Chapter 2
Exposure
I'm going to keep this fairly simple. No graduate study of physics and/or engineering is required to understand photographic principles. Photography at its heart is a control of light. By understanding the three main dynamic settings of your camera you can have control of how much light hits the sensor of your camera and therefore control of your exposure.
The three most important things you must understand to improve your camera skills are (1) Aperture, (2) Shutter Speed and (3) ISO.
Exposure in photography is measured in units called f stops. A f stop is the ratio of of the diameter of the lens aperture and the focal length of the lens as measured in millimeters (mm.) 1 f stop change up in light is a doubling and down is halving of the amount of total exposure. Most digital cameras and some lenses will allow you to control f stops in 1/3 stop increments.
Aperture is an adjustable opening in the lens that allows varying degrees of light to pass through the lens . A more open aperture will pass more light through the lens than a more closed (stopped down) aperture. The lens aperture can be controlled directly by your camera or in some cases manually, by adjusting the lens. For example, older Nikon lenses have manual aperture rings which you can manually dial for a desired setting. Canon EOS lenses only have electrically controlled apertures and must be adjusted at the camera.
When you hear the phrase “stopping down” or “stops”, think in terms of light exposure units being decreased or raised by x amount of f stops. Stopping down means reducing the amount of light. Stopping down a lens means to reduce the size of the aperture and as a result increasing the depth of field.
Typical full f stops are as follows: f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22
Notice, the full f stop values aren't in whole number increments. Though f stops control light by halving or doubling the amount of light, the f stops themselves are representative of the actual ratios.
A lens with an aperture setting of f1 would have a very wide opening, allowing a large amount of light to pass through the lens. A lens with an aperture setting of f22 would have a very narrow opening allowing only a small amount of light through the lens. Aperture adjustments on your lens control more than the amount of light passing through your lens. Your aperture also controls the depth of field of your image.
Depth of field is the amount of area in front of and behind the point of focus that will be in acceptable focus. A very open aperture such as f 2.8 will have a very narrow depth of field, a stopped down aperture, say f16, will have a larger depth of field with more area in focus from near to far. The depth of field is dependent upon distance to subject and focal length of the lens, and is calculable. Depth of field is a fundamental building block of exposure and composition and understanding this effect is critical to understanding how to create a photographic image with proper exposure and proper focus compositional elements.
Most DSLR's have what is called a focal plane shutter, which controls the duration of light from the lens striking the sensor. The faster the shutter opens and closes, the less accumulated light striking the sensor. Shutter speeds can be measured in 1000th's of seconds or can be controlled to remain open for as long as the camera battery lasts.
ISO is a measure of the sensitivity of the sensor in your camera. In the film days, it was a measure of the sensitivity of your film.
Take a look at the controls on your camera body, you'll most likely find a knob on the top that has different letters and mini-icons for different settings. These settings are used to place your camera in an exposure mode of your choosing and each configure your camera & lens to operate a little differently from one another. These settings will normally range from a full automatic mode (the camera makes all decisions) to semi-automatic modes (you tell the camera something, it figures out the rest), and fully manual modes (you make all of the exposure decisions.)
Different modes you're most likely to encounter with most DSLR's are as follows.
Fully Automatic: Green circle or wife mode. Put the camera in this mode, point it and shoot. This is point & shoot mode, the camera does everything but fire the shutter. You get what you get as a result. Camera's aren't all that smart, so about half of what you do in this mode will be usable even under the best of circumstances.
Portrait Mode: Usually denoted by a miniature head icon on the dial. When using this mode, the camera apertures are opened up wider allowing more light through from the lens and decreasing the depth of field (more background blur)
Landscape Mode: Usually denoted by a miniature mountain icon on the dial. When you place the camera in this mode, the camera assumes you need a narrow aperture and want the scene to contain elements in focus both close and in the distance. Depending of course on how close the close things are to the camera and how far away the far away things are from the camera, you'll again get varying degrees of acceptable image quality. Still hit & miss if you know exactly what you want to see in focus.
Macro (Close-up) Mode: Usually denoted by a little flower icon on the dial. In this mode, the camera attempts to keep the aperture at a mid-range value to maintain some depth of field on objects that are being photographed from a very short distance.
Sports Mode: Usually denoted with a tiny running human icon. In this mode the camera will attempt to keep a high shutter speed to cut down on visual motion blur when photographing moving objects such as athletes running. Again, your results will be hit and miss.
Night Scene: Usually denoted with a star icon. In this mode, the camera will try to maintain a slow shutter speed for longer exposures (more light to the sensor) and possibly want to fire your flash to light up the foreground. I've never taken a good night photograph using this setting, you probably won't either.
All of the aforementioned modes are programmed modes for users who don't understand photography and just want to pick up a camera and take snapshots. Understanding what the camera does in these modes; however, can give you a better insight into proper exposure techniques for general types of photographic needs. Once you understand how to control your exposure settings you can make the camera do what you want. The trick of course is analyzing a scene and understanding what you'll want to do and then translating that to a particular setting. We'll delve into all this in a future essay.
Let's move on to the “Creative Settings” on your camera. These modes are where you want to operate your camera from most of the time.
Shutter Priority: This mode may be denoted by a Tv or S, depending on the brand and make of your camera. When set to this mode, you tell the camera the shutter speed to use and the camera will adjust the lens aperture for proper exposure. This mode is useful for low light shooting when you don't want the shutter speed to drop too low, preventing motion blur in your image. You may be photographing moving objects and want to freeze the movement, therefore you set a high shutter speed and again the camera adjusts the aperture to control the exposure. What you lose in this mode is control of the aperture and depth of field.
Aperture Priority: This mode may be denoted by a Av or A, depending on the brand and make of your camera. When set to this mode, you tell the camera to set a specific lens aperture and the camera will adjust the shutter speed for proper exposure. This is a useful mode when you know the depth of field you wish to maintain in your image and your aren't concerned with the shutter speed. Lens aperture setting will also have an effect on the sharpness of your image as lenses aren't equally sharp from lens to lens and from focal length to focal length.
Full Manual Mode: This mode is usually identified as M on the control. In this mode, you take full control of the shutter speed and aperture settings of the camera. The camera light metering still functions but the camera doesn't attempt to change the exposure. If you are wrong with your settings, the image will be improperly exposed. The benefit to manual exposure is that you are in control and do not rely on the camera to get it right or wrong. Very useful mode if you aren't in a hurry and know exactly how you want the image to look. Also very useful mode when using a flash. When shooting a wedding or event, I normally keep my camera in manual exposure mode with a preset aperture and shutter speed and allow the camera to control the flash to fill the scene via ETTL flash metering. You can also adjust your flash power by controlling the camera's FEC (Flash Exposure Compensation.) We'll discuss this in further detail in a later write-up.
Auto ISO: Some of the more modern digital SLR's have a feature called Auto-ISO. This feature is usually enabled via a custom setting within the camera's configuration menu. Your ISO setting is an important consideration when trying to control your exposure range. Lower ISO's are generally used in well lit environments and higher ISO's are generally used in lower light situations. When Auto-ISO is enabled, the camera will attempt to adjust the ISO setting automatically, within a certain range (and not all cameras have the same exact behavior with this feature) to compensate for the measured light in a given scene. Another consideration you'll need to make when deciding on your camera's ISO setting are how much digital noise you want to introduce into your image. Modern DSLR's have what is called a “Base ISO” and this is generally ISO 100 or 200 depending on your brand and model. When I'm outdoors on a well lit day, I normally keep my camera set to ISO 200. At ISO 100 or 200, your camera's sensor is normally operating under premium image quality and the amount of noise in a properly exposed image will be unnoticeable. As you (or the camera) increases the ISO, it is increasing the sensitivity of the camera sensor but in doing so it also increases the amount of digital noise introduced into the image. Most modern DSLR's will produce a very acceptable noise level up to ISO 800 and this is a feature that is improving with each new generation of camera. I regularly use ISO-1600 for indoor photography. The benefit to being able to user higher ISO settings is that it allows you to have faster shutter speeds in lower light situations. Doubling or halving the ISO setting controls your exposure in full stop increments. By increasing from ISO 100 to ISO 200, you gain one full stop of exposure, therefore you can also increase your shutter speed or decrease your aperture size an additional stop. Typical ISO's are 100, 200, 400, 800, 1600, 3200, 6400. Your camera may also allow you to adjust ISO in intermediate increments between these standard settings in half-stops or third-stops.
Histogram: This feature on a camera is probably the best tool you have for evaluating your exposure either during or immediately after taking a photograph.Most current DSLR's have the ability to display a histogram of the image. You'll find this function in your camera's configuration menu and/or the image info button. I highly recommend that you configure your camera to display a histogram for each image. You can use this information to help determine proper exposure.
Here's a sample image with its associated histogram (as seen from Photoshop CS4.)
In a nutshell, the image is a combination of three different colors. Red, Green and Blue.
Your camera may display variations of this information. The graphical image represents the amount of light at each color from dark (left) to bright (right.) By reviewing this information in-camera, you can determine if you are under-exposed or over-exposed. For an optimum exposure, you should use a technique that is called “expose to the right.” If you set your exposure so that the right edge of the data lines up as close to the right edge of the histogram as possible, without going over, you will have achieved as close to a properly exposed image as possible. With digital cameras, overexposing the image will result in clipped highlight information, and you'll lose fine details in the bright areas of your image. Typically, this will be in clouds or bright white sections of your image. Once that detail is lost, it can't be recovered. If the left side of the graph is exceeded, your image will be under-exposed and too dark. Too dark an image will also result in loss of detail in the shadow areas and will also result in excessive digital noise in your image when attempting to brighten things up in photo editing.