Having trouble with your Imatest test results? This article explains the five most common pitfalls in image quality testing, and how to resolve them.
You’re using a chart with overly high contrast, like the obsolete ISO 12233 test chart.
Your chart may have too high contrast edges. The most common example we see of this is the obsolete ISO 12233 test chart, although this can be the case with any test chart. If you are using the ISO 12233 2000 test chart, you should not be.
The ISO 12233 2000 standard test chart has a very high contrast ratio (the white is very white and the black is very black) that can cause clipping in signal and lead to invalid MTF values. This is because the contrast of the chart often saturates the sensor, reporting higher MTF results than there might actually be. Therefore, we recommend using a chart with a 4:1 contrast ratio as revised in the ISO 12233 2014 standard, such as the ISO 12233 2014 chart or SFRplus chart. The lower contrast ratio produces more accurate measurements by providing MTF measurements that do not clip.
If you need help selecting the right contrast in a chart, email us at firstname.lastname@example.org.
You’re using the wrong dynamic range chart.
When you’re testing dynamic range, you need to make sure you’re using a dynamic range chart which has a higher dynamic range than your camera to ensure you’re measuring the camera and not the chart. The common pitfall here is that people tend to use tonal step charts which have lower dynamic ranges than their sensors, such as the old Kodak Q14 or X-rite colorchecker. For example, if you are using a typical matte reflective chart which has a dynamic range of 48dB, and you’re testing a camera which has a dynamic range of 80dB, then the highest result you could get is 48dB. This is inaccurate.
However, if you’re using a chart with 120dB, you’ll get a much more accurate reading because it will measure the camera and not the chart. Before you test, you should check to see that your camera is not exceeding the dynamic range of what your chart is rated for. Once you have verified this, you will know which dynamic range chart to select. Our 36-Patch Dynamic Range chart is the most popular chart we sell with densities exceeding, 50db, 100db, and 150db.
Verifying the quality of your dynamic range chart.
The usage of linear dynamic range charts such as the Stouffer T4410 or Xyla target causes many problems including difficulty selecting dark regions that do not have visible contrast (especially using distorted lenses) as well as the linear density slope which interferes with the radial non-uniformity of the imaging system. The ISO standard dictates that each patch should be the same distance from the center and charts that have radial layouts of patches are much less impacted by light falloff. Also, targets which are very dark are not representative of more luminous scenes and overlook the impact of flare on dynamic range which leads to higher dynamic range performance, which will not hold up when the camera is exposed to a scene which is not mostly blackened. Imatest’s 36-patch targets include an option for a “DarkWorld” mask which can produce both dark and gray scenes using a single target.
If you need help determining which chart is best suited for you, please email us at email@example.com.
Your chart quality is poor.
Similar to the dynamic range chart pitfall, when testing the sharpness of your camera system, you need to ensure the resolution of your chart is high enough for the device you’re testing. If your reflective or transmissive chart’s MTF is too low the accuracy of your camera resolution measurements will be limited by the resolution of the chart.
The culprit is print quality. The print process can affect the results of your test, especially at close distances or with high-megapixel cameras. For example, using a chart printed on a basic laser printer to test a high-resolution mobile phone camera, could reveal inaccurate results, shown by the steps in the MTF graph.
Avoid this pitfall by using a high-quality chart on higher resolution substrates such as inkjet or film, or by applying MTF Compensation to compensate for the chart quality. To ensure the highest quality, we encourage you to get charts printed from Imatest. If you don’t know what quality chart you need, contact firstname.lastname@example.org.
You’re not filling the camera field of view.
When testing, it’s important to ensure you’re completely filling the camera field of view with the test chart. If the chart you are using does not include measurement regions near the corners of your field of view, your analysis will not be comprehensive and you may overlook problems with MTF or signal loss at the extents of the image plane. When measuring distortion, the full field of view can be extrapolated from points available within the center, but this is less accurate than having real points to measure. The following video helps illustrate how the field of view affects your setup.
This is particularly challenging for cameras with wide or ultra-wide fields of view where planar targets can’t fill the FoV. We offer specialized test equipment to measure resolution and uniformity of ultra-wide devices.
You’re not testing at the right focus distance.
Test distance is calculated by focal distance; in other words, the target needs to be set at the distance(s) in which it is in focus. The common pitfall we see is not testing at the right focal distance. A comprehensive test regimen will involve testing between the focal distance and the hyperfocal distance. You can’t always move closer to a target in order to fill the field of view, as this can fall below the minimum focal distance of your lens – or below the distance which a chart of a particular does not negatively impact measurements.
The chart needs to be within a distance to fill the lens field of view in focus. You may need to test at more than one focus distance if you have a lens with variable or automatic focus so that you can validate optimal focus across the range of focus of your device.
To find the minimum focus distance, check your lens specifications or contact the lens manufacturer. To find the maximum focus distance, or hyperfocal distance, see this hyperfocal distance calculator.