Field of View
Field of view (FoV) and focus distances determine which test chart to use and the required space for an image quality test lab. Lenses with normal, wide, ultrawide, and narrow FoV warrant different charts and equipment setups to test key image quality factors properly. Standard planar (flat) test charts are typically suitable for cameras with normal-to-wide FoV up to 160º, but not sufficient for ultrawide FoV camera systems.
Wide FoV typically ranges between 100º and 160º and has fixed-focus distances closer than normal. The lens-to-chart distance is commonly between 18cm and 1m. Predistorted charts are the best solution for testing wide FoV such as those captured by fisheye lenses. These charts are designed with pincushion distortion to correct for the lens’ barrel distortion so features within the captured image will be as rectilinear as possible.
To select an appropriate test chart size, you need to know the pixel dimensions (width and height) of your sensor, diagonal FoV of your lens, and ideal lens-to-chart distance. The Imatest Chartfinder can help determine the size of your imaging plane, which in turn will determine the correct size of test charts for your application.
Ultrawide cameras with a FoV greater than 160º are increasingly common but can be challenging to test. For example, it is difficult to fill the image plane with a planar test chart even with pincushion predistortion. To solve this problem, place SFRreg charts outside of an eSFR ISO resolution test chart (Figure 1). The Imatest software eSFR ISO module will automatically detect these charts for analyzing sharpness and chromatic aberration.
The Imatest Ultrawide Test Fixture is designed to provide an out-of-the-box solution to align charts properly for measuring sharpness and chromatic aberration. THe Ultrawide Test Fixture works with cameras with FoV up to 180° at 1m or 270° at 50cm lens-to-chart distance. For testing ultrawide uniformity, a spherical light source becomes necessary to fill the FoV and avoid the cosine-falloff that will be present on a flat-panel light source.
Long Range and Narrow FoV Testing
Testing narrow FoV lenses, where the focusing range may cover between 5m and up to hundreds of meters, can be costly and complicated to achieve in most imaging labs. When testing requirements extend beyond your available space, collimating optics can produce virtual images at or beyond the hyperfocal distance.
If you are interested in testing narrow FoV lenses, Imatest offers custom solutions for testing long distances within a compact environment. Contact us for more information about simulating long distances.
At the very least, make sure to test beyond your minimum focus distance; however, determining the appropriate test distance depends on your device’s application. It may only be necessary to test your camera system beyond the hyperfocal distance to ensure it is acceptably sharp for far-away objects. Alternatively, you may require multiple test distances, such as one close to the minimum focus distance and another beyond the hyperfocal distance.