorithm and a second-order polynomial fit to the average edge for a more accurate estimate of SFR in the presence of lens distortion. This box is normally left unchecked; it should only be used for comparing normal Imatest calculations to the ISO standard. The difference is typically very small. When entries are complete, click OK . A Calculating... box appears to let you know that calculations are proceeding. Results appear in individual windows called Figures (Matlab's standard method of displaying plots). Figures can be examined, resized, maximized, and closed at will. A zoom function is also available. Their contents are described in the pages on Imatest SFR results: MTF (Sharpness) plot, Chromatic Aberration, Noise, and Shannon Capacity plot, and Multiple ROI (Region of Interest) plot. Warnings A Clipping warning is issued if more than 0.5% of the pixels are clipped (saturated), i.e., if dark pixels reach level 0 or light pixels reach the maximum level (255 for bit depth = 8). This warning is emphasized if more than 5% of the pixels are clipped. Clipping reduces the accuracy of SFR results. It makes measured sharpness better than reality. The percentage of clipped pixels is not a reliable index of the severity of clipping or of the measurement error. For example, it is possible to just barely clip a large portion of the image with little loss of accuracy. The plot on the right illustrates relatively severe clipping, indicated by the sharp "shoulder" on the black line (the edge without standardized sharpening). The sharp corner makes the MTF look better than reality. The absence of a sharp corner may indicate that there is little MTF error. Clipping can usually be avoided with a correct exposure-- neither too dark nor light. A low contrast target is recommended for reducing the likelihood of clipping: it increases exposure latitude and reduces the sensitivity of the MTF results to errors in estimating gamma. Clipping warnings 134 of 451
Imatest Documentation Two precautions when working with figures Too many open Figures Figures can proliferate if you do a number of runs, especially SFR runs with multiple regions, and system performance suffers if too many Figures are open. You will need to manage them. Figures can be closed individually by clicking X on the upper right of the Figure or by any of the usual Windows techniques. You can close them all by clicking Close figures in the Imatest main window. For large batch runs, the Close figures after save checkbox in the SFR Save dialog box prevents a buildup of open figures. Saving the results At the completion of the SFR calculations the Save SFR results? dialog box appears, except in Express mode, which uses previous settings. It allows you to choose which results to save and where to save them. The default is subfolder Results of the data file folder. You can change to another existing folder, but new results directories must first be created outside of Imatest using a utility such as Windows Explorer. (This is a limitation of this version of Matlab.) Figures can be saved as either PNG files (a standard losslessly-compressed image file format) or as Matlab FIG files, which can be opened by the Open Fig file button in the Imatest main window. Fig files can be manipulated (zoomed and rotated), but they tend to require more storage than PNG files. Saved figures, CSV, and XML files are given names that consist of a root file name (which defaults to the image file name) with a suffix added. Examples: Canon_17-40_24_f8_C1_1409_YR7_cpp.png Canon_17-40_24_f8_C1_1409_YR7_MTF.csv The root file name, Canon_17-40_24_f8_C1_1409_YR7, can be overridden by entering another name in the Results root file name: box. Be sure to press the Enter key. This feature does not work with batch runs. Clicking on Figures during calculations can confuse Matlab. Plots can appear on the wrong figure (usually distorted) or disappear altogether. Wait until all calculations are complete— until the Save or Imatest main window appears— before clicking on any Figures. SFR save dialog box Checking Close figures after save is recommended for preventing a buildup of figures (which slows down most systems) in batch runs. When multiple ROIs are selected, the Save results? dialog box appears only after the first set of calculations. The remaining calculations use the same Save settings. Save results? is omitted entirely in an Express run for repeated images. The first four checkboxes are for the figures, which can be examined before the boxes are checked or unchecked. After you click on Yes or No, the selections are saved, then the Imatest main window reappears. Result file names— The roots of the file names are the same as the image file name. The channel (Y, R, G, or B) is included in the file name. If a Region of Interest has been selected from a complete digital camera image, information about the location of the ROI is included in the file name following the channel. For example, if the center of the ROI is above-right of the image, 20% of the distance from the center to the corner, the characters AR20 are included in the file name. 135 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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Imatest Documentation Image Quality
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