Thursday, 31 October 2019

artifacts - Why are red lights in night / city scenes coming out as big red blobs?


I've noticed that in a lot of my night shots of cityscapes, red lights (e.g. neon signs on buildings, etc.) tend to come out as big red blobs:


Not very good example of big blobby red lights


The above was taken with my Canon 500D.


What can I do to reduce this? (Either as I'm taking the photo, or as a post-processing step.)



Answer



What you're seeing in that shot is overexposure. Unlike overexposure in a day time shot,where the blown highlights tend to go pure white, the red light from the sign caused overexposure in the just the red channel. Thus all the different tones of red have become 100% red and detail is lost.


It can be fixed by reshooting at a faster speed / smaller aperture pulling up the shadows, or blending multiple exposures (using HDR techniques).


Did you shoot raw? If so you may be able to fix it without reshooting, by taking advantage of the extra headroom in raw to reduce the exposure. If you have Adobe Camera Raw there is a tool called "recovery" which attempts to fix this kind of blown highlights though it doesn't always work that well.


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