Why are the red light and its hood in my image colored different than they were in reality?

Question

I took this image in a train station with shutter speed 1/8 second and AV mode, the aperture was chosen to be 4.5. The problem in the colors of the spotlight in the right corner of the image. In the image, the light is white and the border (the hood) is red — however in reality, the light was red and the hood was black. Why this change in color?

I'm using Canon Rebel T3i with its EF-S 18-55 IS II lens. I'm using Adobe RBG color space.

Answer

The reason is that the red light is a light source, therefore it's much brighter than any other parts of the scene. The pixels showing it are overblown - meaning there was more light coming than your camera sensor could capture. The light is not pure red, it emits enough green and blue light to blow these color channels of pixels too.

The hood is just reflecting light from it. It was hard to see with a naked eye, because the light beneath it was much stronger and the hood seemed perfectly black compared to it. In photo, however, the intensity of captured light is capped by your camera sensor's dynamic range, and you can see the subtle reflection next to overblown pixels.

Unfortunately, overexposed pixels cannot be restored in post processing.

To capture a similar scene with correctly exposed light source, you should take a second frame with exposure reduced to a level where the light source is exposed correctly, and then either create an HDR image or simply use layer masking in Photoshop/Gimp/etc to select which parts of image should come from which frame.

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