Imatest - Using SFRplus Part 1

Imatest SFRplus performs highly automated measurements of several key image quality factors using a specially-designed test chart. Unlike other modules, the user does not need to manually select Regions of Interest (ROIs). Image quality factors include

Sharpness, expressed as Spatial Frequency Response (SFR), also known as the Modulation Transfer Function (MTF),
Noise,
Lateral Chromatic Aberration,
Distortion (with less detailed output than the Distortion module), and
Tonal response (again, with less detail than Stepchart; no noise statistics)
Color accuracy, when used with an SFRplus that contains the optional color pattern, located above the central square.

SFRplus operates in two modes.

Interactive/setup mode (run in Rescharts), which allows you to set up region selection and calculation parameters and interactively examine results in detail.
Auto mode, which runs automatically with no additional user input. ROIs are selected automatically based on settings saved from the Rescharts interactive/setup mode. This allows images of different sizes and framing to be analyzed with no change of settings. Auto mode is especially useful for automated testing, where framing varies from image to image.

This document introduces SFRplus and explains how to obtain and photograph the chart. Part 2 shows how to run SFRplus inside Rescharts and how to save settings for automated runs. Part 3 illustrates the results.

New in Imatest 3.4.1
Calculations are now much faster. A Speedup checkbox has been added to the SFRplus setup window to further increase speed by eliminating some calculations that most users don't need (SQF, noise histograms, clipping check, etc.).

The SFRplus test chart

The standard SFRplus test chart is illustrated below.

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

Sharpness is derived from light-to-dark slanted edges at the boundaries of the squares, as described in Sharpness: What is it and how is it measured? The new chart enables SFRplus can handle a wide range of camera aspect ratios; the left and right sides of the chat can extend beyond the frame or be inside the frame.

The essential features of the SFRplus chart are

Bars at the top and bottom used for measuring distortion and facilitating edge detection.
A grid of slanted squares with contrast no greater than 10:1. There are 4, 5, or 7 rows of squares. 5x9 and 5x7 are the standard grid sizes. The squares are slanted to facilitate measurement of Lateral Chromatic Aberration using vertical edges (which are near-tangential in orientation) near the corners.
The squares above and below the middle square are reserved for non-spatial measurements. In the standard chart, the square below the middle is a 4x5 stepchart with density steps of 0.1 used to measure tonal response and gamma. The square above the middle square may contain a fine star pattern (used as a focusing aid; not for analysis) or a 4x5 color pattern for measuring color accuracy.
A small number of low contrast (typically 2:1) squares for measuring the effects of nonlinear signal processing. Charts may optionally have squares of a single contrast. This is helpful for 3D plots that map the performance over the image surface.

Advantages of the SFRplus chart over the ISO 12233 chart

Much less wasted area. About 90% of the ISO chart is covered with patterns that have little value for computer analysis.
You can produce a map of sharpness (MTF) over the image surface. This cannot be done with the ISO chart because there are only a few suitable edges— and they are not well-located.
The 10:1 (or lower) contrast edges are less likely to clip than the edges in the ISO chart, which is specified to have a contrast of at least 40:1. The camera operates in a more linear region, and hence results are more consistent and accurate— less affected by overexposure and underexposure. Also, 10:1 edges are more representative of real edges that affect perceived image sharpness.
The low contrast edges (2:1 contrast in a few edges in the standard chart) provide additional information about signal processing in the camera under test. Although Imatest SFR is relatively insensitive to chart contrast (MTF is normalized to 100% at low spatial frequencies), measured SFR is often affected by chart contrast due to nonlinear signal processing in cameras, as described in the box below.
The SFRplus chart is well-suited for automated testing with the Imatet SFRplus module. With the ISO chart, regions of interest (ROIs) must be selected carefully whenever the image framing changes— even slightly.
The measurement is ISO-compliant. ISO-compliant measurements do not require the standard ISO chart. The upcoming revision to the ISO standard will recommend an entirely different pattern of low contrast.
It can measure additional image quality factors, including distortion, gamma (contrast), tonal response, and color accuracy (in charts that have the optional color pattern).

Nonlinear signal processing and chart contrast

SFR (MTF) measurements are often affected by chart contrast due to nonlinear signal processing in cameras, i.e., processing that depends on the contents of neighboring pixels, and hence may vary throughout an image. Nonlinear processing is almost universal in digital cameras (though you can avoid it by using RAW images with dcraw). It improves pictorial quality but complicates measurements. It takes two primary forms.

Sharpening, applied in the proximity of contrasty features like edges. Boosts response at high spatial frequencies.
Noise reduction, applied in the absence of contrasty features. Attenuates response at high spatial frequencies, i.e., removes fine, low contrast detail (texture), which is interpreted as noise. Many cameras increase noise reduction at high ISO speeds.

The signal processing algorithms are proprietary; they are a part of a manufacturer's "secret sauce" for producing pleasing images. Though they vary a great deal, some generalizations can be made.

Most cameras do NOT apply noise reduction and sharpening uniformly throughout an image.
Contrasty edges tend to have better (more extended) MTF than low contrast edges.

For this reason it may be a good idea to photograph both a relatively contrasty edge (though not so high that it causes clipping) as well as a relatively low contrast edge. The SVG charts (above) are excellent choices. Both types of edge can also be produced using Imatest Test Charts. An estimate of chart contrast derived from the average light and dark pixel levels (away from the transition) and gamma is displayed in several places in SFR and Rescharts Slanted-edge SFR. (Estimated chart contrast = (avg. pixel level of light area/avg. pixel level of dark area)^(1/gamma) ).

Nonlinearities are analyzed in depth in the Log F-Contrast module.

SFRplus slanted-edge algorithm The algorithms for calculating MTF/SFR were adapted from a Matlab program, sfrmat, written by Peter Burns (

) to implement the ISO 12233 standard. SFRplus incorporates numerous improvements, including improved edge detection, better handling of lens distortion, and far more detailed output. A description can be found here. The original Matlab code is available on the I3A ISO tools download page by clicking on ISO 12233 Slant Edge Analysis Tool sfrmat 2.0. In comparing sfrmat 2.0 results with Imatest, keep the following in mind: If an OECF (tonal response curve) file is not entered into sfrmat, it assumes that there is no tonal response curve, i.e., gamma = 1. In Imatest, the default gamma is 0.5, which is typical of digital cameras. To obtain good agreement with sfrmat, you must set gamma to 1.

Obtaining and photographing the chart

Summary

Obtain a test chart (Purchase or print)
Mount the chart .
Photograph the chart.
Tips: Lighting | Distance | Exposure | Tips on photographing

You can run Imatest SFR 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, the quickest way to obtain a chart is to print one on a high quality inkjet printer.

Obtain a test chart.

SFRplus standard SVG 5x9 square pattern The standard SFRplus test chart consists of a 5x9 grid of squares, all but four of which have a 10:1 contrast ratio. The contrast ratio of the remaining four (one column off the center) is 2:1. A small 4x5 patch stepchart (densities in steps of 0.1 from 0.05 to 1.95) is located below the central square.

The chart can be purchased a chart from the Imatest store. It should be mounted on 32x40 or 40 x60 inch sheets of 1/2 inch (12.5 mm) thick foam board with spray adhesive or double-sided tape. 1/2 inch foam board stays flatter than standard 1/4 or 3/8 inch board.

Charts are available with a variety of options. The chart on the left below contains a color pattern (an L*a*b* reference file is included with purchase). The single-toned chart on the right below contains a star pattern, which can be used as a focus aid (not for analysis). It be produced as a chrome-on-glass transmission target in very small sizes Details here.

5x9 SFRplus chart with color pattern

Single-toned 5x7 SFRplus chart
with focus star

Photograph the chart.

Frame the chart so that

there is white space above and below the bars (used to measure distortion) at the top and bottom of the images. This space should be at least 0.5% and no more than 25% of the total image height. Ideally the white space should be 2-8% of the image height. The chart should be vertically centered, but this is not necessary for SFRplus to run successfully.
The stepchart pattern is close to the horizontal center of the chart.
The sides of the chart may extend beyond the image (as shown below) or be well within the image. The software is designed to accommodate a wide variety of framing and aspect ratios. Edges closest to the left and right boundaries will always be properly located. If the left and right sides of the chart are inside the image, there should be no interfering patterns in the image that could be mistaken for chart features— the chart should be surrounded with plain white or light gray.

Well-framed SFRplus image

The chart should be aligned correctly using techniques and tricks shown in The Imatest Test Lab. Moderate misalignment is tolerated by SFRplus: a tilt of 1-2 degrees, perspective distortion, etc., but every effort should be made to align the chart properly. Moderate barrel or pincushion distortion (<5% SMIA) is also well tolerated, but Fisheye lenses (with high, intentional barrel distortion) will probably fail.
In Imatest 3.4+ the image can be cropped (starting with) to remove interfering features near the edges, using the Crop button in the SFRplus setup window.

If exposure compensation is available, you may want to use it to get a good exposure: typically by overexposing +1 f-stop.

Bad framing Interfering patterns near borders (Can be cropped in Imatest 3.4+)	Good framing Same pattern; interfering patterns masked out
No white space above top distortion bar	Some tilt, distortion tolerated

Lighting

The chart below summarizes lighting considerations. The goal is even, glare-free illumination. Lighting angles between 30 and 45 degrees are ideal in most cases. At least two lights (one on each side) is recommended; four or six is better. Avoid lighting behind the camera, which can cause glare. Check for glare and lighting uniformity before you expose. A detailed description of the recommended lighting setup, which uses six high quality (CRI > 98) 4700K (near-daylight) 50W SoLux quartz-halogen lamps, can be found in The Imatest Test Lab. SoLux Task Lamps may also be used. The BK Precision 615 Light meter (Lux meter) is an outstanding low-cost instrument (about $100 USD) for measuring the intensity and uniformity of illumination.

Simplified lighting diagram

Distance

Distance and field of view

The camera must be far enough from the chart so you are measuring the sharpness of your camera and lens, not the chart. But remember,

It's the field of view, not the chart-to-camera distance, that counts.

A rough rule of thumb: For an inkjet-printed chart the field of view should be at least

22 inches (60 cm) for a 6-megapixel camera;
35 inches (90 cm) for a 16 megapixel camera.

Details below. For a high-quality photographically-printed chart (such as the charts from Applied Imaging) you can get quite a bit closer.

A letter-sized (8.5x11 inch) chart printed on Premium Luster paper on the Epson 2200 (a high quality pigment-based inkjet photo printer) was analyzed for MTF using the 6.3 megapixel Canon EOS-10D. There was no change when the image field was at least 22 inches (56 cm) wide— twice the length of the chart. Performance fell off slowly for smaller fields.

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, hence printed chart width.

Distance/field width guidelines for high quality inkjet charts
(You can get closer with photographically-printed charts.)

The camera-to-target distance is not critical as long as it is greater than a reasonable minimum.


Sensor sizes
Desig- nation	Diagonal mm.	Width mm.	Height mm.
1/3.6"	5.0	4.0	3.0
1/3.2"	5.68	4.54	3.42
1/3"	6.0	4.8	3.6
1/2.7"	6.59	5.27	3.96
1/2"	8.0	6.4	4.8
1/1.8"	8.93	7.18	5.32
2/3"	11.0	8.8	6.6
1"	16.0	12.8	9.6
4/3"	22.5	18.0	13.5
35mm	44.3	24.0	36.0

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 6-10 megapixel digital SLRs. (25X is the absolute minimum for 6 megapixel DSLRs; 40X leaves some margin.) For compact digital cameras, which have much smaller sensors, the distance should be at least 100X the actual focal length: the field of view is about the same as an SLR with comparable pixel count. The recommended distance is described in more detail in Chart quality and distance, below.

There is some confusion about lens focal lengths in compact digital cameras. They are often given as the "35mm-equivalent," which many photographers can relate to viewing angle. 35-105mm or 28-140mm are typical "35mm-equivalent" numbers, but they are not the true lens focal length, which is often omitted from the specs. What is given is the sensor size in 1/n inches, a confusing designation based on the outside diameter of long-obsolete vidicon tubes. It The table on the right relates the 1/n designation to the diagonal dimension of the sensor.

True focal length = "35mm-equivalent" × (diagonal mm.) / 44.3

Exposure

Good exposure is important for accurate SFRplus 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. Some exposure compensation, typically around +1 f-stop, may be helpful.

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 SFRplus.

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