A camera’s Dynamic Range (DR) is the range of tones in a scene that can be reproduced with adequate contrast and good signal-to-noise ratio (SNR). Camera DR is often limited by flare light, which is stray light in the image, primarily caused by reflections between lens elements. Flare light reduces DR by fogging images; i.e., washing out detail in dark areas. It is the primary reason that the DR of cameras (which include lenses) is poorer than that of image sensors, which can be up to 150dB (30 million:1) for recent HDR (high dynamic range) sensors.
In the past 2 years we’ve learned that flare light in test chart images could, in theory, be mistaken for the image of the test chart, resulting in exaggerated DR measurements. The situation became real in mid-2019, when we started seeing actual images where this error occurred. By studying these images, we have developed techniques to guard against exaggerated Signal-to-Noise Ratio (SNR) measurements caused by flare light. We describe these techniques here. (more…)