Documentation

Using Rescharts slanted-edge modules Part 2

Running Rescharts slanted-edge modules
(Interactive and Auto mode settings)

Imatest Rescharts slanted edge modules— SFRplus, eSFR ISO, SFRreg, and Checkerboard— perform highly automated measurements of several key image quality factors using specially-designed test charts. This document shows how to run these modules in Rescharts and how to save settings for automated runs. 

Four introductory pages —  SFRplus, eSFR ISO, SFRreg, and Checkerboard — describe each module and explain how to obtain and photograph the chart. 

Rescharts Results Part 3 and Part 4 illustrate the results.

Selecting filesSetup windowROI selection & analysis – More settings window
Secondary readout – Settings area – Auto mode windowWarningsSummary 

This page combines instructions for the four Rescharts slanted-edge modules
that have automatic region detection,
 SFRplus, eSFR ISO, SFRreg, and Checkerboard. 
These modules contain many common functions,
making a single page 
easier to maintain than four separate pages.

In many cases we will use SFRplus as an example. The other module may be substituted.

New in Imatest 5.1  Chart MTF compensation has been added to compensate for chart limitations. This can double the maximum megapixel capacity of most charts.

Imatest 5.0  Nonuniform lighting perpendicular to the edge can be compensated. This improves the estimate of summary metrics such as MTF50.

 

SFRplus_rescharts_autoOpen Imatest by double-clicking the Imatest icon Imatest_icon_16 on

  • the Desktop,
  • the Windows Start menu,
  • the Imatest folder (typically C:Program files\Imatest\Master\ in English language Windows installations).
 

SFRplus, eSFR ISO, SFRreg, and Checkerboard operate in two modes:

  1. interactive/setup (in Rescharts) and
  2. Automatic (batch-capable).

Since Automatic runs for each module use settings from interactive/setup runs, an interactive/setup run should always be run first.

The following instructions apply to all Rescharts slanted-edge modules.

To start an interactive/setup run, press [module] setup or Rescharts[module], where [module] is SFRplus, eSFR ISO, SFRreg, or Checkerboard. [module] setup opens a dialog box for reading an image file for the selected module. Rescharts is more general; it lets you open any Rescharts module.

Either of these buttons allows you to analyze an image for the chosen module, examine detailed results interactively, and save settings for the highly automated SFRplus Auto,  eSFR ISO Auto, SFRreg Auto, or Checkerboard Auto runs (or the even more automated Imatest IT versions). The module should be run interactively at least once prior to the first Auto run. Settings can also be updated by pressing Settings, [module] Auto settings from the Imatest main window.

 

Selecting file(s)

The portion of the Rescharts window used for opening files is shown on the right. You can open a file by clicking on Read image file if the correct chart type is displayed, or by selecting the required Chart type. One or more files may be selected, as shown below. If you select multiple files, they can be combined (averaged), and you’ll be given the option of saving the combined file.

If the folder contains meaningless camera-generated file names such as IMG_3734.jpg, IMG_3735.jpg, etc., you can change them to meaningful names that include focal length, aperture, etc., with the Rename Files utility, which takes advantage of EXIF data stored in each file.

The folder saved from the previous run appears in the Look in: box on the top. You are free to change it. The file name from the previous run is displayed at the bottom. You can open a single file by simply double-clicking on it. You can select multiple files for combined runs (for Interactive or Auto runs) or for batch runs (for Auto runs-only) by the usual Windows techniques: control-click to add a file; shift-click to select a block of files. Then click Open. Three image files are highlighted. Large files can take several seconds to load.

Imagest SFR: Open slanted-edge input file dialog box
File selection

Multiple file selection Several files can be selected in Imatest Master using standard Windows techniques (shift-click or control-click). For interactive (Rescharts) runs, files can be combined to reduce noise or (in some instances) observe the effects of camera shake or image stabilization. For [module] auto runs, you can run large batches of images. The multi-image dialog box gives you the option of saving the combined file, which will have the same name as the first selected file with _comb_n appended, where n is the number of files combined.
 
RAW files   Imatest can analyze raw files from cameras (using dcraw) or from development systems (using Generalized Read Raw). The files can be demosaiced or Bayer raw: standard files (TIFF, etc.) that contain undemosaiced data. Undemosaiced files are infrequently useful for measuring MTF because the pixel spacing in each of the four image planes is twice that of the image as a whole; hence MTF is lower than for demosaiced files. But Lateral Chromatic Aberration can be severely distorted by demosaicing, and is best measured in Bayer RAW files (and corrected during RAW conversion). Details of RAW files can be found here

Setup window

When the file (or files) have been opened, the Setup window, shown below, appears. This window allows you to select groups of regions (ROIs; shown as violet rectangles) for analysis. It also lets you select the size of the regions, whether to analyze vertical or horizontal edges, and much more. Pressing More settings on the left opens the SFRplus More settings, which allows you to select additional settings that affect the calculations and display. Pressing Auto mode settings lets you select the output figures and files (for SFRplus auto mode runs). The light yellow-orange rectangles are for calculating the Color/lightness uniformity profiles.

SFRplus setup window; 46 regions (23H + 23V) selected for analysis

Setup window   Settings relevant to speedup are shown with red background.
Settings area (upper-left)
Channel Select channel to analyze: R, G, B, Y, R-only, G-only, B-only, Y-only, Mean, Mean-only. (Y is the Luminance channel: normally recommended). Selecting one channel-only can significantly speed up calculations. It can also help where other channels are dark or may not contain valid data. 
Gamma Assumed Gamma (contrast) of the chart. Has a small effect on the MTF results. Default is 0.5. Overwritten (only used for an approximate initial estimate) if the Use for MTF checkbox (below Chart contrast) is checked. Described in more detail below.
Chart contrast
(for gamma calc.)
Chart contrast– for the contrasty squares (i.e., most of them). Used to estimate gamma from the image for charts with contrast ≤ 10.
Use for MTF (Checkbox, normally unchecked) When checked, gamma derived from individual slanted-edges on the chart is used for MTF calculations. This may result in a small improvement in accuracy.
Orientation [SFRplus, eSFR ISO] Selections are Standard orientation [default], Mirror (H or V), Rotate (180, 90 CCW, 90 CW) Applies to the next read. 
Reg mark detection [SFRreg-only] Selections are Autodetect # SFRreg or 1-30 regmarks. Entering the number may be somewhat faster and more accurate in some situations, but isn’t generally necessary.
More settings Open the More settings window, shown below.
Auto mode settings Open the Auto mode settings window, shown below, for controlling Rescharts Auto runs.
ROI Selection & analysis area (lower-left)

Region selection
(Selects which regions to locate. Actual ROIs are located
automatically)

Note that selections are different for each module

Select the regions (ROIs) to analyze. Choices below. The number of regions is in parentheses. This is a particularly important setting. We encourage users to become familiar with the settings below. Note that the squares above and below the middle square are usually not selected for analysis; they are reserved for other purposes (step chart, color chart, and/or focusing pattern). At least 9 regions are required for 3D plots (but 13+ are recommended).

SFRplus eSFR ISO
1. Center (1)

Definitions for eSFR ISO region selections

“9 iso sqs” refers to the 9 squares from the ISO 12233:2014 specification, i.e, the squares included in the Standard E-SFR chart.

“15 sqs” refers to all 15 squares in the Enhanced or Extended charts. These selections (9-14) should not be used with the Standard chart.

Selections 1 and 4-15 are for various combinations of center and outer edges. The descriptions in the eSFR ISO setup window are self-explanatory.

2. Center & corners (5)
3. Center, corners, part-way (9) (Part-way on diagonal between center & corners)
4. Center, L, R, T, B (5)
5. Center, corners, L, R, T, B (9)
6. Center, corners, part-way, L, R, T, B (13)
7. 5 rows, 5 columns (except step & color ROIs) (23) (edges on a 5×5 grid, omitting the squares above and below the middle). Highly detailed results, well-suited for 3D displays. Often a good compromise between speed and detail.
8. Inner squares (max 3×5), outer edges A 3×5 grid (omitting squares above and below the middle) representing the central region of the image.
9. All inner squares, outer edges. A tight grid of squares covering most of the image, except near the edges. 1. Ctrs and outer corners (6 or 9 ISO squares) (the default). Note that two regions are always selected for the center square and center regions are included in all region selections except for the two for Edge ID files.
10. All squares, inner & boundary edges (best 3D map). A highly detailed grid, recommended for detailed 3D displays where squares have a single contrast. The edges tend to be non-overlapping.
11. All squares, edges (many; overlapping; slow) All available edges. Slow; not generally recommended. 2. Read an Edge ID/Zone selection file. Explanation below.
12. Center, part-way to corners (for mfg test) (5) This is a minimal pattern for manufacturing testing, where high speed is required. 3. Use most recent Edge ID file.
13. All squares, inner & bdry except low contrast (good 3D map). A highly detailed grid, recommended for detailed 3D displays for charts that have two contrast levels (mostly high and a few low). The edges tend to be non-overlapping. 1, 4-8. These selections work for all charts including the Standard (minimal) Chart, which has only 9 squares
14. Skype spec: center, above-L, & below-R ctr. See the Skype support page. 9-15. These selections work with the Enhanced or Extended charts, which have 15 squares. They should not be used with the Standard (9 square) chart. 
15. Read an Edge ID/Zone selection file. Explanation below.
16. Use most recent Edge ID file.
17. Center, 75% of way to sides  
18. NO regions: fast geometry calculation  
 
SFRreg Checkerboard
SFRreg has separate settings for
Vertical and Horizontal regions
1. Center (1)
2. Read Edge ID/Zone selection file. Explanation below.
Vertical settings: All, Top, Bottom, None, Inner, Outer 3. Use most recent Edge ID file.
4-9. Various combination of Center, corners, par-way, LRTB.
Horizontal settings: All, Left, Right, None, Inner, Outer  10. All squares
11-14. Alternate squares, starting at one of four positions.
  15,16. Alternate columns. Two starting rows.
  17. Select radial ROI zones (% from ctr): 70 80.
  18. NO regions: fast geometry calculation.

 

EdgeID/Zone selection file.

An Edge ID/Zone selection file contains a list of identifiers for slanted edge ROI locations or selection zones, one per line, similar to lists in CSV and JSON output files. There are two types of data line: for individual ROIs and for ROI selection zones. It allows you to freely select edges or zones.

This window opens when you click
 Read EdgeID/Zone selection file. Click to view full size

Click Read EdgeID file to read the file. Then click OK.

Individual edge ROI selection lines:  The format is  Column_Row_Side,  where

  • Column is the column number (x) relative to the center, e.g., -3, 0, 3, etc.
  • Row is the row number (y) relative to the center, e.g., -2, 0, 2, etc. NOTE: Row number increases in a downward direction, e.g., -1 is ABOVE center.
  • Side is L, R, T, or B  (Left, Right, Top, or Bottom) (SFRplus and eSFR ISO modules). (H or V are used instead of L, R, T, B for the Checkerboard module-only).
  • The final field from the results files ( _C, _E, etc. for Corner, Edge, etc.) is omitted.

Example: -2_1_L selects the left edge of the square two columns left of center and one row below center.

Zone selection lines (Imatest 5.0+):  The format is Z ii nn or S ii nn, where

  • Z denotes zone for edge selection based on % center-corner distance.
  • S denotes zone for edge selection based on % center-side distance.
  • ii is the minimum % distance. nn is the maximum % distance.

Example:  Z 60 80 selects all edges with centers between 60 and 80% of the way between the image center and the corner. Several zones can be selected.

Other selections may be added on user request.

Vertical, Horizontal edges (or both) Chooses between Vertical and Horizontal edges (or both). Usually Vertical, but Horizontal is useful on occasion. Use both for Lens-style MTF plots. For 3D plots it’s best to plot Horizontal or Vertical-only.
Step chart (checkbox) [SFRplus, eSFR ISO] (√) Perform step chart analysis. Should be unchecked for charts that do not include the step chart (such as transmissive chrome-on-glass charts). Checked by default.
Full/Reduced image for detection [Checkerboard] Use full, 1/2X, or 1/3X image for detecting checkerboard corners. Full is the normal default, but a reduced image may speed up detection, which can be slow.
Color chart [SFRplus, eSFR ISO] Perform color analysis for charts with a color pattern or (SFRplus-only) analyze the square above the center. Be sure the correct Color reference (file name) and Color space has been entered in the Settings region of the More settings window. Unchecked by default.
Crop function (Normal/Exclude) [SFRreg-only] Selects the crop function. Normal crop means analyze the crop area. Exclude crop from detection means exclude the crop area— analyze only outside the crop.
Rows
(Auto or number)
[SFRplus-only] The number of rows of squares in the chart (between the top and bottom bars) or Auto for automatic row detection (the default). In rare cases Automatic row detection may fail and the actual number of rows (4, 5, or 7) may work better. If there are no bars, select Auto- no bars, and the regions will be similar to what you would get with the bars at the top and bottom of the image. Distortion and Field of View won’t be calculated.
Extra reg marks [eSFR ISO-only] The number of extra registration marks outside the eSFR ISO chart to be used for MTF analysis. Default is 0.
Negative [SFRplus-only] (√) Check this box for negative images (light squares on dark background). 
Speedup (checkbox) (√) Speed up the run by eliminating some calculations that many users don’t require, including SQF/Acutance, noise statistics and histograms. Recommended.
ROI size Slider that determines the size of the ROI. Use the largest value that keeps a save distance from edges of squares and top and bottom bars. May need to be reduced where barel distortion is severe.
Crop borders Allows borders to be cropped to remove interfering patterns that might otherwise be included in the image. This button is tinted pink whenever the image is cropped.
ROI width
(below ROI size slider) and Extra smoothing
Width of ROI selection. Available settings: Normal, Wider, Widest, without or with Extra smoothing. Use Normal width for standard rectangular ROIs. Choose Wider or Widest for fuzzy edges, for enhanced noise analysis, or for more extended low frequency response. Use one of the Extra smoothing options for wide and rough or noisy regions (including shaken images), for example when measuring inkjet test charts.
Other controls (lower-right)
Title Title. Defaults to file name. You can add a description.
Help Open this web page in a web browser.
Display … image Adjust image for Setup window display: Original (RGB) image, R, G, or B channel, as well as numerous settings for lightening dark images, boosting color saturation, tone-mapping, etc.
Save & close
Save settings (for use in auto SFRplus), but do not continue with run.
OK Save settings and continue with run: Calculate results for all selected region. You will be able to view results interactively.
Cancel Cancel run; do not save settings

After you’ve finished making settings, click OK to save settings and continue with the run. You can Click Save settings to save the settings without continuing.

Speeding up runs
Setting Value(s) for fast operation ini file for IT Effect
Channel [S] One channel (typically Y (luminance))-only RGBY_value = 8 Major
Speedup [S] Checked (on). Turns off slow noise & histogram calculations. speedup = 1 Major
Step chart [ct] [S] Unchecked (off, unless needed) stepchart = 0 Minor
Analyze color patches [ct] [S] Analyze square above center recommended (unless color needed) colorchart = 0 Minor
Edge roughness Unchecked edgerough = Min Minor
Acutance analysis Unchecked SQF = Min Minor
Image stabilization [SFRplus-only] Unchecked   Minor
Wedge [eSFR ISO-only] [S] Unchecked (off), unless needed   Moderate
Noise settings
[eSFR ISO-only]
Opens Noise settings window. For speed, Noise calculations (near top) should be set to 1 or 2 (No or Elementary calculations). Visual Noise display settings should be set to 1. No Visual noise.   Major for visual noise
[ct] means SFRplus, eSFR ISO, SFRreg with center chart-only (the others lack color and grayscale patterns).
[S]
 means this setting is in the Setup window (most are also in More settings). Others are in More settings-only.
IT users: See the knowledge base post, Speeding up Imatest IT with INI File Settings .  INI file settings (above) are for the [sfrplus] section of the ini file. 

More settings window

The More settings window, shown below, opens when

  • More settings is pressed in the Setup or Rescharts windows.
  • Settings, SFRplus (eSFR ISO, SFRreg, or Checkerboard) settings is pressed in the Imatest main window, then  More settings is pressed in the Setup window.

The settings are read from imatest.ini, and saved when OK is pressed. Settings are similar to the settings in the SFR input dialog box.

More settings window

You can select either Expert or Simplified mode for the settings window. In Simplified mode many settings are grayed out so they can’t be set. If you press Standard settings, commonly used settings (recommended for beginners) are selected.

This window is divided into sections: Title and Help on top, then Display options, Settings, Optional parameters, and finally, OK or Cancel at the bottom.

Title defaults to the input file name. You may leave it unchanged, replace it, or add descriptive information for the camera, lens, converter settings, etc.— as you please.

Help opens a browser window containing a web page describing the module. The browser window sometimes opens behind other windows; you may need to check if it doesn’t pop right up.

 

Display options (near the top of the window) contains settings that affect the display (units, appearance, etc.).
MTF plot units  selects the spatial frequency units for MTF plots and for summary plots. Units are described in the Spatial frequency units section of the Sharpness page Choices include 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). (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 the Pixel size— either in pixels per inchpixels per mm, or microns per pixel. For more detail on pixel size, see the box below.
Pixel size and units (Same as pixel pitch for μm/pixel units) Required for cycles per distance (mm, inch), angle, or object distance, but not for C/P, LW/PH, and LP/PH output. Also required for calculating Field of View (FoV) in units of distance (mm, inches).
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, but not for imaging systems where the edge is spread over several pixels. In such cases, a lower maximum frequency produces a more readable plot. Choices include 1x Nyquist (0.5 c/p), 0.5x Nyquist (0.25 c/p), and 0.2x Nyquist (0.1 c/p).

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 third is used only for Lens-style MTF plots.

Clicking Change readout opens the window shown on the right, which lets you select the readout and save them by pressing OK.

  • The upper radio button (MTF) for each readout selects MTFnn, the spatial frequency where MTF is nn% of its low frequency value.
  • The second radio button selects MTFnnP, the spatial frequency where MTF is nn% of its peak value: useful with strongly oversharpened edges.
  • The third radio button (MTF @ …) selects MTF @ nn units, where nn is a spatial frequency in any of the supported units (Cycles/pixel, LP/mm, Cycles/angle, etc.). Some units (cycles/mm, etc.) require that you enter Pixel size, as described above. A reminder message is displayed if it has been omitted. 
  • The fourth radio button (MTF Area) selects the area under the MTF curve (below the Nyquist frequency). When it is normalized to the peak MTF it tracks MTF50 for low to moderate sharpening, but, unlike MTF50, it remains constant for oversharpened images. Described in Slanted-Edge MTF measurement consistency
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. 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.
  2. 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 (σ2) is displayed.
  3. Edge pixel profile is proportional to the edge profile in pixels, which includes the effects of gamma encoding.
  4. Edge linear, unnormalized is similar to Edge profile (1.), but not normalized. Useful in diagnosing situations where a channel may be saturating (and affecting MTF measurements).
Lines & arrows

[Checkerboard-only]  Dropdown menu that lest you select (1) No arrows, (2) Display lines & arrows in the corrected image, or (3) Display lines & arrows in both images. Here is an example.

  
  Settings (middle of the window) affect the results as well as the display.
 

The Settings area for SFRplus is shown on the right.

(√) indicates a checkbox.

The left column is

blank for settings that apply to all modules.

[ct] for modules that have tonal and/or color charts (SFRplus, eSFR ISO, and SFRreg with center chart).

[s+] for SFRplus,  [e] for eSFR ISO,  [sr] for SFRreg, and  [ch] for Checkerboard.

  Speedup (√) Speed up calculations by eliminating some calculations that many users don’t require, including noise statistics and histograms. Recommended. Checking Speedup (recommended) can significantly speed up calculations.
  Edge roughness (√) Calculate edge roughness (for including in CSV output). Slows calculations slightly. Required for Edge roughness plot to be displayed.
  Acutance analysis (√) Calculate acutance or SQF. Required for SQF output. Slows calculations slightly.
  MTF noise
reduction (mod apod)
 
(√) Reduce noise using the modified apodization technique. Improves MTF accuracy, especially with noisy images, but not an ISO standard calculation. Generally recommended.
  Nonuniformity
MTF correction
(√) corrects MTF for nonuniform illumination perpendicular to the edge. Should normally be unchecked. Described here.
[ct] Color reference 
Color space
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.
[ct] Grayscale
reference
 
allows you to enter a reference file for the grayscale (tonal response) patches. For Standard values you can choose between Std ref values from target values used to create the print and standard values for several media.
  Channel specifies the primary channel to display. All channels are analyzed unless “channel-only” is selected. It’s normally left at it’s default value of Y (luminance), where Y = 0.2125*R + 0.7154*G + 0.0721*B. R, G, and B color channels can also be selected. Selecting one channel-only can significantly speed up calculations (recommended only when speed is critical) and may help in rare instances where bad data in one channel causes a crash.
  Image orientation Selects image orientation for the next run. Options include Standard, Mirror (H-flip), Mirror (V-flip), and Rotate 180 degree. For selections other than Standard, the window background is yellow
  Acutance/SQF Options Open the Acutance/SQF settings window, described in Acutance and SQF (Subjective Quality Factor). Acutance is a subjective measure of sharpness that includes viewing conditions. 
[ct] 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.

[s+]

[e]

[ch]

Bar-to-bar chart height in cm    [SFRplus] 
Reg. mark vertical spacing cm  [eSFR ISO] 
Square spacing (cm)                  [Checkerboard]

These are used to calculate the Field of View (FoV) when Pixel spacing (pitch) has been entered. They are optional. For SFRplus the bar-to-bar chart height is measured from the top of the top bar to the bottom of the bottom bar. For pre-distorted charts, use the center distance, as shown on the right. FoV is displayed in Image & Geometry and reported in the CSV and JSON output file if calculated.

  Lens-to-chart distance in cm used to calculate the actual lens focal length when Pixel spacing (pitch) and bar-to-bar chart height have been entered. It is optional. The distance from the lens to target is most conveniently measured with a laser measuring device. Focal length is displayed in Image & Geometry and reported in the CSV output file if calculated.
  Gamma Gamma is the average slope of log pixel levels as a function of log exposure for light through dark gray tones). It is used to linearize the input data, i.e., to remove the gamma encoding applied by the camera or RAW converter. It defaults to 0.5 = 1/2, which is typical of digital camera color spaces, but may be affected by camera or RAW converter settings. Small errors in gamma have little effect on MTF measurements (a 10% error in gamma results in a 2.5% error in MTF50 for a normal contrast target). Gamma should be set to 0.45 or 0.5 when dcraw is used to convert RAW images into sRGB or a gamma=2.2 (Adobe RGB) color space. It is typically around 1 for converted raw images that haven’t had a gamma curve applied. If gamma  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.

If the chart contrast is known and is ≤10:1 (medium or low contrast), you can enter the contrast in the Chart contrast (for gamma calc.) box, then check the Use for MTF (√) checkbox. Gamma will be calculated from the chart and displayed in the Edge/MTF plot.

If chart contrast is not known you should measure gamma from a grayscale stepchart image. A grayscale is included in SFRplus, eSFR ISO and SFRreg Center ([ct]) charts. Gamma is calculated and displayed in the Tonal Response, Gamma/White Bal plot for these modules. Gamma can also be calculated from any grayscale stepchart by running ColorcheckStepchart , Multicharts (interactive), or Multitest. A nominal value of gamma should be entered, even if the value of gamma derived from the chart (described above) is used to calculate MTF.

Gamma
Gamma is the exponent of the equation that relates image file 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 ~= exposuregamma_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 on gamma can be found in SFR Instructions, part 2.

  Wavelength (um) for diffraction-limited MTF plot. If a number is entered (0.555 um is the default for visible light systems) and pixel spacing is also entered, the diffraction-limited MTF is displayed on the MTF plot and the diffraction-limited edge or Line Spread Function (cross-section of Airy disk) is shown on the edge plot as a faint dashed line – – – – – – –.
  MTF compensation lets you to compensate MTF measurements for chart MTF limitations and/or sensor MTF loss (to get closer to lens measurements). Chat compensation requires an MTF compensation file for the specific chart medium. Opens the Chart and sensor MTF compensation window described in detail in Compensating MTF measurements.
[s+] Distortion parameters [SFRplus] Two parameters, printed on pre-distorted charts, that allow distortion, hance Fields of View, to be correctly calculated. Currently (5.1) works correctly only with SFRplus, but we plan to add it to other modules.
  Distortion calculation
Calculation
3rd Oder
5th Order (3,5)
arctan/tan
Best of above
No distortion calc.
5th Order (ALL 2-5)
Best of (a)tan, 5thALL
7th Order odd
7th Order ALL
9th Order odd
11th Order odd

Distortion, FoV calculation  This dropdown window lets you select the equation for calculating distortion.

[Checkerboard]  All the calculations shown on the right are available.
[SFRplus]  Only the first seven (3rd order through Best of (a)tan, 5thALL) are available
[SFRreg]  None are currently available.

For the standard multiplication model), odd (2n+1)-order polynomials have the equation,

\(r_u = r_d + k_1 r_d^3 + … + k_n r_d^{2n+1} = r_d(1 + k_1r_d^2 + … + k_n r_d^{2n} )\) 

For the division model (Division model (below) checked), “odd”-order polynomials have the equation,  \(r_u = r_d / (1 + k_1 r_d^2 + … + k_n r_d^{2n})\).

For both models, all terms (rather than just odd) can be selected for 5th or 7th order polynomials (4th or 6th for Division). We have not seen much advantage for these extra terms.

Higher order polynomials (n = 7 and above; 3 or more polynomial coefficients) should be used with extreme care because they can become unstable for Checkerboard grids where the maximum dimension has fewer than n points or where the features are not close to the edge of the image.

  Distortion decentering (√) Calculate center of distortion. Works extremely well (always recommended) with SFRplus and Checkerboard.
  Division model (√) Use the Division model for polynomial distortion, \(r_u = r_d / (1 + k_1 r_d^2 + … + k_n r_d^{2n})\). We have found it to be slightly more accurate than  the standard multiplication model for extremely barrel-distorted images. Polynomial exponents are displayed as even numbers, 1 lower than the corresponding term for the standard multiplication model. If checked, the setting is displayed as red with yellow background.
[e] Noise settings Settings for calculating noise from the circular grayscale pattern (eSFR ISO-only). Opens the Noise settings window described in Multicharts/Multitest/eSFR ISO Noise.
  Zone weights Weights of the center, part-way, and corner zones. Used for calculating weighted means of key results, displayed in the Multi-ROI and 3D plots. The defaults of 1 (center), 0.75 (part-way), and 0.5 (corners) are for typical pictorial photography; corners should probably be given more weight for technical photography.
[e] Wedges [eSFR ISO] Select wedge analysis. None or 4 basic for standard charts (no extra wedges). Additional options are available for charts with extra wedges are 4+4 inner, 4+4+8 corners, 4+4+10 corners.
[e] [sr] Number of SFRreg targets [SFRreg, eSFR ISO] For SFRreg, the number of registration mark patterns to search for. For eSFR ISO, the number of SFRreg patterns to be detected and analyzed outside the main eSFR ISO pattern. 
[e] [sr] Minimum SFRreg mark diameter, Maximum SFRreg mark diameter  [SFRreg, eSFR ISO]  (optional). 
                 Can improve detection in complex images.
  Standardized sharpening (Display oversharpening-only is recommended) Standardized sharpening, which displays results as thick red curves and readouts in the edge and MTF plots is not recommended. CSV and JSON summary files are unaffected. See SFR instructions for more details.
  Reset  restores the settings in More 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 setup window or to save settings and return to the Imatest main 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.

Auto mode settings window

Auto mode settings affect plots and output files for Auto runs as well as Imatest IT. These settings do not directly affect interactive (Rescharts) runs. The Auto mode settings window, shown below, opens when

  • Auto mode settings is pressed in the Setup window.
  • More settings is pressed in the Rescharts window, then Switch to Auto mode setup is pressed in the More settings window.
  • Settings (dropdown menu), [SFRplus, eSFR ISO, SFRreg, or Checkerboard] Auto settings is pressed in the Imatest main window.

eSFR ISO auto mode settings.
Other modules have different Multi-ROI selections.

The upper box allows you to select single and multi region figures to be plotted and saved in SFRplus auto output. For the most part they are self-explanatory. Note that all figures plotted in Auto mode are saved.

Saved figures, CSV, and JSON 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

When 3D plot is checked, SFRplus auto plots the last 3D plot displayed in Rescharts unless the More 3D plots button has been pressed and one or more plots has been selected. More 3D plots will be displayed in pink in this case. This allows several 3D plots to be displayed and saved by SFRplus auto.

Close figures after save should be checked if a large number of figures are to be displayed. It prevents a buildup of figures, which can slow processing.

Figures can be saved as PNG or FIG files. PNG files (a losslessly-compressed image file format) are the default— they require the least storage. Matlab FIG files allow the data to be manipulated– Figures can be resized, zoomed, or rotated (3D figures-only), but FIG files should rarely be used because they can be huge. PNG files are preferred if no additional manipulation is required.

A CSV summary file is saved for all runs. An JSON file is saved if Save JSON results is checked. XML is not recommended; it will be deprecated in 2019.

You can select either Save CSV files for individual ROIs or Save summary CSV file only (the summary file is always saved).

Save folder determines where results are stored. It can be set either to subfolder Results of the image folder or to a folder of your choice. Subfolder Results is recommended because it is easy to find if the image folder is known.

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 over 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 MTF 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 strong clipping, indicated by the sharp corner near the “shoulder” on the black line. The MTF measurement is better than reality. The absence of a sharp corner indicates that there is little MTF error.Clipping can usually be avoided with a correct exposure— neither too dark nor light— and by avoiding high contrast targets (like the old ISO-12233 chart). The maximum recommended edge contrast is 10:1; 4:1 contrast (recommended in the upcoming revision to the ISO-12233 standard) is even better. Low-contrast targets are more reliable overall: in addition to better exposure latitude (reduced risk of clipping), they tend to have less sharpening in cameras with variable signal processing, and MTF results are less sensitive to errors in estimating gamma.  

Clipping warnings

 

Summary

  • SFRplus analyzes images of the SFRplus test chart, ideally framed so that there is white space above and below the horizontal bars in the chart, i.e., so neither bar runs off the top or bottom of the image. The white space should be between 0.5% and 25% of the image height. SFRplus can work without the bars, but distortion and field of view are not calculated. The bars may run off the sides of the image. Interfering patterns (bars, etc.) outside the chart should be kept to a minimum.
  • eSFR ISO should be framed to the registration marks are well inside the image. Where possible the entire chart should be inside the image. eSFR ISO has less spatial detail than SFRplus, but has wedges and larger tonal patches that allow much more detailed noise analysis.
  • SFRreg is highly flexible, and can include individual charts facing the camera with angular fields of view well over 180 degrees.
  • Checkerboard uses a simple checkerboard (chessboard) pattern that works over a wide range of distances: chart framing is not critical. 
  • Lighting should be even and glare-free. Lighting and alignment recommendations are given in The Imatest test lab.
  • The first time any of these modules is run, it should be run through Rescharts. This allows
    • parameters to be adjusted and saved for later use in the automatic version, which is opened with a (module) Auto  button in the Imatest main window.
    • results (listed above) to be examined interactively in the Rescharts window.
  • The Auto buttons in the left column of the main Imatest window runs the corresponding module in full automatic mode using settings saved from the most recent Rescharts run. Batches of files can be run.
Table of Rescharts slanted-edge MTF modules. (All also available as batch-capable fixed modules.)
All feature automatic region detection, ISO-compliant MTF (sharpness) analysis, and Lateral Chromatic Aberration calculation.
Feature   SFRplus
  eSFR ISO
eSFRiso_enhanced_200W
     SFRreg
Optikos_Meridian_200W
  Checkerboard
checkerboard_ideal_framing-200W
Sharpness map detail High Medium Depends on arrangement High
ISO-standard chart design
Color analysis *
Tonal response (OECF) *
Noise analysis Limited * Limited Limited
Distortion Limited
Geometry *
Features and
recommended uses
Imatest’s original automatically-detected chart, in use since 2009. robust and versatile. Some white space recommended above and below top and bottom bars. More spatial and distortion detail than eSFR ISO.
ISO-standard chart design. Includes wedge analysis. Supports detailed noise analysis.

Several individual charts are typically placed around the image field. Works with
-extreme fisheye lenses (>180º)
-the Optikos Meridian system.
-Charts at different distances to test focus & depth of field.
-extreme high resolution cameras (>36MP)

*Available with SFRreg center chart in Imatest 5.0+. FoV is omitted from geometry.

Relatively insensitive to framing: can zoom in or out as long as distance is large enough so chart quality is not an issue and there are detectable corners.

♦ denotes strong support;  – denotes no support.

Next: Part 3: Slanted edge Results  Part 4: Other results

Pixel size

Pixel size is closely related to image quality. For very small pixels, noise, dynamic range and low light performance suffer. Pixel size is rarely given in camera spec sheets: it usually takes some math to find it. If the sensor type and the number of horizontal and vertical pixels (H and V) are available, you can find pixel size from the table on the right and the following equations.

pixel size in mm = (diagonal in mm) / sqrt( H2 + V2 )
pixel size in microns = 1000 (diagonal in mm) / sqrt( H2 + V2 )

Pixel size in microns (microns per pixel) can be entered directly into the SFR settings box. Example, the cute little 5 megapixel Panasonic Lumix DMC-TZ1 has a 1/2.5 inch sensor and a maximum resolution of 2560x1980 pixels. Guessing that the diagonal is 7 mm, pixel size is 2.1875 (rounded, 2.2) microns.You can find detailed sensor specifications in pages from Sony, Panasonic, and Kodak.

Sensor
Designation

(Type)
Diagonal mm. Width mm. Height mm.
1/4″ 4.5    
1/3.6″ 5.0 4.0 3.0
1/3.2″ 5.68 4.54 3.42
1/3″ 6.0 4.8 3.6
1/2.7″ 6.59 5.27 3.96
1/2.5″ 6.9 – 7.2    
1/2″ 8.0 6.4 4.8
1/1.8″ 8.93 – 9.1 7.18 5.32
2/3″ 11.0 8.8 6.6
1″ 16.0 12.8 9.6
4/3″ 22.5 18.0 13.5
35mm 44.3 24.0 36.0