Now that you have a "neutral" shot, take your first test shot. Make your first test photo under
exposed.
Check out the histogram. See how the spikes are more to the left hand side of the screen? That's
the side that shows the dark areas of a photo and since this one is under exposed, it is darker than
normal.
Now do a photo of the same type of subject but greatly over expose it.
See how the histogram spikes have shifted to the right side? That is the side dealing with the
light areas in your photo.
Now look at your original "correctly exposed" photo. The spikes are likely to be more in the
middle area.
Spikes at either end of the spectrum tend to indicate a photo that is over or under exposed and
you will want to make adjustments - but not always.
Caution... the temptation at this point is to automatically set every photo so that the histogram
spikes are in the middle. This would NOT be correct. Each photo has its own
right or wrong settings - determined by the subject matter and your artistic vision.
If you are doing a photo showing a lot of snow... a histogram heavily skewed to the right is
correct.
A scene of a coal bin would correctly produce a histogram heavily skewed to the left.