Modules

SFR

Measure sharpness from slanted-edges in any image

Imatest™ SFR measures image sharpness and Lateral Chromatic Aberration (LCA) from simple slanted-edge images, illustrated below. In Imatest Master it can analyze raw files.

SFR measures Spatial Frequency Response (SFR), which is also called the Modulation Transfer Function (MTF). The primary summary metric for sharpness is MTF50, the spatial frequency where contrast drops to half its low frequency value. SFR and MTF are introduced in What is image sharpness and how is it measured?

Concise instructions for using Imatest SFR and interpreting its results can be found in How to test lenses with Imatest. In many cases it will be advantageous to use SFRplus, which is a newer module that uses a specially-designed test chart to measure sharpness, LCA, and several additional image quality factors with with a high degree of automation.

Comparison of SFR with SFRplus
SFR SFRplus
Measures sharpness from slanted-edges in any image Requires the SFRplus test chart
Regions must be manually selected. Can be automatically refined (to a limited degree) for batches of similar images (which must have the same pixel size). Regions are automatically selected based in preset criteria. Work for batches of images that can be framed quite differently and have different pixel sizes.
Complicated to map sharpness of an image surface Very easy to map the sharpness of an image surface
Also measures Lateral Chromatic Aberration and noise (in light and dark areas near edges) In addition to all SFR measurements, measures distortion, tonal response, and color response. Produces very nice 3D plots
To measure sharpness,

  • Purchase a test chart from the Imatest Store or print one on a photographic-quality inkjet printer from a file created by Test Charts.
  • Assemble one or more test images into a target. The standard version of Imatest SFR analyzes images tilted between 2 and 7 degrees from horizontal or verticall. The tilt is required to obtain a good distribution of sampling phases so results are not sensitive to the edge relative to the pixels.
  • You can analyze edges of any angle in Imatest Master.
  • Photograph the target under good conditions: even glare-free light, sturdy camera support.
  • Run Imatest SFR. Unless the image has been cropped beforehand you’ll need to select a region of interest (ROI), i.e., crop it. Typical crop sizes are 120×80 pixels or larger, but crops can be as small as 10×10 pixels (with reduced accuracy).

Sharpness

The Figure below illustrates the primary results of running Imatest SFR: plots of sharpness in two domains spatial (the average edge profile) and frequency (spatial frequency response (SFR/MTF) curves). The two plots present similar information from two different viewpoints. Input data is shown on the right of the Figure: a thumbnail of the complete test image, the selected Region of Interest (ROI), and selected EXIF data (information recorded by the camera). The industry-standard ISO 12233 test chart is shown, but you can print your own charts on a high quality inkjet printer.

The upper left plot shows the cross section of an average edge. The narrower this edge, the better the performance. The black curve is the original edge for the selected channel, usually Y (luminance). The thin, light, dashed curves are the edges for the Red, Green, Blue and  Luminance channels. The dashed red curve is the edge for the selected channel with standardized sharpening— the output of an algorithm that sharpens (or de-sharpens) the image so the edge overshoot is similar for different cameras. This allows
cameras with different amounts of built-in sharpening to be compared fairly. Standardized sharpening tones down the original edge of this particular camera, which is strongly oversharpened.

The primary spatial domain performance indicator is the 10-90% rise distance, indicated in black for the original edge and red for the edge with standardized sharpening, labeled 10-90%(corr) for “corrected.” Shorter is better.

The lower left plot shows the Spatial Frequency Response (SFR), also called modulation transfer function (MTF), i.e., the image contrast relative to contrast at low spatial frequencies. Black is for the original; dashed red is with standardized sharpening. The thin, light, dashed curves are the MTFs for the Red, Green, Blue and Luminance channels. The primary performance indicator is MTF50 (the spatial frequency where image contrast drops to 0.5 = 50%). The higher the better, though excessive response above the Nyquist frequency (0.5 Cycles/Pixel; the highest spatial frequency where information can be conveyed) may indicate aliasing problems (low frequency artifacts). “Ideal megapixels” (shown in brackets […] ) is the number of megapixels with ideal response (MTF50 = 0.5 Cycles/Pixel) that would have equivalent sharpness. Practical digital cameras rarely reach this level; MTF50 around 0.33 Cycles/Pixel is very decent.

One or two secondary readouts are available. It defaults to MTF30 (the spatial frequency for 30% contrast), but it can be set to any contrast level or to MTF at a specified spatial frequency in cycles/pixel, line pairs/inch or line pairs/mm. For example, you might select the half-Nyquist frequency, 0.25 cycles/pixel.

The uncorrected response (MTF50 or 10-90% rise distance) is appropriate for evaluating the performance of different lenses on a single digital camera. The response with standardized sharpening is appropriate for comparing
different digital cameras.

These measurements begin to make sense when cameras are compared, as they are here. The page on Sharpness has more details on MTF. SFR Results: MTF (sharpness) plot describes this Figure in detail.

Subjective Quality Factor (SQF)

MTF is a measure of device or system sharpness, only indirectly related to the sharpness we perceive when we observe a print. To estimate perceived sharpness for a print of a given size, we need a formula that includes assumptions about viewing distance (usually proportional to the square root of the print height, but other options are available) and the human visual system (the human eye’s Contrast Sensitivity Function (CSF)). Such an formula, called Subjective Quality Factor (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 was added to Imatest in October 2006.

SQF (Subjective Quality Factor)

A portion of the Imatest SFR SQF figure is shown above, with SQF plotted as a function of print size. Viewing distance (pale blue dashes, with scale on the right) is assumed to be proportional to the square root of picture height. SQF is shown with and without standardized sharpening. SQF is extremely sensitive to sharpening, as you should expect since sharpening is applied to improve perceptual sharpness. Since this camera is rather oversharpened— enough to have significant “halos” near edges, standardized sharpening reduces the MTF peak, and hence SQF. An interpretation of SQF is give here. Generally, 90-100 is excellent, 80-90 is very good, 70-80 is good, and 60-70 is fair (these numbers may be shifted as more data becomes available). A peak over about 105 may indicate oversharpening (strong halos near edges), which can degrade image quality. SQF measurements are more valid when oversharpening is removed, which is accomplished with standardized sharpening.

Chromatic aberration

SFR measures lateral chromatic aberration, also known as “color fringing,” which is most visible on tangential boundaries near the edges of the image.

The curves for the R, G, and B color channels are shown below. The visibility of the chromatic aberration is proportional to the area between the highest amplitude transition (in this case, red) and the lowest amplitude transition (in this case, blue).

Chromatic aberration

 

This area is called the area chromatic aberration. It is displayed in magenta. Since the edge amplitude is normalized to 1, area CA can be measured in pixels— a simple unit of length. But since CA in pixels tends to be proportional to the distance from the image center, a better indication is obtained by expressing CA as the percentage of the distance from the image center. This has the advantage that it is relatively independent of the measurement location and the number of pixels in the camera. CA under 0.04% of the distance from the center is insignificant; CA over 0.15% can be quite visible and serious. 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.

 

See Chromatic
Aberration
for more details. Chromatic Aberration, Noise, and Shannon Capacity plot describes this Figure in detail.

Multiple runs and regions

Batch runs  Imatest SFR allows you to perform batch runs on several files of the same size and same framing by simply selecting several files (Pro only). This can be particularly convenient when testing a lens at several f-stops.

Repeated runs  Imatest Master can automatically refine ROI (region of interest) selection during repeated runs and batch runs. This can be especially valuable when different cameras and lenses are being tested and chart alignment cannot be perfectly maintained.

Multiple ROI runs  You can also select several ROIs in a single run, then display one of the two summary plots shown below.

SFR Multiple ROI 2D plot

Multi-ROI 2D summary plot, showing Center-corner distance,

MTF50, MTF20, and CA superimposed on image.

This display can be particularly useful because lenses are not always centered properly. (If they were, performance would be a simple function of the distance from image center.) You can quickly scan the summary results, then look at the detailed results for the individual region.

Multiple ROI (Region of Interest) plot, describes this Multi-ROI figures in detail.

You can learn more about Imatest SFR from
What is image sharpness and how is it measured?How to test lenses with Imatest

Standardized sharpening | Chromatic aberration

Shannon information capacity

Instructions and reference
Using Imatest SFR: Part 1: Setting up and photographing the target

Using Imatest SFR: Part 2: Running SFR

MTF (Sharpness) plot

Chromatic Aberration, Noise, and Shannon Capacity plot

Multiple ROI (Region of Interest) plot, describes this Multi-ROI figures in detail.