Using SFRreg Part I

Imatest SFRreg performs highly automated measurements of sharpness (expressed as Spatial Frequency Response (SFR), also known as Modulation Transfer Function (MTF)) and Lateral Chromatic Aberration from images that contain registration mark patterns (circles with two light and two dark quadrants). Unlike standard test charts, these patterns do not need to be located on a flat (planar) surface. Since Region of Interest (ROI) detection is automatic, they may be positioned where needed. Depending on the image source, they offer several advantages. You can

Test images at infinity distance (or any distance of choice) using a compact projection system such as a collimator or a printed target. Measure the sharpness of extreme wide angle (fisheye) lenses (with angular fields of view over 180 degrees), whose MTF near the image boundaries cannot be measured with a single flat target. This is accomplished with an array of test charts, each consisting of an individual registration mark. They work best when facing the camera. Measure sharpness at several distances in a single image. Useful for testing focus accuracy and depth of field. If there is a large difference in distances we recommend using larger charts at greater distances. Measure sharpness where an extremely large Field of View (FoV) is required (for example, at very large distances)— much larger than can be printed on a single chart. Several large registration mark charts can be placed over a large area, as needed. Measure sharpness of extremely high resolution cameras (>50 Megapixels) using multiple large charts covering a larger Field of View than is practical with a single chart. The charts don’t have to be as large as SFRplus or eSFR ISO charts.

Registration mark

Registration mark charts, such as the one shown on the right, may be purchased from the Imatest store or printed on a high-quality inkjet printer.

Details of the regions to be analyzed are based on user-entered criteria (similar to SFRplus or eSFR ISO, which it closely resembles).

Sharpness is derived from light/dark slanted edges inside the registration marks, as described in Sharpness: What is it and how is it measured? SFRreg can handle a wide range of camera aspect ratios and chart arrangements.

SFRreg operates in two modes.

• Interactive/setup mode (run by pressing Rescharts in the Imatest main window, then selecting 10. SFRreg, pressing SFRreg Setup , or pressing the Interactive/Postprocessors dropdown menu, then selecting SFRreg setup), allows you to select settings and interactively examine results in detail. Saved settings are used for Auto Mode.
• Auto mode (run by pressing SFRreg Auto or by pressing the Fixed modules dropdown menu in the Imatest main window, then selecting SFRreg), runs automatically with no additional user input. ROIs are located automatically based on settings saved from the interactive/setup mode. This allows images of different sizes and framing to be analyzed with no change of settings. Auto mode works with large batches of files, and is especially useful for automated testing, where framing may vary from image to image.

This document introduces SFRreg and explains how to obtain and photograph the chart. Part 2 shows how to run eSFR ISO interactively inside Rescharts and how to save settings for automated runs. Part 3 and Part 4 illustrates the results for slanted-edge and other measurements, respectively.

Image acquired from a 9-projector system

SFRreg images from arbitrary arrays of printed registration mark charts

SFRreg also works with printed registration mark patterns, which can be placed anywhere in the image. For extreme wide-angle (fisheye) lenses, they should be oriented facing directly towards the camera. You can add other charts— typically color or grayscale— to the image for additional measurements.

Fisheye lens image with registration mark charts oriented facing the camera

The SFRreg Center chart

Though the primary purpose of the SFRreg targets is to allow MTF measurements at arbitrary locations in the camera Field of View (FoV)  (and at arbitrary distances, for maximum versatility), users may need a dense set of MTF measurements near the camera’s center of FoV as well as tonal response, noise, and color measurements. In order to to make this more convenient, we have developed the SFRreg Center chart (which is similar to the ISO 12233:2014 chart).

NOTE: This target does not automatically perform color. tone, noise, tilt, rotation, FoV or Distortion calculations in Imatest versions prior to 5.0. All but FoV and Distortion will be added in 5.0.

This chart can be placed in the central portion of the image, typically subtending 90 to 100 degrees horizontally (but other angles are available). The 90 degree chart is shown on the right. It is used along with single SFRreg charts located elsewhere in image, and typically facing the image (not in the center chart plane). Top views representing some potential arrangements (for a 240 degree camera FoV) are shown below. The charts on the sides typically represent two (or more) charts located in the corners (and possibly sides) of the camera’s field. These are merely suggestions: other arrangements may work better, depending on the angular field of view and camera’s pixel resolution. For example, for smaller angular FoVs, it might be sufficient to have only one set of added charts (at each of the 4 corners).

The SFRreg single chart circles should have circle diameters approximately equal to the outer ellipses in the Center Chart.	The SFRreg single chart circles should have circle diameters approximately equal to the inner circle in the Center Chart.

 

When either of these arrangements are photographed with a fisheye camera with the expected Field of View, the resulting image looks something like this.

A portion of the SFRreg setup window for an image with an SFRreg center chart and four individual charts.
Field of View is approximately 180 degrees. 19 registration marks have been selected.

Tips for ensuring auto-detection and accurate measurements

When using the SFRreg Center Chart, take care to photograph it with the same recommendations as for other resolution-determining test charts. This involves lighting the chart evenly, choosing the correct size and distance, and ensuring reasonable exposure.

Similarly, any single SFRreg targets should be placed at least at a great enough distance away from the camera so that their printing resolution is not a factor. Due to the ability to place SFRreg targets anywhere in a scene, it is not possible to recommend a typical lighting set-up. While even lighting on a target is important to have for both slanted edge analysis and automatic detection of SFRreg targets, it may not always be possible to achieve. 

The SFRreg marks should be positioned to avoid pure horizontal, vertical, or 45 degree edges. Typical edges are 2-7 degrees off of horizontal or vertical, though other orientations like sagittal/meridional edges are acceptable. 

The following guidelines should be followed as closely as possible for the automatic detection routines to work correctly:

• Light each registration mark as evenly as possible.
• Avoid glare (shiny specular reflections). Matte printing helps in this regard. 
• For fisheye (highly barrel-distorted) lenses, the targets should face the camera. (A pre-distorted elliptical SFRreg target may also be desired in such cases, in order to aid detection and maximize the area available for slanted-edge MTF analysis.)
• Care should be taken to avoid interfering patterns that could be mistaken for registration marks.
• The registration marks should have reasonable contrast in the image (regardless of what the printed contrast was) in order to be detected. This means having the difference of the light and dark areas of the marks be at least ~20% of the possible dynamic range of the image.
• The registration marks should be large enough in the image to detect, with diameter at least ~4% of the image height. 
• The image can be cropped to remove interfering features near the edges, using the Crop borders button in the SFRreg setup window.