Histograms and light meters are two built-in tools that are available on SLR cameras for photographers to check their exposure before and after they take a photo. But how exactly do they help and where can you find them on your camera?
The light meter appears when you are looking through your camera’s viewfinder. It’s activated by pressing the shutter button and measures the intensity of the light at various points across the entire scene. The light meter displays an overall reading of the light as a number below or above zero, which is considered to be the centre. Below zero indicates darkness or underexposure, and above zero indicates brightness or overexposure. It's up to you to decide what to do after reviewing the information the light meter gives you. When shooting in manual mode, it's usually a case of tweaking your camera settings (ISO, aperture and shutter speed) until you're happy.
Unlike the light meter, the histogram is something that you’ll need to switch on to be able to see. Very simply, the histogram is a graph that you can choose to display on your camera’s LCD screen that helps you read the light better. Too much dark and the histogram will bunch up on the left side, too much light and its bunches up on the other. Once you get the hang of the histogram, it almost becomes like a watch that you glance at regularly, to evaluate the quality of the exposure that the camera has made after each photo you take. A histogram is especially useful when the light frequently changes, as a reminder to adjust your camera settings.
By reviewing your images using these two tools, you’re better able to judge which areas of the photograph are incorrectly exposed across the entire scene and make adjustments until the light meter gives a reading of 0 or the histogram shows no peaks on either end of the graph.
However, both light meters and histograms can be confused by high key/high contrast and low key/low contrast scenes. Photos that have extremely bright tones with very few shadows, are referred to as high key images. Conversely, low key photos use predominantly dark tones and very few highlights to create dramatic looking images.
Histograms are designed to show possible extreme contrasts, which is why even though high key and low key photos may not be evenly exposed according to the histogram, they may actually be fine. This is why you can't rely on the histogram alone.
The good thing about light meters is that they can be overridden manually to overexpose or underexpose to best suit the conditions you’re shooting in, and all types of light. This can be done in a number of different ways such as choosing a suitable metering mode, making an exposure compensation or using bracketing. Check your camera manual to find out how to do these things and see which one works best for you in high key and low key scenes.