Modulation transfer function (MTF) is a well defined and accepted method of measuring image sharpness. The slanted
edge test, as defined in ISO12233 is a standard method of calculating MTF, and is widely used for lens alignment and
auto-focus algorithm verification. However, there are a number of challenges which should be considered when
measuring MTF in cameras with fisheye lenses. Due to trade-offs related Petzval curvature, planarity of the optical
plane is difficult to achieve in fisheye lenses. It is therefore critical to have the ability to accurately measure sharpness
throughout the entire image, particularly for lens alignment. One challenge for fisheye lenses is that, because of the
radial distortion, the slanted edges will have different angles, depending on the location within the image and on the
distortion profile of the lens. Previous work in the literature indicates that MTF measurements are robust for angles
between 2 and 10 degrees. Outside of this range, MTF measurements become unreliable. Also, the slanted edge itself
will be curved by the lens distortion, causing further measurement problems. This study summarises the difficulties in
the use of MTF for sharpness measurement in fisheye lens cameras, and proposes mitigations and alternative methods.
KEYWORDS: High dynamic range image sensors, Spatial frequencies, Sensors, Image sensors, Microlens, High dynamic range imaging, Modulation transfer functions, Phase measurement, Computer architecture, Sensor technology
Split-pixel HDR sensor technology is particularly advantageous in automotive applications, because the images are captured simultaneously rather than sequentially, thereby reducing motion blur. However, split pixel technology introduces artifacts in MTF measurement. To achieve a HDR image, raw images are captured from both large and small sub-pixels, and combined to make the HDR output. In some cases, a large sub-pixel is used for long exposure captures, and a small sub-pixel for short exposures, to extend the dynamic range. The relative size of the photosensitive area of the pixel (fill factor) plays a very significant role in the output MTF measurement. Given an identical scene, the MTF will be significantly different, depending on whether you use the large or small sub-pixels i.e. a smaller fill factor (e.g. in the short exposure sub-pixel) will result in higher MTF scores, but significantly greater aliasing. Simulations of split-pixel sensors revealed that, when raw images from both sub-pixels are combined, there is a significant difference in rising edge (i.e. black-to-white transition) and falling edge (white-to-black) reproduction. Experimental results showed a difference of ~50% in measured MTF50 between the falling and rising edges of a slanted edge test chart.