Objects surrounded by a brighter area appear darker, and if the brightness of the area increases further, then the objects appear even darker? An ex: Sun spots appear black.

I observed the following phenomenon today: in my room there is a stuffed toy placed in front of a wall. The ambient lighting doesn't cause any shadow on the wall currently.

Scene 1: Then I switched ON a torch and pointed it at the stuffed toy which was kept in front of a wall and a shadow appears on the wall.

Scene 2: I increased the light intensity of the torch falling on the toy and observed that the shadow on the wall has become darker. However, I didn't observe this phenomenon when I switched off all the lights of my room and repeated the experiment.

I captured Scene 1 and Scene 2 using a camera (all settings auto). I compared these two images using a program. The shadows pixels are of different intensity in the two images as it was observable by the naked eye also.

Further, in flash light, the ambient shadows tend to disappear. How this happens? The ambient shadow is still there but in flash light it just does not appear. Why?

Can someone formally explain why and how this happens, either in context of human eye or a digital camera (preferred)? I think it has to do with how our eyes adapt to changing brightness in the scene and exposure compensation in digital cameras.


5 Answers 5


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.

  • Can you also explain the fact that why I don't observe this phenomenon where ambient light is very low (image taken in ambient light is very dark and noisy)?
    – kunal18
    Dec 14, 2015 at 9:01
  • @stalin I think it depends on a) Metering Mode, and b) camera's program (it may be able to recognize a dark room but not able to recognize and/or correctle expose bright spots and shadow in the darkness).
    – Zenit
    Dec 14, 2015 at 9:09
  • Can you elaborate on how exactly this compensation happens? I mean what's really going on inside when I suddenly increase the flash intensity which results in shadows to appear darker?
    – kunal18
    Dec 14, 2015 at 9:16
  • 1
    @stalin because the flash is orders of magnitude stronger than the ambient light: it simply overpowers the ambient shadows
    – null
    Dec 14, 2015 at 12:15
  • 2
    @stalin: because it is so much stronger. If you are listening to music with headphones very loudly, you will not hear your phone ringing.
    – null
    Dec 14, 2015 at 14:08

It's all a matter of exposure. If you expose for the brightest part of the image, the other parts of the image will be underexposed. To understand this better, try using the camera in FULL MANUAL mode to be fully aware what exposure and what aperture settings you have.

  • So if I go to manual mode and don't change anything after increasing the flash intensity, the shadows should not become darker. Right?
    – kunal18
    Dec 14, 2015 at 9:29
  • If you increase the flash intensity, wouldn't the shadows also become more (indirect) light from the extra power of the flash? But theoretically, yes, if you manage to isolate the shadows and not get light reflected on those areas, the shadows should remain the same in the picture (if camera is set to full manual and you don't change the exposure, the aperture, the position of the camera and the position of the light source)
    – Dragos
    Dec 14, 2015 at 11:34

Employees of the State University of New York College of Optometry investigated the cause of the visual illusion that was first discovered by Galileo Galilei. The study was published Feb. 10, 2014 in the edition of Proceedings of the National Academy of Sciences. Optical illusion makes considered Venus (lowest point) is bigger than Jupiter (top point) Optical illusion makes considered Venus (lowest point) is bigger than Jupiter (top point)

Neuroscientists have found an explanation of optical illusion, first described by Galileo 400 years ago during the observation of the planets of the Solar System it. By studying the planets through a telescope and with the naked eye, the astronomer was surprised to see that in the first case, the apparent size of Venus is less than the size of Jupiter, and in the second case - on the contrary.

"Galileo was the first to suggest that our eyes distort the reality", - said Dr. Jose Manuel Alonso from the College of Optometry in New York (USA). Galileo found that distortion occurs in the human eye. Bright planets observed directly on a dark background seem to gain more and radiant crown, which makes for Galileo Venus eight times larger than Jupiter, despite the fact that when viewed through a telescope, Jupiter appeared four times more. In the future, many astronomers have noted that the angular resolution when viewed with the naked eye above for faint objects than bright. Galileo wrote that this is "or from the fact that their light is refracted on the moisture, which covers our pupil, or due to the fact that it is reflected from the edges of the eyelids, and then scattered by the pupil or by some other reason. "

The reason turned out to be really in the man, the physiological reasons blur bright objects have tried to explain many scientists, including the famous German doctor and physicist Hermann Helmholtz.

Helmholtz was closer to the truth, realizing that the increase in the apparent size of bright objects related to our perception of light signals, not the optics of the eye.

"Our research has shown that the perception that Helmholtz explained this phenomenon - the nonlinear response of the visual system, when objects are visible on a dark background," - said Alonso, author of the article published in the journal Proceedings of the National Academy of Sciences.

In their experiments, the researchers tested the activity of neurons in the thalamus and the cerebral cortex responsible for the perception of light in cats, monkeys, and humans, using implanted electrodes.

At the time of the experiment to humans and animals, which were under anesthesia, showed three types of image: the dark figures on a light background, bright shapes on a dark background, as well as light and dark figures on a gray background.

Today we know that a person perceives the bright and dark signals (no light) with the help of the so-called on and off-channel in the retina and the optic thalamus. By measuring electrical signals from these channels, the researchers found that the off-neurons react predictably and linearly on the appearance of dark figures on a light background: the sharper the contrast between figure and background, the more active the neurons. However, on-the neurons responded to an increase in the brightness of light objects on a dark background disproportionately.

In other words, with the same contrast of figure and ground the second option gives you more excited neurons.

The most absurd assumptions in the mouths of scientists look more natural than the narrow-minded comments about the dangers of invading aliens and ... → According to researchers, this explains the asymmetry in our assessment of the size of bright and faint objects, and this feature has arisen in the course of evolution is not just.

"When you're in a very dark place, it allows you to see the weakest light sources" - said Alonso.

Once it used to help people detect danger in the dark. However, in the afternoon, on the contrary, view this property allows us to see fine details of the dark for a bright background. The same effect also plays a role in everyday life, every woman knows that her slender dark clothing, smoothing figure flaws. Now, this is a scientific explanation: a dark figure on a light background gives the excited neurons less than the figure of light on a dark background.

The researchers believe that their discovery will help better understand the nature of diseases such as nearsightedness

  • 2
    How does it answer the question and explain why shadows pixels have different intensity?
    – Zenit
    Dec 14, 2015 at 9:41
  • What is the source of this text?
    – Dragos
    Dec 14, 2015 at 12:55

Our homes and offices are likely illuminated with diffused light. Lamps from fixtures are shielded by translucent diffusers. Additionally lamps are usually located on or near the ceiling. Our surrounds are thus illuminated by both diffused direct and indirect light. The indirect light reverberates from ceiling and walls. Thus it is highly diffused. Diffused lighting has no specific origin. Shadows cast by more direct light rays are filled or softened to absence by the indirect diffused light. Thus no distinct shadows are observed.

Scene 1 is now illuminated by a torch (flashlight) that outputs collimated rays (parallel). These cast hard shadows to dense to be filled by the ambient light.

Scene 2 is illuminated by more intense collimated rays. The shadows cast appear harsh under the ambient room lighting but not as harsh when viewed only by the light of the torch. This is likely due to Lateral Brightness Adaptation. Sensitivity changes in local areas of the retina are sometimes accompanied by similar changes in adjoining areas of the retina. This is a sideways of lateral brightness adaptation that is exemplified by what happens when we look at moderately dark areas surrounded by considerably brighter areas. The sensitivity of the retina is decreased in light areas but at the same time a decreased sensitivity is apparent in the dark areas.

See page 59 Color as seen and Photographed Kodak Publication E-74H Likely derived from textbooks authored by Ralph M Evens "Introduction to Color Photography".

  • This is also observed in images taken by camera. Any references in context of camera?
    – kunal18
    Dec 15, 2015 at 9:50

I believe you are speaking of the term "Veiling luminance" which can cause it's surrounding to appear darker. Increasing light intensity may cause distortion to which may reduce the surroundings contrast. This can be seen by flashing your flashlight directly at the camera. You can also use a low-budget non-digital camera to see it first hand or any of that sort that is used manually.

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