Cameras do not see the way we do. We can see details in the shadows and bright sunlight, but cameras can only see one or the other. This explains why there are parts of a photograph overexposed and parts underexposed or visa versa. The picture of the flower below illustrates the point.
There is not a backdrop used in the photograph. The flower was in bright sunlight, but the background was in shadow. The camera could not see the shadowed area because the camera was set to see the flower. If the camera had been set to see the background the detail on the flower would have been overexposed. The lesson here is that you can't have it both ways. If you want to shoot both the shadow and the light then either more light needs to be added to the shadows or the lighted area needs to shaded. This is the reason so many different light sources are used.
The most used source of artificial light in photograph is the flash. It is also the light source I see misused the most. There are three misuses of the flash I'm going to discuss.
The first problem is understanding the lack of flash power. I frequently see pics like this:
Here is another problem.
Yes I have looked better, but that is not the deal here. It is that blob of light. All you young ladies taking that pic into the mirror. Well it has gone to the ugly. Not as bad as me of course but none the less. The same thing will happen if you try to shoot through glass. It will be ugly. Control your selves do not do it. If there is glass between you and the subject there will be ugly. The same thing will happen if there is glass behind the subject. Again, try turning off the flash and adjusting the ISO, shutter speed and F-stop.
And yet another problem:
Notice the wrinkles in the forehead, the slits where eyes belong, the crinkled nose, the shadow of the leaves on my head. This happens frequently with the subject looking into the sun. The photograph is ugly. Turn the subject so they face away from the sun, or place them in the shade and turn on the flash
Oh, look how handsome he is now. Well he isn't looking at the camera, but what do you expect? So there you have it sometimes in doors you should turn the flash off. I know everyone and their brother has told you inside use the flash, but try it. Going outside, hey, try turning the flash on. Inside flash off, outside flash on. This isn't always going to work. Most of the time flash inside will work, and no flash outside will work, but consider the situation. SO before you shoot think flash on, flash off; flash on, flash off . thanks Miyagi