why shadows become dark when light intensity increases? I observed following phenomena today: in my room with some lightings, there is a stuff toy placed in front of a wall. The ambient lighting doesn't cause any shadow on the wall currently. 
Scene 1: Now, I switched ON a torch and pointed it to a stuff 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 phenomena when I switched of all the lights of my room and repeated the experiment.
I captured Scene 1 and Scene 2 using a camera. I compared these two images using a program. The shadows are of different intensity in the two images as it was observable by the naked eye.
A practical example: Sun spots appear black, but they aren't actually black
It seems camera also adapts and works similar to our eyes. Can somebody formally explain this to me? I am more interested in the image formation by the camera in this case, not the human eye and biology related things.
 A: Ok, let's put some numbers to the scene. They are arbitrary numbers, in an arbitrary scale, just for illustration purposes, but I hope that will make my explanation clear.
I will use light units (lu) as an absolute scale for light intensity, and apparent units (au) for the perceived light intensity.
Scene 1: You have your room lit with a room light. The toy projects a shadow to the wall. The un-shadowed wall will have a light intensity of 100 light units (lu), while the shadowed wall will have 10 lu (because of ambient light, diffuse reflections in other walls, etc.).
Scene 2: You switch on your torch and point to the toy. The torch will make the un-shadowed wall brighter, with extra 1000 lu, that added to the room lamp gives a total of 1100 lu. The shadowed wall will get a bit of extra diffuse light, but not much because the torch has a narrow beam, so it will be about 15 lu.
That is:


*

*Shadow 1: Wall of 100 lu, shadow of 10 lu.

*Shadow 2: Wall of 1100 lu, shadow of 15 lu.


Now, your eyes, and your camera, compensate for the full intensity of the room, that is they usually try to normalize the light intensity of the full scene (also the colors but that is out of topic).
And since the wall is what dominates the scene, your eyes/camera will normalize the situation in this way, more or less. This is actually an oversimplification, since human eyes are non-linear, that is their response to light is logarithmic, but that detail is not necessary for this explanation, and will only complicate the computations without changing the broad conclusion.


*

*Scene 1: divide by 100.0


*

*100 lu -> 1.0 au (apparent units)

*10 lu -> 0.1 au


*Scene 2: divide by 1100


*

*1100 lu -> 1.0 au

*15 lu -> 0.014 au



As you can see, although in scene 2 the shadow is absolutely brighter (10 lu vs 15 lu), it looks a lot darker (0.1 vs 0.014 au).
