Imatest™ SFRplus measures image sharpness and several additional image quality factors, including Lateral Chromatic Aberration (LCA), noise, distortion and tonal response, using a special test chart that provides a high degree of automation.

The primary sharpness indicator is MTF50, the spatial frequency where contrast drops to half its low frequency value. Spatial Frequency Response (SFR), also known as Modulation Transfer Function (MTF), is introduced in What is image sharpness and how is it measured?

Full instructions for using SFRplus and interpreting its results can be found in Using SFRplus Part 1: Setup, Part 2: Running, and Part 3: Results.  To use SFRplus,

  • Purchase or print an SFRplus test chart and mount it on a rigid substrate (typically 1/2 inch foam board).

SFRplus test chart
Standard SFRplus test chart: 5x9 grid, 10:1 and 2:1 contrasts,
with 20-step 4x5 stepchart
(0.1 density increment).

  • Photograph it so that there is a small amount of white space on the top and bottom. It should be centered, but the sides don’t matter. Lighting should be even and glare-free.
  • Run the Rescharts SFRplus module. The SFRplus parameters & setup window allows you to choose criteria for automatic ROI selection as well as several additional
    settings. Pressing Display options brings up even more settings.

SFR parameters & setup window
SFRplus parameters & setup window showing well-framed SFRplus chart image;
9 regions selected for analysis

  • When settings are complete press OK to save the settings and run Rescharts SFRplus. You can also press Save settings to save settings for use in the fully automatic version of SFRplus, which is run by pressing the SFRplus button in the Imatest main window. If SFRplus is run inside Rescharts a large variety of displays is available.

SFRplus multiple region summary display
SFRplus results in Rescharts window: Multiple region (ROI) summary

The multi-ROI summary results shown in the Rescharts window (above) is the best summary of SFRplus results. It is described in detail in Multiple ROI (Region of Interest) plot. The upper left contains the image in muted gray tones. The selected regions are surrounded by red rectangles and displayed with full contrast. Four results boxes are displayed next to each region. There is a legend below the image. Distortion statistics are shown in the lower left. SMIA TV distortion is the simplest overall measure of distortion.

Edge and MTF display
Edge and MTF display in Rescharts window

The Edge and MTF display is identical to the SFR Edge and MTF display. MTF is explained in Sharpness: What is it and how is it measured? The average edge (or line spread function) is plotted on the top and the MTF is plotted on the bottom. There are a number of readouts, including 10-90% rise distance, MTF50, MTF50P (the spatial frequency where MTF is 50% of the peak value; differing from MTF50 only for oversharpened pulses), the secondary readout (MTF20 in this case; selectable to MTFnn or MTFnnP at any contrast level nn or MTF at a spatial frequency specified in cycles/pixel, line pairs/inch or line pairs/mm.), and the MTF at the Nyquist frequency (0.5 cycles/pixel).

Chromatic Aberration

Lateral Chromatic Aberration (LCA), also known as “color
fringing,”
is most visible on tangential boundaries near the edges of the image. Much of the plot is grayed out if the selected region (ROI) is too close to the center (less than 30% of the distance to the corner) to accurately measure CA.

The area between the highest and lowest of the edge curves (shown for the R, G, B, and Y (luminance) channels) is a perceptual measurement of LCA. It has units of pixels because the curves are normalized to an amplitude of 1 and the x-direction (normal to the edge) is in units of pixels. It is displayed as a magenta curve.

Chromatic Aberration display
Lateral Chromatic Aberration

Perceptual LCA is also expressed as percentage of the distance from center to corner, which tends to be more reflective of system performance: less sensitive to location and pixel count than the pixel measurement. Values under 0.04% of the distance from the center are insignificant; LCA over 0.15%
can be quite visible and serious.

Information for correction LCA (R-G and B-G crossing distances) is also given in units of % (center-to-corner) and pixels. LCA can be corrected most effectively before demosaicing. Results are explained in Chromatic Aberration … plot.

SQF (Subjective Quality Factor)

SQF is a perceptual measurement of the sharpness of a display (monitor image or print). MTF, by comparison is device sharpness (not perceptual sharpness). SQF includes the effects of the human visual system’s Contrast Sensitivity Function (CSF), print (or display) size, and viewing distance (which is assumed to be proportional to print height, by default).

See Introduction to SQF for more detail. SQF was developed by Eastman Kodak scientists in 1972. It has been verified and used inside Kodak and Polaroid, but it has remained obscure until now because it was difficult to calculate. Its only significant public exposure has been in Popular Photography lens tests

 

SQF (Subjective Quality Factor)
SQF (Subjective Quality Factor)

Noise spectrum & Shannon capacity

This may be one of the less frequently referenced displays. It is explained in detail in Chromatic Aberration, Noise, and Shannon Capacity plot. See also Noise. Calculations are made in portions of the ROIs (Regions of Interest) away from the transitions.

Noise spectrum contains qualitative information about noise visibility and software noise reduction, which generally reduces high frequency noise below 0.5, typical of demosaicing alone.

Noise and Shannon capacity
Noise spectrum & Shannon capacity

Shannon capacity is an experimental measurement of image quality based in the image’s information-carrying capacity. It is an experimental measurement whose accuracy can be severely affected by software noise reduction and sharpening. Noise reduction (lowpass filtering) improves the measured SNR (Signal-to-Noise Ration), which improves Shannon capacity numbers, but it actually reduces the information in the image by removing low contrast detail at high spatial frequencies. Shannon capacity should be interpreted with caution.

Tonal response & gamma

This display is derived from the 4x5 stepchart pattern, located just below the central square of the SFRplus test chart. It resembles the third figure in Stepchart. The upper plot shows the tonal response for all colors. The lower plot shows instantaneous gamma— the slope (derivative) of tonal response. The value of gamma may differ slightly from the values in the Edge response and MTF display because it’s calculated differently– based on the average slope of the light to middle tone squares of the stepchart.

Tonal response & gamma
Tonal response & gamma

Histograms and noise statistics

This display contains histograms of pixel levels for individual ROIs (original on top and linearized using input gamma on bottom). The black (background) histogram contains pixel levels for the entire ROI. The red histogram is for the light region, away from the transition, used in the noise statistics calculation. The cyan histogram is for the dark region. Sharpening may cause extra bumps to appear in the black histogram.

Histograms and noise statistics
Histograms and noise stats

You
can learn more about Imatest SFR from
What
is image sharpness and how is it measured?

How to test lenses with Imatest

Chromatic aberration  | Shannon information capacity

Instructions and reference
Using Imatest SFRplus: Part 1: Setting up and photographing the chart
Using Imatest SFRplus: Part 2: Running SFRplus
Using Imatest SFRplus: Part 2: Results
MTF (Sharpness) plot
Chromatic Aberration, Noise, and Shannon Capacity plot

Multiple ROI (Region of Interest) plot

Page updated: 2013-10-16