You said Histowhat? I said Histograms!

One thing that you will heard mentioned time and time again is that you should get the photograph 'right' in the camera before you take it home for post processing.  I suppose that in a perfect world you would not need to do any post processing at all. 

Fig. #1One tool that can be quite useful to that end is a Histogram.  Most cameras today have them, certainly if you use a DSLR you have one for sure (See Fig. #1).  Histograms also show up in many post processing tool such as Photoshop CS and Lightroom just to mention two, but these histograms are used during post processing. (See fig. #3).

A histogram is just a graph that will show you the tonal mapping of any give photograph you are looking at.  It is relatively easy to understand once you've used it.

This tonal map reads from dark (black) to light (white) from left to right.  See figure #2.  On your camera it is less dimentional, meaning it does not show you the color channels of blue, red and green.  It's primary purpose is to help you identify any photograph that you have taken that is either over or under exposed.  If there is a dramatic rise in either blacks or white atthere respective sides then photograph may have a significant amount of informatiion that is lost.  See figure #4, many highlights have been lost.

Figure #3The third histogram (fig. #3), was taken from a photograph in Lightroom.  This histogram contains the tonal mapping of all the color channels which you might use if you wanted to adjust each channel individually, but this is another subject.

In this histogram you can see that there are a lot of dark tones very close to but not touching the left hand side.  This photograph is dark but almost no information in the darks has been lost.  

Remember this is just a tool, a guide that you should become familiar with.  You may very well want to under or over expose any give shot.  

The next time you pick up your camera play with it.  Learn how to find your histogram and use it after each shot.  If you've learned even some basics about your camera and can use it in manual mode, take a lot of shots (of the same subject) adjusting the exposure in each shot.  See how it effects the histogram for each photograph.  

You may not always use this tool but it will be one that you are familiar enough with should you need it.

For another article on Histograms head to this Digital Photography School's article.

Next we'll talk about "Exposure Compensation" or "Exposure Bias" in which you need to know about the use of histograms.