Why do shadows look darker when the surrounding area gets brighter?
Asked 12/14/2015
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When I shine a torch on a toy in front of a wall, the wall shadow seems to get darker as I increase the torch brightness. If I photograph both scenes with the camera in auto mode, the shadow pixels also end up darker in the brighter-lit scene. But if I repeat the experiment in a dark room, I don’t notice the same effect as much. Also, when I use flash, some ambient shadows seem to disappear even though they should still be there.
Why does this happen? Is it mainly due to human vision adapting to scene brightness, or due to camera auto exposure? I’d especially like the explanation in terms of a digital camera.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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If you'll look at the exposure data of both images you will see, that they are not equivalent. That means that the shadow spot was exposured differently and thus has different intensity.
That's how auto exposure work. Generally speaking, it "thinks" that the average intensity of all pixels in a picture must be grey and so adjusts the exposure for such a result. When you increased the light intensity of a torch, bright areas became brighter. Auto program then "compensated" it by changing the exposure (so that the average pixel intensity still remains grey). Try to repeat your experiment with manual settings not changing them between taking pictures.
This is why many camera have program "Snow". If you make a photo of sunny landscape with a lot of snow using general auto settings, the picture will be underexposed - camera does not know that there is a lot of white bright snow in the picure and will expose it to get "grey average", let me call it this way.
Originally by user44894. Source · Licensed CC BY-SA 4.0
user44894
10y ago
0
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Mostly, this is an exposure issue, especially in your camera test.
With auto exposure, the camera tries to make the overall scene average out to a mid-tone. When you brighten the torch-lit areas, the camera reduces exposure to keep the whole frame from getting too bright. The shadow then records darker because it is being exposed less than before. That’s why the shadow pixels differ between the two auto-shot images.
To test the effect properly, use full manual exposure and keep shutter speed, aperture, and ISO identical for both shots. Then only the brighter illuminated areas should change, while the shadow itself will not magically become darker unless less light is actually reaching it.
Flash can make ambient shadows seem to disappear because the flash adds much more light from a new direction, filling in darker areas and overwhelming weaker ambient-light shadowing.
Your eyes also adapt to scene brightness, so perceived shadow darkness can change depending on how bright the surroundings are. But in the camera example you described, auto exposure is the main reason.
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