A few years ago when film was developed the developers, pardon the pun, neede to determine how the film reacted to light and what constituted correct exposure. Somewhere in this development phase it was determined that middle grey would be the tone used to determine correct exposure. Middle grey obviously means the tone that is perceptually (Wikipedia) half way between pure black and pure white. The full Wikipedia definition is (reference http://en.wikipedia.org/wiki/Middle_gray):
“In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about halfway between black and white on a lightness scale; in photography, it is typically defined as 18% reflectance in visible light.
Middle gray is the universal measurement standard in photographic cameras. To calibrate light meters, whether in a camera or hand held, the 18% gray card was conceived. It is assumed that the measurement taken by a meter gives the exposure for a shot so that some of the light reflected by the object measured is equivalent to middle gray. However, many note that modern cameras generally treat 12-13% gray as “middle gray”.
In the sRGB color space, middle gray is equivalent to 46.6% brightness. In 24-bit color, this is rounded to RGB value (119,119,119) or #777777.“
Now there are a few technical terms used there such a sRGB etc which one can discover by suitable internet searches what they mean as it is beyond what I am writing about today.
So to continue what happens in the modern camera is that following the old film tradition it exposes for middle grey. Now for tones and colours that in black and white sit in the greys that is ok. However for pure whites and blacks some care is needed.
For instance if you have a lovely landscape scene full of white snow how does one expose? Now I don’t have any snow here to photograph and I don’t think at this time there is any snow within 500km’s of where I live so I just can’t duck out and take a snap. However what I can do is photograph a white piece of paper.
With the settings adjusted so that the light meter of the camera is indicating a normal exposure this is what we get.
Trust me this has not been altered in any way it is straight from the camera and it looks like a nice grey instead of a white sheet of paper. One gets a similar result if it is black paper as per this image:
Clearly not black but it is further away from the middle grey than the white paper.
So what does one do, and this is important when taking photographs of high contrast scenes, such as snow as a backdrop to dark objects or even if one is photographing a wedding, which personally is something I will never do. Well one has to determine what they want, is it nice white snow for example or is it a correctly exposed object that is the attention of the photograph. Assuming that it is a general landscape and predominately white then one must over expose and that may be by up to two stops.
This is where the histogram is very useful. Take a shot, look at the histogram and ensure that the graph is well to the right with out any clipping. Then the white snow will look white or very close to white. Final adjustments can be done in ones favourite processing application.
Hope this helps.