The slanted-edge test image

Imatest SFR measures the spatial frequency response, also known as the Modulation Transfer Function (MTF), of digital cameras and digitized film images.

ISO 12233 chart with crop indicated by red rectangle
ISO 12233 test chart with Imatest SFR cropping indicated by the red rectangle.

It uses a simple slanted-edge (black-to-white) target described in Sharpness: What is it and how is it measured? Although it doesn't provide the as strong a visual indication of MTF as the test described in Lens testing, it provides a far more accurate quantitative measurement.

Imatest SFR also measures Chromatic aberration, noise and Shannon information capacity— a useful but unfamiliar indicator of image quality.

Imatest SFR uses a standardized sharpening algorithm to compensate for the different amounts of sharpening in different cameras, allowing them to be fairly compared.

The algorithms for calculating MTF/SFR were adapted from a Matlab program, sfrmat, written by Peter Burns to implement the ISO 12233 standard. The original Matlab code is available on the I3A download page on ISO 12233 Slant Edge Analysis Tool sfrmat 2.0. You can write Peter at .

A horizontal slanted-edge for measuring vertical MTF performance is shown below; a vertical slanted-edge for measuring horizontal MTF performance is shown on the right. These edges were cropped from an image of the PIMA/ISO 12233 test chart taken by the Canon EOS-10D digital SLR, shown above. A crop area is indicated by a red rectangle.

Cropped slanted-edge image for measuring horizontal MTF
Cropped slanted-edge image for measuring vertical MTF

Printing, assembling, and photographing the target

Although Imatest SFR works splendidly with the IMA/ISO 12233 target (available from Sine Patterns; expensive), you can run it with any clean, sharp, straight black-to-white or dark gray-to-white edge. The solid areas need to be smooth and uniform: white on one side and black or dark gray on the other. Since Imatest does not yet sell a printed chart (it's planned), the quickest way to obtain a chart is to download a file and print it on a high quality inkjet printer.

Download a test chart.

You can download a printable test chart by right-clicking on one of the thumbnails below. (These charts are included in the samples folder of the Imatest installation.) The chart on the left, which has pixel levels 47 and 255, has a contrast ratio of  (255/47)2.2 = 41 when printed at gamma = 2.2 (the normal setting). This is close to the minimum recommended by the ISO 12233 standard. The chart on the right, which has pixel levels 0 and 255, prints with the maximum possible contrast ratio for the printer/paper combination, typically between 80:1 and 100:1. This may result in clipping near the edges in some cameras. For this reason the reduced contrast chart on the left is recommended.

Right-click to download printable edge chart.
Edge_chart_low.png (reduced contrast;
pixel levels 47, 255: recommended)
Right-click to download printable edge chart.
Edge_chart.png (full contrast:
pixel levels 0,255; not recommended)

A printable vector-graphics version of the ISO 12233chart is available courtesy of Stephen H. Westin of the Cornell University Computer Graphics Department. If you use it, you should print it as large as possible so edge sharpness is not limited by the printer itself. (There may be some jaggedness in the slanted edges; not a problem with the recommended target.)

Print the charts

 

with a high quality inkjet photo printer on glossy, semigloss, or luster paper. I recommend printing at least two copies: one for measuring center sharpness and one for edge sharpness. The printed image size should be 8x10 inches (20.3x25.4 cm) or smaller— its size is not critical. Be sure the edges look clean and sharp to your eyes; examine them with a good magnifier or loupe. Chart quality is described in geeky detail in Chart quality and distance, below.

The charts are intended to be tilted approximately 5.7 degrees (anywhere between 4 and 7 degrees is OK) when they are photographed. It is tilted 5.7 degrees when the tick marks, located near the edges, are vertically or horizontally aligned with the center. This is illustrated by the red horizontal and vertical lines on the right.

Properly tilted test chart
The use of guide marks for tilting the chart

The charts are printed straight and physically tilted because the edges print sharper that way. If they were printed at an angle, the printer dot pattern would result in a slight jaggedness. A 5.71 degree angle ( = tan-1(1/10)) is an offset of one part in 10.

Assemble the charts into a target to be photographed.

If you are using other Imatest modules, such as Colorcheck or Q-13, you may want to combine charts.

I used a 32x40 inch sheet of 1/2 inch thick black foam board for the assemblage. The 1/2 inch foam board stays flatter than standard 1/4 or 3/8 inch board. Black board results in less flare light than white board. (Flare light is light that bounces between lens elements and off the inside of the lens barrel, reducing image contrast)

I use Velcro to attach the Colorchecker and Q-13 charts to the foam board. That makes them easy to remove. With Velcro, the surface of the Colorchecker is about 1/4 inch (6 mm) in front of the board.

There is no need to remove the inexpensive printed charts, so I mount them on sheets of 1/4 inch foam board using 3M Photo Mount Spray Adhesive (anything similar will do), then I mount the foam board using RTV silicone seal, which can be removed without much damage.
 
An image of a horizontal or vertical edge on an LCD monitor (desktop or laptop) can also be used as a target. The camera should be tilted with respect to the monitor. (Thanks to Scott Kirkpatrick for the suggestion.)
 
Click here to load an LCD test image. To minimize artifacts (Moire, etc.) the camera must be far enough from the monitor so the sensor pixel "frequency" (1/(2*pixel spacing) at the image sensor) is at least 30% above the Nyquist frequency of the LCD screen. A good rule of thumb is that the LCD screen image should take up no more than 1/3 to 1/4 the image width (1/9 to 1/16 the area), as shown on the right.

My inexpensive Toshiba laptop, shown on the right, displays 1024 pixels horizontally. The Canon EOS-10D has 3072 horizontal pixels. In this case, the laptop image should take up no more than 1/4 the image width.

Click here to view a web page with a suitable LCD test image (dark gray and white rather than black and white to minimize clipping).

Photograph the target.

I placed the the upper right test chart near the center of the image and the  lower left test chart near the corner. The Colorchecker image is suitable for analysis, but the the Q-13 is smaller than optimum. The dominant light was from a skylight. The light was not as even as I would have liked. 

Lighting

The chart below summarizes lighting considerations. The goal is even, glare-free illumination. Lighting angles around 30 degrees are ideal. Al least two lights (one on each side) is recommended; four is better. Beware of lights behind the camera, which can cause glare. Check before you expose.

Distance

Using a letter-sized (8.5x11 inch) chart printed on Permium Luster paper on the Epson 2200 (a high quality pigment-based inkhet photo printer), I analyzed the MTF performance of the 6.3 megapixel Canon EOS-10D. I found no change if the image field was at least 22 inches (56 cm) wide— twice the length of the chart. Performance falls off slowly for smaller widths. Choose a camera-to-target distance that gives at least this image field width. The actual distance depends on the sensor size and the focal length of the lens. The minimum image field is illustrated on the right.

Cameras with more pixels, and hence higher potential resolution, should should have a larger image field width. Some gudelines for the minimum field width are,

Image field width (in inches) > 8.8 * sqrt(megapixels)     ( >  means  "greater than.")
Image field width (in cm)      >  22 * sqrt(megapixels)
                                          — or —
There should be no more than 140 sensor pixels per inch of target or 55 sensor pixels per centimeter of the target.
                                          — or —
The distance to the target should be at least 40X the focal length of the lens for digital SLRs. (25X is the absolute minimun; 40X leaves some margin.) For compact digital cameras, which have much smaller sensors, the distance should be at least 100X the focal length: the field of view is about the same as an SLR with comparable pixel count. The recommended distance is described in geeky detail in Chart quality and distance, below.
The camera-to-target distance is not critical as long as it is greater than a reasonable minimum.

Exposure

Proper exposure is important for accurate Imatest SFR results. Neither the black nor the white regions of the chart should clip— have substantial areas that reach pixel levels 0 or 255. The best way to ensure proper exposure is to use the histogram in your digital camera. Blacks (the peaks on the left) should be above the minimum and whites (the peak(s) on the right) should be below the maximum.

The above image (taken from the Canon File Viewer Utility) is close to a perfect exposure.

Tips on photographing the chart
Distance doesn't matter as long as the target far enough from the camera so sharpness is limited by the camera and lens, not by the target. For a target printed on the Epson 2200 printer, a distance that gives at least a 24 inch (horizontal) field of view seems to be sufficient.
The target should be evenly lit and free of glare.
White balance should be approximately neutral.
Use a sturdy tripod and a cable release. If possible, use the mirror lock. You can use Imatest SFR to find the difference made by a good tripod or mirror lock— to sharpen your technique, literally ( pun intended ).
Be sure to expose the image so detail is maintained in both light and dark areas. Neither should be blocked (clipped). Use your camera's histogram. If more than 0.5% of the pixels are at levels 0 or 255, Imatest SFR will assume that clipping has taken place and issue a warning message. This has no effect on the calculations— it's just a warning that accuracy may be compromised.
Be sure the camera is correctly focused on the chart.
Place slanted-edge images near the corners of the field as well as near the center.
You may find it instructive to photograph the slanted edge target along with a target from Lens testing, but there's no need to do so.

Save the image as a RAW file or maximum quality JPEG. If you are using a RAW converter, convert to JPEG (maximum quality), TIFF (without LZW compression, which is not supported), or PNG. If you are using film, develop and scan it. The file name should be descriptive and should indicate the parameters you are testing. Use dashes and underscores ( - and _ ), but avoid spaces. (Spaces work with Imatest, but they can be troublesome in DOS command lines and web pages.) An example would be Canon_EOS10D_70-200f4L_100mm_f8_ctr.jpg.

You are now ready to run Imatest SFR.

Chart quality and distance
Three printed charts were scanned with the Epson 3200 flatbed scanner at 1200 dpi— a much higher scan density than normal for reflective documents (300 or 400 dpi). At a typical monitor resolution of 96 dpi, the images are enlarged 12.5x. The charts were tilted around 5.7 degrees (a 1.1 inch offset on an 11 inch letter-sized print) for analysis with Imatest SFR.

Epson 2200
Premium Glossy 1440 dpi
High contrast

Epson 2200
Premium Glossy 1440 dpi
Reduced contrast

HP LaserJet 5L
bond paper
Reduced contrast
The high contrast chart appears to be the sharpest. The reduced contrast chart is still very good: its dot pattern (always a concern) is not a problem. The laser print is clearly unsuitable.

Results of running Imatest SFR on the high contrast printed chart are shown below. The SFR input dialog box was set to display cycles/millimeter for 1200 pixels/inch (scan density). The key result is MTF50 (50% contrast spatial frequency) = 5.15 cycles/millimeter (131 cycles/inch). Results in LW/PH and with standardized sharpening (indicated by (corr.); dashed red lines) are not applicable.  
As expected, the reduced contrast chart, which doesn't appear as sharp, has a lower MTF50: 4.31 cycles/mm (109 cycles/inch). It would benefit from sharpening in the image editor.

The chart MTF50 affects the minimum recommended camera-to-target distance. For example, the Canon EOS-10D (and 300D) has a pixel spacing of 7.4 microns. Its Nyquist frequency is 1000/(2*7.4) = 68 cycles/mm. When the target is 40x the lens focal length, so the magnification at the sensor is approximately 1/40 (recommended above), the MTF50 imaged on the sensor for the reduced contrast chart is 4.31*40 = 172 cycles/mm, which is 2.5 times the Nyquist frequency. The imaged target contrast is approximately 0.9 in the critical region around 0.6*Nyquist (0.3 cycles/pixel). This is adequate, though a little more distance) (e.g., 50x focal length) wouldn't hurt.
A print of Stephen H. Westin's printable vector-graphics version of the ISO 12233chart, made on the Epson 2200 at 1440 dpi on Premium Luster paper and scanned by the Epson 3200 at 1200 dpi, has been analyzed to compare slanted and straight edges. The comparison is of interest because Stephen's chart uses vector graphics, hence the slanted edge is as fine as the printer can make; it is not limited by image pixels. Straight and slanted edges where compared with the print scanned straight and tilted.
Document straight
Document tilted

Slanted edge

Straight edge

Slanted edge

Straight edge
The straight edge is clearly smoother than the slanted edge, though the sharpness of the individual edges (horizontal scan lines) is about the same. This MTF results are surprizing: MTF50 for the slanted edge scanned straight is 4.13 cycles/mm, not very different from MTF50 for the straight edge scanned tilted, 4.24 cycles/mm. The difference is more pronounced at lower MTFs (higher spatial frequencies). MTF20 for the slanted edge scanned straight is 8.6 cycles/mm while MTF20 for the straight edge scanned tilted is 10.6 cycles/mm. I clearly perfer the straight edge, tilted. Although the numbers suggest it would make little difference, I recommend a 20% greater distance if the slanted edge is used.