Todays DLSRs can display a histogram (a graphical reading of the tonal range of the photo) on your camera monitor, but if you don´t know how to read one, it won´t do much good.
Personally I only use it to make sure I haven´t clipped of any details in the highlights. But how do I know if I am clipping any details? Well you don´t want the graph to go too far to the right. See the below graph in the church photo.
Do you see the above photo and the histogram? The graph is way off to the right. This means the photo is overexposed, or too much light was let in. In this case you can see this by just looking at the photo. It represents the sky. But this is just an example; there will be some photos you take where you need to check the histogram to make sure of the exposure.
Yes, you can fix this, with levels or curves in Photoshop, but is this what we are all about? Using an editing program? Personally I prefer to get in right in camera. Yes I do edit, but I do my very best to get it right first. The above photo was part of three photos for an HDR, that’s why I overexposed it. But, if the photo is way too overexposed you cannot get the information back in an editing program no matter what.
In the above photo the histogram shows what a perfect exposure is.
After you take a photo you can review if on the screen with the histogram. If it is too overexposed try using the exposure compensation control on the camera to override what the camera thinks is best and lower the exposure by one or two stops depending on the original reading, then take another photo and review. Keep doing that until the clipping goes away.
An example of too dark, (the left of the histogram) and of clipping the highlights.(the small thin line on the left of the histogram)
So what happened if you are taking a photo of a sunset for example, there will be clipping because the sun is such a bright subject, but it´s ok, as, as far as we know
There is an option on the view screen of the camera to see the highlights in each photo. If there are parts of the photo flashing it means there is clipping and you need to use the exposure compensation control and use a faster shutter speed. The histogram shows there is too much light, but the highlights screen shows exactly what part is overexposed, this way you can tell if it is an important part of the photo, ie, the white petal of a flower or something that doesn´t matter as much, like the sun.
An example of too dark, but in this case it´s ok because the histogram represents the dark background. So the histogram is just an idea. Don’t get too hung up on it.