On our recent trip to BC Canada, we had an run in with another photographer trying to (with frustration) explain to someone why the back of her camera has black things blinking on it. Which led to explaining the histogram, which quickly ended the discussion.
So if you fully understand the blinkies or the histogram, you can probably skip this. But, if you find that your images are often noisy, blown out, or way too dark.... well keep reading.
First lets start with the histogram. There are whole books on it, so to make things a little easier here are some basics.
Peaks to the tops are not always bad. High peaks represent the number of pixels at that exposure level. So if you took a picture of an exposure card (black, gray, white) you see 3 peaks. Those peaks are not your exposure. Your exposure is read left to right. The more to the left, the less exposed; and the more to the right the more exposed.
Most photographers today prefer to expose to the right. Which basically means to over-expose as much as possible without over reaching beyond the right edge of the histogram.... Just touching the edge - maybe a little gap, and disregard how much if any of the histogram reaches the left. That provides the most amount of detail in the shadows (which usually shows noise as you lighten those up in your editor.)
Why might you want to expose to the right? lets do a test.
First you need to find out how to see your histogram in camera (that can be found by reading the pesky manual.)
Now as an experiment we will take a few images (for comparing be sure to focus on the same object with the same lighting conditions - use a tripod if you can, and only adjust the shutter speed)
Start by putting your camera in your Manual (M)
1. take a shot at even (0) exposure compensation
2. take a shot at +1 exposure compensation
3. take a shot at -1 exposure compensation
As you can see in the previews above, we have a zero, a +1, and a -1 compensation, all shot at f4.0 and 400 ISO. While we are here we can repeat this at high ISO values to test our own personal ISO limits. So, go ahead and grab 800, 1200, and 1600... If you have the latest and greatest camera, you might be surprised with 2500 and 3200.
Just in case you have been using 800 ISO at a slower shutter speed or underexposed because you were a afraid of that higher ISO.
Time to compare the images.
Now we can hope that your camera got it right at (0) but you might find that with the Histogram showing while reviewing the images in camera, your +1 is very close (or even a flat) to the right. But the image looks really bright on the camera's preview screen.
The real value can be found when looking at these on a computer. First lets look at comparing the 400 ISO images by editing them to give the same end result.... Take the +1 images and move the exposure slider -1. On the -1 image, increase the exposure slider by +1. Now all 3 images should show similar histograms with regards to the exposure - some peaks might show different. Now we want to find the noise, up the shadows of all 3 images by the same amount (50% on these samples)
Zoom in to 100% view (using the compare tool in LR) look around for noise. You should notice that the +1 image has less noise than the (0) image, and much less than the -1 image.
+1 on the left vs -1 on the right
Now for the real test, take your 800 ISO image that is +1 and compare it to your 400 ISO -1 using the exact same editing steps. You can repeat this for the 800 -1 to the 1200 +1 and 1200 -1 to the 1600 +1, and so on, until you reach a point that you think there is simply too much noise to worth it anymore.
Using it in the real world....Practice, Practice, Practice.
You may need to ask yourself, do I want the shadow's to be pure black or the highlights to be pure white. Snow is a good example of something that you will usually blow out (to a degree) so you can get the details in the shadows.
In the words of our friend Michael Wigle, "Eventually you will gain a sense of Reading the Light." When you gain this sense, you might stop thinking about these tools all together.
If you want to have a better grasp of when to over or under expose, that will be covered in Metering Modes (coming soon) and since I went pretty long on this, look for "Blinkies" soon as well.