I will always remember using black and white film and the trepidation waiting to have the films developed. I would worry that I hadn’t nailed the focusing or the exposure would be off. Early on I started manually using exposure compensation to make sure that I would have a good exposure of a scene. This meant that a roll of 36 frames would really only have the potential of 12 scenes.
With digital we can turn on chimp mode and check the exposure on the LCD screen to make sure we have captured a good shot, yet there are some other things we can do to make sure we can get the best exposure.
In photography, the exposure is the amount of light allowed to fall on the sensor or light sensitive material. The exposure is controlled by three factors; Shutter speed, aperture and ISO, also known as the exposure triangle. By changing one of those elements the exposure will be over or under and to bring it back into balance you have to change one or both of the other elements. There is no right exposure theoretically, as the exposure can change the mood/ emotion of the scene.
There are two ways to measure exposure; Through The Lens (TTL) or with a hand held meter. The TTL meter measures the exposure and it takes into consideration the Lens, filters and extension tubes you may have on your camera.
A light meter can measure light in two ways; reflective – light coming off a subject, and incident- the light falling on a subject. Incident light meters tend to lead to a better exposure as it measures the light independent to the subject’s reflective quality. Although it measures a more accurate exposure you do have to hold the light meter next to the subject, that in some circumstances is not an option; for example wildlife photography. A reflective light meter, like the TTL one in a camera, measure the amount of light reaching the camera ( without taking into consideration the Lens ect). A reflective light meter gives you an exposure for a midtone grey. In a scene with a lot of light and bright colours the meter would incorrectly measure the scene and give you a reading that will lead to an image being underexposed.
A reflective meter and therefore the TTL meter in your camera is good but not great.
Cameras have differing ways in which it cam measure the reflective light of a scene or subject. The main ones are;
Multi segmen– good for scenes with no central subject; busy street scenes or a roaming pack/herd of animals.
The mode reads the exposure form multiple parts of the frame and then calculates an average exposure reading.
Center weighted– good for portraits
Also known as average metering, it places the emphasis of the meter reading on the centre of the frame while taking readings from the edge in a 75/25 split.
Spot meter – High contrast scenes.
Has the best level of control, measures a precise area with a 5-6 degree angle. This allows you to emphasise a specific area in the scene. It also means that you can meter a subject and then recompose the scene, leaving the subject accurately exposed while with the rest of the scene either under or overexposed.
Even using the best exposure mode, reflective metering can leave an image looking muddy and grey. A good example of this is when it snows. Snowy scenes apart from being quite monochromatic are also bright due to the reflective light from the snow. When taking a picture the reflective metering measures this brightness of the snow as grey, resulting in the snow being underexposed. One way to help correct the metering is to carry a grey card with you and take a reading of it or if you don’t have a grey card use the green of the grass to get a midtone exposure reading. With a snowy scene you don’t have the green of the grass to take advantage of but you can use exposure compensation.
Exposure compensation is when you tell the camera to automatically take a reading with exposure a certain amount higher or lower, usually in increments of stops or fraction of a stop. Alternatively you can bracket your shots. Bracketing is where instead of taking one picture you take three, one over exposed, one under and exposure given by the camera. This can be useful when you have a landscape with a bight sky and midtone foreground, as you can either select the preferred exposure of merge them together.
You can also use the histogram of the image to measure how “correct” the exposure is. Using the preview function on your camera you can select to preview the histogram alongside the. A histogram is an information graph that shows how many tones there are from dark to light. A technically correct exposure should have a nice curve from the shadows to the midtones and then slopping down to the highlights. An overexposed image would have the histogram weighted towards the highlights and an underexposed image weighted towards the shadows. Although there is a technically correct histogram this may not be correct for every scene. You really need to look at the histogram and interpret what it is saying. A darker scene would have more dark tones and so it should be expected that the histogram has more tones in the shadows, similarly you would expect a bright scene to have more tones in the highlights.
You can also set up on the preview of the image a flash to warn of tonal clipping. Clipped tones are tones that have fallen beyond the dynamic range of the camera and are represented as absolute blacks and absolute whites. With this preview on you can see where the clipping is taking place and adjust the exposure as needed.
Checking histograms and clipping warnings can leave you spending more time looking at the back of the camera than through the lens. I have a 3 sec preview of each image set up on my cameras. This allows me at a glance, if need be, at a preview of my last shots to make sure the histogram looks ok, and that there are no major flashes warning of clipping.
In saying this if you shoot raw you have quite a wide degree of error and can rescue what may have once been seen as lost shots. I would say it is generally better to overexpose slightly than under as noise is more prevalent in the shadows of an image and this lowers the image quality, but it is harder to rescue tones from an overexposed shot.
To achieve a good exposure of a scene I recommend using the best exposure mode for the scene, shoot for the midtones and check histogram that it matches the scene as well as bracketing to ensure that the best exposure has been captured. Also don’t be afraid of merging images to create the scene before you, to stretch the dynamic range of the camera and get an excellent exposure of the highlights and shadows as well as the midtones in between.