The Histogram… and metering for correct exposure
Understanding the Problem
(Please familiarize yourself with ‘Down the Rabbit Hole – Part 1’ before you read on).
After taking a shot have a look at the image on the rear LCD screen of your camera and press the info button until you see the histogram for that particular image. What the histogram tells you is important because correct exposure is what we all aim for. Many photographers either ignore or don’t understand the histogram: they think it’s unimportant because they rely on the camera getting it right. And to some extent the camera will do a reasonable job… but it won’t get it exact. So what is a histogram, what does it tell us and, more importantly, how does it help us get the correct exposure?
‘Help‘ is the key word here because on its own the histogram does nothing: it is simply a graphical representation of the tonal values of your photograph. Take a look at the image below:
Remember that your camera ‘sees’ the world in approx 255 shades of grey, ranging from ‘0’ on the extreme left (pure black) to ‘255’ on the extreme right (pure white). The histogram is divided into five sections (four for Nikon) with each section showing how much data there is in the photograph in terms of a)the number of pixels and b)how those pixels are distributed. The number of pixels is shown on the vertical axis and their distribution on the horizontal axis. Simply put, the more the distribution is to the left of the histogram the darker the picture and the more the distribution is to the right the brighter the picture. Too many pixels to the left and we might say that the picture in underexposed. Too many pixels to the right and we might say that the picture is overexposed. It seems then that the better the distribution of pixels across the whole histogram the better the exposure; as shown in the image above. This is a massive generalization because it does not take into account photographs with more light tones than dark, and vice versa. And if we use auto mode and evaluative metering, then we have to cross all our fingers and toes in the hope that the camera will get it right. But there’s a problem…cameras don’t take photographs…YOU do. And cameras rarely get exposure right on their own! So logically, it makes no sense to rely on the camera to get it right especially if the subject moves in front of different backgrounds of different brightness: Take a shot of a bird in flight against a bright white sky and then a shot of the same bird against a woodland background and you’ll see different histograms for the same bird (If you’re a landscape photographer and you take two shots of the same scene, and one image has much more sky than ground than the other image, you’ll have exactly the same problem). Now here’s the conundrum…if the bird is in the same light then surely your camera should be able to tell the difference between the two backgrounds and be able to keep the bird exposed the same in each shot, yes? No! I’m afraid not, especially if you rely on a general metering pattern e.g. Evaluative. That bird against the bright sky will not be exposed the way it should be because the camera takes into account the bright sky and evens out the exposure across the whole image. Likewise, with the bird against the dark woodland the camera exposes across the whole scene taking account of the background because it is the prominent tone in the image. The upshot of this is that your bird is exposed differently in both images. If you understand this then you’re half way there to getting your exposure right in every shot.
Metering
Choosing the correct metering pattern is crucial to the outcome of your photograph. Crucial? Absolutely, because it measures the brightness of the scene to make a correct exposure. And to get the best out of the metering system in our DSLR we need to understand the basics of metering light.
Your camera has a built-in light metering sensor. This sensor measures the overall brightness of the scene and then chooses what it thinks is the correct aperture, shutter speed and ISO to get the right exposure. (And generally, as I mentioned before, it does a pretty good job, but it doesn’t get it exact.)
Types of Light Meter
Two types that measure different light:
- Incident-light meter
- Reflectance-light meter
The first measures the light falling onto the subject and gives an accurate exposure. However, using this type of meter is not always practical especially for outdoor photographs of, say, landscapes or wildlife because the incident meter has to be placed on or very near to the subject in order to measure the light.
The second is much more practical because it measures the amount of light reflected off the subject back to the camera. Thus, we don’t need to be close to the subject to meter correctly. This is the type of light meter built into your camera.
Meter the Light 