Alley in Pisa, Italy

by Lars Kotthoff

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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.

enter image description here

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up vote 18 down vote accepted

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. You could try simply painting the pixels with solid color or slight texture. As evidenced by blowing all color channels, the color wasn't pure red, so it might take a bit of playing with colors to get it looking right. Judging by the glow, the color seems to have more blue than green.

To capture a similar scene with correctly exposed light source, you should take a second frame (from tripod in same place) with exposure reduced to a level where the light source is exposed correctly, and then either create an HDR image (might not work too well due to steam changing between frames) or simply use layer masking in Photoshop/Gimp/etc to select which parts of image should come from which frame.

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While the pixels can't be restored really, in this particular case, it looks like the light could be retouched simply by carefully drawing in a replacement red textured area. May not fit some standards for image integrity, but could be more accurate to the human impression of the scene. –  mattdm Nov 26 '11 at 4:16
    
@mattdm thanks, edited my answer to include that solution. –  Imre Nov 26 '11 at 4:37
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