Webster's defines dynamic range as:
|Main Entry:||dynamic range1|
|Part of Speech:||n|
|Definition:||the ratio of a specified maximum possible level of a parameter to the minimum detectable or acceptable value of that parameter|
And expressed another way:
dynamic range –noun Audio. the ratio of the loudest to faintest sounds reproduced without significant distortion, usually expressed in decibels.
This audio definition is good enough for the photographic arena as well with a few term modifications. Instead of the loudest to faintest sound it would be brightest to least bright light. It is commonly accepted that the eye has a general dynamic range of around 20 stops. Remember that any one stop is 100% brighter than the stop below it. That means that the eye has an extraordinary ability to see all the different intensities of light in a scene. Film has the ability to see around 7 stops of light in a scene and camera sensors have around 5 stops of ability.
While this will no doubt improve over time (sensors anyway, there is probably not going to be much in the way of research dollars going into film technology anymore) at the moment it forces you to make decisions about the scene you are looking at. You must decide what the important part of the scene is and make sure that range of stops is properly done in your photo.
Now that you know this, how can you combat it? Knowledge my friend. As mentioned before, you must make some decisions about what is important and what is not. You must make sure your subject falls within your dynamic range in order for it to be seen properly. In addition to that, you can "compress the dynamic range" by making brighter things darker through the use of neutral density filters. Alternately you can make darker things lighter though the use of strobes. We'll explore those concepts in depth in a later entry just as long as you have the concept down for now.