Imatest SFR measures the spatial frequency response, also known as the Modulation Transfer Function (MTF), of digital cameras and digitized film images.
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.
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.
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.
pixel levels 47, 255: recommended)
|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.)
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.
The use of guide marks for tilting the chart
The charts are printed straight and physically tilted because the
print sharper that way. If they were printed at an angle, the printer
pattern would result in a slight jaggedness. A 5.71 degree angle ( = tan-1(1/10))
is an offset of one part in 10.
| 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
them on sheets of 1/4 inch foam board using 3M Photo Mount Spray
(anything similar will do), then I mount the foam board using RTV
seal, which can be removed without much damage.
of a horizontal or vertical edge on an LCD monitor (desktop or
can also be used as a target. The camera should be tilted with
to the monitor. (Thanks to Scott
Kirkpatrick for the suggestion.)
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).
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.
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.
Using a letter-sized (8.5x11
inch) chart printed on Permium Luster paper on the Epson 2200 (a high
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
at least 22 inches (56 cm) wide— twice the length of the chart.
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.
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.
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.