What is a Histogram and what does it mean to me?
The histogram is one of those things that people have seen everywhere, it’s in your camera and available in almost every software program but there is still a lot mystery surrounding it. The histogram is basically your photo in a graph. It will show you the tonal range of a photo and it is used to help you see if you are capturing the entire dynamic range of an image.
The histogram is display as a playback option in your camera's playback menu. When you playback an image the histogram of that image is displayed in addition to other shooting data. The histogram is a great guideline to check to see if your exposure is right in the camera.
Reading a Histogram If you look at the very far left that shows the shadows or blacks of the photo. When you look to the very far right that shows the highlights or whites of the photo. The peaks and valleys that are in between show the tones that exist between the blackest black and the whitest white. There are a total of 256 shades of grey that can possibly exist in a photo. Depending on what the photo is, will dictate where the tones show on the histogram. For the most part you shouldn't concern yourself so much about what happens in the middle of the histogram(as every image is different). However, you should pay attention to what happens at the ends of a histogram.
What is a "good" histogram?
A good histogram is one in which the graph reaches both ends but does not creep up the side. If a histogram, falls short on the shadows end the photo is overexposed and too bright. When the histogram falls short on the highlights end the photo is underexposed and too dark. Additionally, if the histogram reaches the end in either direction but shoots up the side and goes through the roof thats not good news either. When it creeps up on the shadow end or the left side of the graph it is an indication that parts of the photo are too dark, there is no detail in the shadow areas. Most editing software will allow you to recover some of these dark areas especially if it is a RAW file. (see post “Should I be Shooting JPEG or RAW?”). However, when a histogram shows the highlights all the way to the right and going up the side to the top it means that the highlights are completely blown out. This will also be displayed ‘in camera’ under the highlight clipping warning playback menu. This is not recoverable in any software program the highlights were not captured at all.
Why does a histogram fall short one way or the other?
Generally speaking it is because the exposure is wrong due to the meter being fooled. When a camera’s meter is dealing with difficult lighting, a lot of contrast, big dynamic range, or a predominately bright photo or conversely a very dark photo it has a hard time giving an accurate exposure.
What can I do in camera to get a more correct histogram?
overexpose or underexposure the image
use exposure compensation
auto bracketing
change up metering modes
use a hand held meter
Get it right in camera. Be aware of your histogram but don’t be beholden to it. Its a great guide to use and when you learn to read it in camera and use it post-process you will be sure to produce images with an entire dynamic range of tone.