Centering test charts is critical for image quality testing. If the camera is not properly aligned with the test chart, your image may not be suitable for analysis; you may not know if the results are truly indicative of the camera system, or from an error in positioning the chart.
If the optical axis is offset relative to the center of the target, the resolution regions of interest can become asymmetrical, shifting from the ideal radial field distance and introducing error in your map of sharpness across your imaging plane. If the device is tilted relative to the target, measurements of keystone distortion and convergence angles will have reduced accuracy. Controlling the alignment of the camera relative to the test chart is the best practice to prevent these errors.
Understanding and maintaining proper alignment is crucial for the centering process. The x and y axes that run vertically and horizontally through the camera should be aligned to be parallel to the test chart. The z axis, or the optical axis, should be perpendicular to the chart. Imatest recommends two methods for accurate chart alignment.
The first method, which utilizes the Imatest Modular Test Stand, is a helpful tool for centering the test chart to establish proper alignment. This is Imatest’s recommended procedure. The procedure is as follows:
Once the camera is mounted on the tripod head and the chart is set up, you can begin the centering process.
Make sure the camera is level by adjusting the tripod head’s roll, pitch and yaw until each angle reads 0º, and the bubble level on the tripod head is centered. This brings the camera’s nominal optical axis perpendicular to the chart.
Start by sliding the camera post close to the chart. As seen in the video at 2:14, the camera lens is not centered with the center of the chart. Because the Modular Test Stand is already aligned along the optical axis, the only variables that need adjustment are in the x and y direction.
Make adjustments by moving either the chart or camera to center the camera on the chart. On the Modular Test Stand, Y axis adjustments are more easily achieved by moving the camera, and X axis adjustments are more easily achieved by moving the chart.
Start by making coarse adjustments along these axes, then make finer adjustments as the lens becomes more aligned with the chart center.
Now that the camera has been centered, slide it to your desired working distance.
Evaluate the chart framing using a live feed or a captured image. If there is tilt or rotation present in the image, it could be an issue with your sensor alignment with your device.
The advantage of this method is that the camera will always be aligned along the nominal optical axis.
Another method for chart centering utilizes a tripod and a mirror. This process usually requires an extra set of hands unless you are able to mount the mirror:
Similar to the first method, begin by setting up your camera on the tripod and properly leveling it.
Taking care to avoid touching the chart surface, have someone hold the mirror very close to the center of the chart. The mirror should be as parallel as possible with the chart.
Begin aligning the camera so that, when taking an image, the camera lens is visible in the center of the mirror. It is best to complete adjustments for each individual axis before moving onto the next axis to avoid confusion.
Although this method can work, it is more labor intensive, and it forces the lens and sensor to be parallel, which may prevent you from being able to measure the tilt of the sensor installed in your camera body. It’s also very easy to accidentally misalign any of the axes. Tripods can be sturdy, but can easily be bumped, disorienting the whole setup. As long as care is taken with this method, it can enable measurements of image quality factors besides tilt and rotation to provide you with acceptable results.
Learn more about test charts and how to get started with image quality testing here.