Exposure is the key concept that governs all forms of photography where light in some form or the other is used. Regardless of the camera, lens or other gear you may have, or post processing skills you may possess, if you are unfamiliar with the concepts of exposure and are incapable of getting a good exposure 9 out of 10 times, in camera, you will never be a good photographer. Even if your concept is a good one, a poor exposure is going to ruin the image.
What Is Exposure?
So, even before we progress any further, we need to have a good understanding of what exposure is. The term exposure is loosely tossed around often synonymously with the words image and picture. An exposure in reality is the quantity of light per unit area that reaches a photographic media. Three things govern how much light will enter the camera and reach the photographic media. Shutter speed, aperture and the average luminance of the scene.
There is a fourth element too and that is the sensitivity of the imaging media. In the older days when film was used, photographers would choose between ‘slow’ or ‘fast’ films. A ‘slow’ film is one that is less sensitive to light and thus is ideally suitable to photograph images in bright conditions where there was abundant light. On the other hand a ‘fast’ film is one that needs less light to make a proper exposure. However, faster film were also prone to noise. Modern day digital cameras have sensors in place of film. However, the same concepts of photography and media sensitivity are still applicable here. Sensitivity of a digital sensor is referred to as ISO.
Under / Over-Exposure
That brings us to the next important matter of discussion. What is under or over-exposure?
For the same shutter speed and aperture, if the scene is too dark (moonlight) an image will be under-exposed while if the scene is too bright (outdoors, bright mid-day conditions) the image will be over-exposed.
Under-exposure is when either the shutter speed or the aperture value or both have been incorrectly set in reference to the luminance of the scene, resulting in an image that is too dark losing details in shadows. Over-exposure is the exact opposite. In this, the aperture value and shutter speed depending on the scene has resulted in an image that is too bright, washing away details in the highlights.
A good way to check whether you nailed the exposure is the histogram. Always remember, if you have a histogram that is leaning towards the right that means the image has too many bright tones and may be even bordering on over-exposure. If the histogram is leaning more towards the left it means you have an image that has too many dark tones or bordering on under-exposed. It is the middle ground that indicates a proper exposure. There are however exceptions.
If You Have A Choice Would You Under-Expose Or Over-Expose?
Please note, that both over and under-exposure have their own disadvantages, but realistically speaking if you have a scene where you are unsure about what exposure to set, always under-expose to be safe. When you under-expose a scene much of the details that appear to be lost in the shadows on first look can actually be retrieved. However, if the scene is over-exposed almost all of the details in the over-exposed areas of the image are lost forever. You may also want to shoot in RAW as that leaves all of the data captured by the sensor unprocessed. This is a big advantage when you are post-processing your images in Photoshop. Speaking of post-processing if you regularly shoot in RAW and want to be able to outsource your post-processing chores, try out Phowd.
The Holy Trinity of Photographic Exposure
While on the subject of exposure we cannot overlook three important constituents that make up what exposure is. Aperture, shutter speed and ISO taken together are what constitutes the holy trinity of exposure. If you master each of these elements individually and their mutual relationship, you will have mastered much of the concepts of exposure.
A camera is a light-tight box, which can be compared with a room. Let’s presume that the room has a single window. This window is the equivalent of camera aperture. The time frame for which you keep the window open is the shutter speed.
If you open only one pane of the window, you are controlling the amount of light that is entering the camera by controlling the size of the window. This is similar to how aperture is controlled in a camera. If the scene is too dark you open the aperture wider, here you would open both panes to allow more light to enter the room. If it is too bright you open only one pane.
ISO as we have already discussed above is the sensitivity of the sensor. Before sensors, the film was the photographic media (we are not going too back to the days of wet plate and dry plates). Back in those days photographers needed to select films of different speeds in order to counter dark or bright conditions. These days photographers have a much easier time. All they need is to select the right ISO using the controls of a camera to either increase or decrease the sensitivity.
Aperture value and shutter speed have an inverse relationship. Let’s take an example to demonstrate this. You set out on a bright day to capture some portrait photos. You set your camera to aperture priority and aperture to f/5.6. The shutter speed is automatically calculated by the camera to 1/200th of a second. A few shots later you decide to open up the aperture to capture some nice bokeh. You switch to manual mode and reset the aperture to f/2.8. But that is a full two stops wider than f/5.6. If the shutter speed is kept at 1/200th of a second the image will be washed out. Though the camera’s metering system will automatically figure this out and select the appropriate shutter speed (in aperture priority mode), you could manually do this as well (when in manual mode). At f/2.8 the shutter speed should be two stops faster to about 1/800th of a second. As you can see, from this example, when the aperture value increases to two stops wider, the shutter speed gets faster (reduced) by two stops.
A Word on Exposure Value
What we learnt above is also referred to as exposure value. To define exposure value, it is any combination of shutter speed and aperture that is likely to give the same exposure. In the above example f/5.6 and 1/200th of second is likely to give the same exposure that f/2.8 and 1/800th of a second will. In both the cases, however, ISO number must remain the same. Understanding of exposure value will help you when you are trying to tweak the shutter speed for long exposures and or want to freeze movement using faster shutter speeds.
How the Digital Sensor Works
The digital sensor is where the light falls on after travelling though the barrel of the lens and the front part of the camera. This is where light energy is converted into electrical signals. Then via the delicate internal circuitry that information is passed on to the image processing unit.
The Metering System of a Typical DSLR Camera and When It Can Go Wrong
A digital sensor tries to look at the world around it as if it is 18% grey or middle grey. Middle grey has an average reflectance. In other words if there are equal shades of pure white and pure black in a scene a digital sensor will attempt at averaging everything making them appear grey. This is the reason why, when you are shooting in auto exposure mode, a bright white background appears dull grey in the final image and a perfect black dress appears grey.
Of course the word ‘proper’ depends on what the scene is all about. Such as when you are shooting a snow scene or a subject is wearing black clothes and standing against a black wall you will have the histogram pointing to the right or left respectively. In such situations you will simply have to ignore the histogram and go with your gut feeling.
The Use of Exposure Compensation
In the above example a subject wearing black dress and standing against a black wall is likely to result in a histogram that is more towards the left. If you shoot at auto exposure mode the camera will try to compensate for all that black and over-expose the scene in line with its middle-grey philosophy of the built-in metering system. But the problem is you want to capture the scene for what it is. You know that the scene is too dark because there are a number of dark elements in it. You will need to override the metering system of the camera and tell it what should be the proper exposure. There are two ways of doing it.
In aperture and shutter priority modes, as well as in programmed auto mode you can use the exposure compensation option to override the camera’s metering system. The exposure compensation button is shown in the image above that has the signs (+/-). Usually for the black scene above you may have to use negative exposure compensation by 2 to even 3 stops. For the snow (bright scene) you may have to over compensate by 1-2 stops.
The second option is to manually change the exposure by adjusting your shutter speed or aperture. Please note, in manual mode exposure compensation is not available.
A Word on Post-processing
While it is recommended that you get the exposure correct as much as possible in the camera itself, there may be instances when, for any reason, you are unable to nail it. Either the lighting is tricky or you got the exposure value incorrect. In any case, you deserve a second chance with your images. Especially if they have emotional value to you. In such situations upload your photos to Phowd.com and independent photo retouchers can help you salvage your image.