Jul 17, 2011

Photography 101 - Exposure

Exposure is one of the most important camera and lens functions that a photographer needs to understand and master when pursuing photography. A correct exposed photograph conveys an image of clarity that retains details and colors in all areas including light or dark areas. Correct exposure is always subjective and while I prefer slightly underexposed images that boosts colors and saturation, others may not.

Correct exposure is a fine combination of ISO, shutter speed and aperture camera and lens settings. By selecting a certain ISO setting we determine the image sensor's sensibility to light. Usually we choose lowest ISO settings to minimize camera noise and enhance image quality. The camera shutter speed or exposure time controls the amount of light that strikes the sensor or in the older days the film. The shutter is right in front of the sensor and when we press the shutter release button to capture an image the shutter or curtain opens for a certain amount of time allowing light to strike the sensor before it closes again. The slower or longer the shutter speed, the more light strikes the sensor and vice versa.

The lens aperture or f-stop setting allows a certain volume of light to pass through the lens into our cameras. For example a small aperture (large f/numbers such as 11 and greater) provides a small opening for the light to pass through; therefore limiting the volume of light to pass through. On the other end of the scale, a large aperture (small f/numbers such as 5.6 and less) provides a large opening for the light to pass through; hence allowing more light to strike the sensor.

There are 4 camera modes that allow us to take control over exposure:

1. Manual Mode
In manual or M mode we are able to select ISO, shutter speed and aperture to achieve a particular effect. I usually shoot in manual in low light applications where I look for longer exposure times, mainly city skylines at twilight.

2. Semi Automatic P Mode
The semi automatic P mode selects ISO, aperture and shutter speed combinations depending on the available light. Compared to the fully automatic modes such as portrait, landscape, or flower, the P mode allows us to change the combination to influence depth of field or how we capture objects in motion.

3. Shutter Speed Priority Mode
In the shutter speed priority mode the camera automatically selects an aperture setting to match the chosen ISO setting and brightness of the object. This may be the preferred mode when shooting sport events at fast shutter speeds where we prefer to freeze the action in front of us. It is also an option for moving photography where we strive for silky water effects and require longer exposure times.

4. Aperture Priority Mode
In the aperture priority mode we choose an aperture in combination with an ISO setting while the camera determines an appropriate shutter speed. Aperture priority mode is my favorite mode because it provides me with total control over depth of field and shutter speed. For example, at a given low ISO setting for optimum image quality I usually choose either a large aperture or large f/numbers such as 11 and greater when I shoot landscapes or seascapes where we like to create a sharp image from front to end. On the other hand I select a small aperture or f/numbers such as 5.6 and less for wildlife or flowers where we like to create a shallow depth of field. Shutter speed is less important to me since I always bring my tripod and use it in nearly every photo I take. Depending on my photographic goals I only fiddle around with the aperture to adjust the depth of field and the appearance of my images.