Log F-Contrast

Revised Log F-Contrast chart: works with automatic region detection

Introduction

Log F-Contrast (short for Log Frequency-Contrast; not in Imatest Studio) measures the effects of signal processing— noise reduction and sharpening— on imaging system performance using a chart that varies in spatial frequency on the horizontal axis (log frequency increases with x) and in modulation ((max-min)/(max+min) signal, sometimes called Michelson contrast) on the vertical axis. Modulation is proportional to (y/h)² for image height h. Because the Log F-Contrast chart is sinusoidal (rather than a bar pattern) its modulation is equivalent to SFR or MTF.

Log F-Contrast can be run through the Rescharts interface or as a fixed module (capable of analyzing batches of images).

This module is particularly valuable for measuring the loss of fine, low-contrast detail (texture) due to software noise reduction, which can result in a “plastic” appearance in areas where fine texture is missing. This type of nonuniform image processing, which is common in consumer cameras, especially those with tiny pixels (< 2 microns) such as camera phones, is often implemented as a bilateral filter, which can be simulated and analyzed in the Imatest Image Processing module. The chart can be purchased at the Imatest store or 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. The chart design was modified in July 2014 to work with automatic region detection in Imatest 4.0+.

Log F-Contrast Image, Canon EOS-20D camera, 24-70mm f/2.8L lens set at 42mm, f/5.6, ISO 100
(pre-Imatest 4.0 version).  Click on image to load full-size image.

Log F-Contrast divides the chart 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 and readability of the MTF calculations. Spatial frequency is derived from the pattern.

Test charts window, showing the revised chart (works with auto region detection in Imatest 4.0+)

Creating and printing the chart

The Log Frequency-contrast can be purchased from the Imatest Store, downloaded, or printed on a high quality photographic printer from a file created with Imatest Test Charts.

Test Charts has a number of options. The dialog box and recommended settings are shown on the right. The post-July 2014 chart design, which includes registration marks near the corners, is shown.

Example: The Log Frequency-contrast chart shown on the right occupies 857 of 3504 horizontal pixels of an original image taken with the Canon EOS-20D. f = 857/3504 = 0.2446. The chart was created with m = 1000 cycles/pixels. The maximum spatial frequency in the chart image is m_image = m/fW = 1.17 cycles/pixel— slightly higher than optimum, but not excessive because the highest frequencies occupy relatively little chart real estate. The chart image spatial frequencies are detected automatically.

When the settings are correct, select the TIFF output file name using the Test chart directory and File name boxes on the lower-right and click Create test chart. Do not print from the preview image. Print the image from the TIFF output file from an image editor or viewer, paying careful attention to the printed image size and color management settings. More details can be found on Test Charts. Examine the printed chart carefully for defects and for image quality, especially at high spatial frequencies. The tones in the 16-step grayscale step chart at the bottom of the test chart should be compared with a standard chart such as the Kodak Q-13/Q-14 with density steps of 0.1. (There may be some divergence in the darkest zones, but zones 1–12 should be very close.) The sine pattern tones will be accurate if the tones match visually.

Photographing the chart and running the program

Mount the chart on a flat dark board— 1/2 inch foam board works well; thinner board warps more easily. Depending on the number of horizontal pixels in the chart to be analyzed, the chart should occupy 1/3 to 1/4 of the horizontal frame. Other charts can be mounted along with it.

Orientation. Although the orientation shown on this page is recommended, the pattern may be rotated by multiples of 90 degrees: it can have portrait as well as landscape orientation.

Photograph the chart using the sort of lighting described in Imatest Lab or How to test lenses, taking care to avoid glare. Save the image in any one of several high quality formats, but beware of JPEGs with high compression (low quality), which will show degraded quality, unless, of course, you are testing JPEG degradations. (Unlike the slanted-edge, the Log f-Contrast pattern reveals JPEG losses quite clearly). RAW images decoded with dcraw tend to produce very different results from in-camera JPEGs.

For Rescharts (interactive) analysis: Open Imatest, then click on Rescharts  for an interactive analysis. Select a pattern to analyze (in this case, Log F-contrast) by clicking an entry in the popup menu near Chart type or by clicking on Read image file if Log F-contrast is displayed. The Read image file button and popup menu (shown on the right) are highlighted (yellow background) when Rescharts starts. Select the image file to read.

For fixed (non-interactive; batch capable) analysis: Open Imatest, then click on the Log F-Contrast button in the Imatest main window. Select the file or batch of files to analyze.

If the pixel size is the same as the previous Log F-contrast run, you’ll be asked if you want to use the previous ROI, adjust the previous ROI, or crop anew.

Cropping The pattern should be cropped to leave a small margin (about 1/2 – 1% of the image height) at the top and bottom of the pattern, as shown below. The fine adjustment window may be maximized to facilitate fine selection.

Crop for the Log Frequency-contrast chart. Leave a small margin
(1/2 – 1% of the image height) at the top and bottom.

If Yes, Continue (not Express mode) is selected, the settings window shown on the right appears.

Gamma is used to linearize the test chart. It can be measured by Stepchart or Multicharts using the 16-step (0.1 density increment) grayscale below the Log F-C pattern. 0.5 is a typical value for color spaces intended for display at gamma = 2 2 (sRGB, Adobe RGB, etc.).

Channel is R, G, B, or Y (luminance; the default).

MTF plots selects the x-axis scaling. If Cycles/inch or Cycles/mm are selected, the pixel spacing (um/pixel, pixels/inch, or pixels/mm) should be entered.

X-axis scaling for linear plots selects the maximum spatial frequency to be displayed.

Plot (not for Rescharts) The five plots in this section are available as output figures. They are referenced as Plot n in fixed module below. EXIF data appears on the right of plots 1, 2, 3, and 5.

Don’t worry about getting all settings correct: You can always open this dialog box by clicking on More settings in the Rescharts window.

After you press OK, calculations are performed and the most recently-selected display appears.

Output

The Display box in the Rescharts window, shown below, allows you to select any of several displays. Display options are set in boxes that appear below Display. All displays except Exif data have a channel selection option (Red, Green, Blue, or Luminance (Y) (0.3R + 0.59G + 0.11B).

Pattern (original and linearized)

Pattern: original and linearized. Similar to Plot 1 in fixed module

The entire Rescharts window is shown. The upper plot is the normalized pixel level (pixel level/255 for bit depth of 8) of the top of the image (row 1 of 25; where modulation is maximum). The lower plot (linearized using the gamma setting) allows the row to be selected: The cyan plot is the top of the chart (the highest modulation). Note that row 10, which is displayed in black, has much less sharpening (much less of a peak around 0.2 cycles/pixel) than maximum modulation row 1.

MTF

The display on the right shows unnormalized MTF on a linear frequency scale for several contrast bands (rows) of the chart, which is divided into 25 rows for analysis, with the highest modulation at the top and lowest at the bottom. MTF is displayed for rows 1, 4, 7, 10, 13, 16, 19, and 22.The dark lines have been strongly smoothed. The pale dashed lines are the unsmoothed results: the fine detail consists of calculation artifacts caused by phase variations and noise; it has no significance.An unnormalized or normalized plot may be selected in a dropdown menu in the plot settings area.The maximum display frequency, the display scale (cycles/pixel, cycles/mm, cycles/in, or LW/PH (Line Widths per Picture Height, where 2 Line Widths = 1 cycle or line pair)), and gamma can be set to any of several options by pressing the More settings button.The text beneath the plot shows the modulation and contrast (at low spatial frequencies) and MTFnn and MTFnnP (the spatial frequencies where response falls to nn% of the low frequency and peak values, respectively) for nn = 50, 20, and 10%, and for rows 1, 4, 7, 10, 13, 16, and 19. The same results appear beneath the 2D pseudocolor and MTFnn displays.

MTF: Panasonic TZ1, ISO 200, Linear frequency scale

The display on the right shows the same MTF results, normalized and plotted on a logarithmic frequency scale. Normalization makes it easier to compare MTF falloff and to visually determine MTFnn. A vertically squeezed version of the image is shown just above the plot. Row 22 (very low modulation) is omitted because normalization exaggerates noise unacceptably. The modulation, contrast, MTfnn, and MTFnnP for each row is shown in the table below the image.

MTF: Panasonic TZ1, ISO 200, Log frequency scale

MTF/contrast (2D pseudocolor contour)

This selection produces the most detailed results. It has several options:

Parameter determines which parameter to display. Normalized contrast level is the most generally useful choice.

Parameter	Description
Modulation (unnormalized)	Displays MTF, which decreases with chart modulation (vertical position in the 2D pseudocolor display). The chart is divided into 25 rows for analysis.MTF at low spatial frequencies is equal to chart modulation.
MTF (normalized – most useful display)	Displays MTF normalized to 1 at low spatial frequencies for all chart contrast levels.This is the most generally useful display because it clearly shows how response changes with contrast level. In a camera with uniform signal processing (i.e., the same sharpening and noise reduction throughout the image, regardless of the presence of edges), the contour lines would be vertical. Most cameras have some sharpening (high frequency boost) in the presence of contrasty edges and noise reduction (high frequency attenuation) in their absence. This is evident in the display on the right (explained in more detail below). The table below contains the relationship between the modulation and chart contrast scales. See Log F-Contrast from Imaging Resource for numerous examples.	Plot 4 in the fixed module
Normalized contrast loss	Displays MTF normalized to 1 at low spatial frequencies for all chart contrast levels, and at the maximum contrast level for all spatial frequencies (the top of the chart). Results can be difficult to interpret, especially in the presence of noise.

Style selects the frequency scale and whether to show a color bar to the right of the main display.

Sharpness and Texture Analysis using Log F‑Contrast from Imaging-Resource compares the the effects of sharpening and noise reduction in several cameras using images downloaded from Imaging-Resource.com. using the MTF (normalized) plot. The differences in signal processing are very revealing.
The displays on the right show the Normalized contrast level (MTF normalized to 1 at low spatial frequencies for all chart modulation levels) for the Panasonic TZ1 camera with ISO speed set at 80 (right) and 800 (right, below). Spatial frequency is displayed on a linear scale with a maximum frequency of 0.5 cycles/pixel = 1920 LW/PH (the Nyquist frequency; selectable by pressing More settings ).The image on the right (ISO 80) shows significant sharpening at moderate to high modulation levels: peak normalized modulation is over 1.1 for chart modulation (shown in the scale on the left) greater than 0.18 (about 1.3:1 contrast ratio; shown inside the image, just to the left of the row number). The amount of sharpening decreases very gradually with chart modulation, indicated by the moderately slanted contour lines between 0.9 and 0.1 at modulations over 0.2.

Plot 4 in fixed module

Normalized contrast: Panasonic TZ1, ISO 80

The difference between the two images is striking. At ISO 800 the TZ1 has much less sharpening and more noise reduction— and correspondingly less detail. The amount of sharpening decreases more rapidly with contrast than for the ISO 80 case, as shown by the more slanted contour lines.

Normalized contrast: Panasonic TZ1, ISO 800

MTFnn (frequencies where MTF = nn% of low frequency or peak values)

This selection produces a concise summary of results for this module. It has several options:

The plots on the right show MTF70 through MTF10 in units of Cycles/Pixel for the Panasonic TZ1 camera with ISO speed set at 80 (right) and 800 (right, below). They are derived from the same images as the Normalized contrast level plots, above.The MTFnn units (Cycles/pixel to Cycles/mm, Cycles/in, or LW/PH) can be selected using the More settings button.

MTF70-MTF10: Panasonic TZ1, ISO 80

At ISO 800 the TZ1 has much lower MTFnn for all values of nn from 70 through 10. Note that the vertical scales are different: 0.5 (above; ISO 80) vs. 0.35 (right; ISO 800).It also falls off much more rapidly with contrast; evidence of stronger noise reduction, i.e., nonlinear signal processing.The MTFnn display shows the changes in performance between different settings (ISO 80 and 800 in this case) more clearly than the 2D pseudocolor contour plots, above, even though (or perhaps because) it contains less detail.

MTF70-MTF10: Panasonic TZ1, ISO 800

 

Calculation details

Second-order interpolation, illustrated below, is used in detecting peaks at spatial frequencies between 0.06 and 0.5 cycles/pixel It increases the peak amplitude for detected peaks between sampling points.. At lower frequencies it does little to improve accuracy because the signal varies slowly. It also has no benefit above the Nyquist frequency (0,5 cycles/pixel), where the signal consists of noise and aliasing. The red dots in the illustration show the peak amplitudes corrected with second order interpolation (with uncorrected peak locations).

The peak and its two adjacent points are assumed to fit the equation, y = ax² + bx + c. The actual peak location is x_p = –b/2a (x₂ is shown in the above plot). The peak amplitude is a_p = b²/2a + c.

Smoothing  The MTF & Moiré displays contain two curves: unsmoothed (thin dashed lines) and smoothed (thick solid lines). The smoothed curves are emphasized because the roughness in the unsmoothed curves is entirely an artifact of the calculations, resulting from sampling phase and noise— irregularities can result from noise on a single peak. Irregularities are reduced, but not eliminated, by second-order interpolation. It is important to realize that the features of the unsmoothed curve have no physical meaning.

Frequency display  Let x₁ and x₂ be the left and right x-axis limits and let f₁ and f₂ be the minimum and maximum display frequencies. Then,

Display frequency = \(\displaystyle f = f_1 10^{k(x-x_1) / (x_2-x_1)}\)     where   \(\displaystyle k = log_{10}(f_2/f_1)\)

Frequencies are detected automatically in Log F-Contrast. The frequency ratio (f₂ / f₁) is specified when creating the chart in Test Charts.