What is a purple border? Why are there purple edges?

Purple edge refers to the dispersion phenomenon at the junction of light and dark parts in the shooting process of digital camera, which is called purple edge phenomenon.

The condition of purple edge phenomenon is that when the light ratio is very strong, if the bright part and the dark part suddenly meet without transition, then the junction (such as eaves) is prone to dispersion. The reason of purple edge phenomenon is also related to the lens' ability to control diffusion, the area of image sensor (the larger the pixel per inch, the easier it is to diffuse), the processor algorithm inside the camera and other hardware performance.

Knowing the above, of course, high-quality photographic equipment should be given priority. For example, SLR cameras with large CCD/CMOS area and lenses with low dispersion lenses can be improved a lot. If there is no premise of changing equipment, we should weaken the conditions of purple edge and try not to shoot scenes with big contrast. If you have to take pictures, try to reduce the light ratio with some skills. For example, if the light ratio is large, use a reflector or a flash to illuminate the dark part. At the same time, the aperture is reduced (large aperture is more likely to produce dispersion). These methods will improve the reduction of purple edge phenomenon.

From the perspective of photography skills, it is the first priority to try to avoid taking pictures under particularly strong light conditions, such as backlight photography under strong light. If the dark part is not filled with light, even professional lenses with low dispersion lenses will have purple edges without exception, because low dispersion does not mean no dispersion.