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Imatest Documentation Display options (to the right of Plot) contains settings that affect the display (units, appearance, etc.). MTF plots (individual and summary) selects the spatial frequency scale for MTF plots for for the summary plot. Cycles/pixel (C/P), Cycles/mm (lp/mm), Cycles/inch (lp/in), Line Widths per Picture Height (LW/PH), and Line Pairs per Picture Height (LP/PH) are the choices. (Note that one cycle is the same as one line pair or two line widths.) If you select Cycles per inch or Cycles/mm, you must enter a number for the pixel size— either in pixels per inch, pixels per mm, or microns per pixel. For more detail on pixel size, see the box below. Maximum MTF plot frequency selects the maximum display frequency for MTF plots. The default is 2x Nyquist (1 cycle/pixel). This works well for high quality digital cameras, not for imaging systems where the edge is spread over several pixels. In such cases, a lower maximum frequency produces a more readable plot. 1x Nyquist (0.5 cycle/pixel), 0.5x Nyquist (0.25 cycle/pixel), and 0.2x Nyquist (0.1 cycle/pixel) are available. Secondary readout controls the secondary readout display in MTF plots. The primary readout is MTF50 (the half-contrast spatial frequency). Two secondary readouts are available with several options. The first defaults to MTF30 (the spatial frequency where MTF is 30%). The third is used only for SFRplus Lens-style MTF plots. Clicking Change opens the window shown on the right. Secondary readout settings are saved between runs. Choices: The upper radio button (MTF) for each readout selects MTFnn, the spatial frequency where MTF is nn% of its low frequency value. The middle radio button selects MTFnnP, the spatial frequency where MTF is nn% of its peak value: useful with strongly oversharpened edges. The lower radio button (MTF @ ) selects MTF @ nn units, where nn is a spatial frequency in units of Cycles/pixel, LP/mm, or LP/in. If you select this button, the pixel spacing should be specified in the Cycles per... line in the Plot section of the input dialog box, shown above. A reminder message is displayed if the pixel spacing has been omitted. Edge plot selects the contents of the upper (edge) plot. The edge can be cropped (default) or the entire edge can be displayed. Three displays are available. 1. 2. 3. Edge profile (linear) is the edge profile with gamma-encoding removed. The values in this plot are proportional to light intensity. This is the default display. Line spread function (LSF) is the derivative of the linear edge profile. MTF is the fast fourier transform (FFT) of the LSF. When LSF is selected, LSF variance (σ 2 ), which is proportional to the DxO blur unit, is displayed. Edge pixel profile is proportional to the edge profile in pixels, which includes the effects of gamma encoding. Settings affect the results as well as the display. Color reference and Color space are used when a color analysis is to be performed (if the image contains the color pattern and the Color chart box is checked in the SFRplus setup window). The Color reference dropdown menu allows you to browse for the reference file, typically a LAB data file in CSV format, supplied with color SFRplus charts. It should be stored in a convenient location. The last selected file is saved. Gamma is used to linearize the input data, i.e., to remove the gamma encoding applied in the camera or RAW converter. Gamma (input) defaults to 0.5 = 1/2, which is typical of digital cameras, but is affected by camera or RAW converter contrast settings. It should be set to 0.45 when RAW images are read into Imatest (to be converted by dcraw), but there is little loss in accuracy if it is left at 0.5. If is is set to less than 0.3 or greater than 0.8, the background will be changed to pink to indicate an unusual (possibly erroneous) selection. Chart contrast For a medium or low contrast charts (contrast
Imatest Documentation Gamma Gamma is the exponent of the equation that relates image pixel level to luminance. For a monitor or print, Output luminance = (pixel level) gamma_display When the raw output of the image sensor, which is linear, is converted to image file pixels for a standard color space, the approximate inverse of the above operation is applied. pixel level = (RAW pixel level) gamma_camera ~= exposure gamma_camera The total system gamma is gamma_display * gamma_camera. Standard values of display gamma are 1.8 for older color spaces used in the Macintosh and 2.2 for color spaces used in Windows, such as sRGB (the default) and Adobe RGB (1998). In practice, gamma is equivalent to contrast. More detail on gamma can be found in SFR Instructions, part 2. Channel is normally left at it's default value of Y for the luminance channel, where Y = 0.3*R + 0.59*G + 0.11*B. In rare instances the R, G, and B color channels might be of interest. Incident lux (for ISO sensitivity calculations) When a positive value of incident light level (not blank or zero) in lux is entered in this box, ISO sensitivity is calculated and displayed in the Stepchart noise detail figure. More details are on the ISO Sensitivity and Exposure Index page. Standardized sharpening If the checkbox is checked, standardized sharpening results are displayed as thick red curves and readouts in the edge and MTF plots. If it is unchecked, standardized sharpening are omitted, which reduces the visual clutter. Results for individual R, G, and B channels are displayed with more prominence (in Imatest Master), and edge noise is displayed. The MTF .CSV summary file is unaffected. See SFR instructions for more details. Reset restores the settings in Options and Settings to their default values. 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 to return control to the SFRplus settings & options window. When all entries are complete, click either Save settings , OK , or Cancel . Save settings saves the settings for use in automated SFRplus runs, which can be initiated from the SFRplus button in the main Imatest window. OK saves the settings then calculates results for interactive viewing. A sequence of Calculating... boxes appear to let you know how calculations are proceeding. When calculations are complete, results are displayed interactively in the Rescharts window, as shown below. 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. 171 of 451
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Imatest Documentation © 2009 Imate
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Imatest Documentation Using SFRplus
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