1. What is the histogram?
The histogram is a graph on your LCD screen that shows what the exposure looks like after you take a picture. If it's too dark, the graph goes to the left. If it's too bright, it goes to the right. If it's a good exposure, it's in the middle.
Now to find the histogram you'll probably need to click info on the picture you're viewing in playback. Check your manuals. The thing to remember is that the histogram is your sometimes friend. It's just a guide. Sometimes the reading is right... sometimes totally off.
To be honest I almost never pay attention to this thing. Most of the time it's all wrong (especially if I'm deliberately shooting dark stuff, like when light painting.) Instead I'm used to how my images look on my LCD screen and how it compares to how they look at home when I upload them on my computer. LCD screens are different though, and if you have one that is hard to judge, you can check your histogram to see what's up.
Here are some examples...
This picture is too dark, I can't even see my dogs eyes. The histogram is towards the left (too many darks.) Totally an accurate histogram reading.
According to the histogram, this is overexposed (the graph is towards the right - too many "lights") but really, it's a pretty good exposure. You can see some of the details are pretty bright in the white wall behind Ava pup towards the left. But my subject is properly exposed just how I want it. So in this case I wouldn't give two fiddle sticks about what my histogram says.
When the histogram stays to the middle and starts to resemble a "mountain" it's supposed to be a good, correct exposure with even lights and darks. The bigger / fatter the mountain, the better the light. Usually we agree! This isn't the best example of a histogram mountain. But like I said, I really don't pay much attention to the histogram. In fact... I kinda/sorta/forgot what it was called. I had to ask my husband B. True story. Mr. Technical saves the day.
Well that's it! This is really a very simple thing in DLSR photography. Just remember that if you want an idea if your exposure is good, take a look at your histogram. It can sometimes be your friend when you're shooting. I never really pay attention to mine but sometimes it helps.