Friday, December 18, 2009

Histograms Part 2

Read Histograms Part 1 first.

As I explained yesterday, our cameras' digital sensors can only record a limited range of brightness values at a given time. Anything that does not fit into that range will show up as either white, if it is brighter than the range, or black, if it is darker than the range. Here is an illustration that I hope will help you to understand this principle.

Imagine that you are working on a paint-by-number picture in gray-scale. You have eight shades from black to white to paint with. When you look at the picture you notice that there are sections numbered from one to ten. Now, you only have eight shades, so you can't paint all ten different shades that are in the picture. You have two choices.
  1. You can use your darkest shade as #1 and let anything marked with #8, #9 or #10 all be filled in with your lightest shade.
  2. You can use your lightest shade as #10 and let anything marked with #1, #2 or #3 all be filled in with your darkest shade.
Either way, you are going to lose some detail that is supposed to be in your picture. Either the three lightest shades will all be one shade, or the three darkest shades will all be one shade. Look and see . . .

Here is your paint by number picture:

If you had ten shades, it would look like this:

But since you have only eight, you must choose one of the following options:

In the first option, the lighter sky shades are combined so you can't distinguish between the sky, clouds and birds. In the second option, the darker shades are combined so the middle of the flower runs together.

I simplified this picture from yesterday to illustrate this point in a real image:

If my D40 sensor could have recorded more levels, there could have been more detail in the pipe. But anything darker than "1" was all recorded as "1."

I hope this is helping you understand a little bit more about how your camera is working. Please let me know if you have any questions. Ultimately, this knowledge will enable you to judge your exposure much more quickly and accurately. Read Part 3 next.