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Understanding Exposure

Introduction:
Exposure is not the main component of a good image, composition and framing are. You can have a well composed image with the exposure a little off and still have a good photo. A badly composed photograph with perfect exposure is still a bad photo and there is nothing you can do to fix it.
 
Exposure:
Exposure is essentially defined as the amount of light hitting the imaging sensor, be it film or a digital imaging chip. In a film camera, the section of film used to capture the image reacts chemically to the incoming photons forming an image. In a digital camera, each pixel can be thought of as a bowl that captures the incoming photons and measures them. The more you fill the bowl, the brighter that pixel will be. Fill it to the brim and you will overflow bowl, resulting in a pixel that is pure white. Leaving the bowl empty results in an underexposed pixel, black.

The bowl must be filled to a certain point for it to produce a pixel with correct exposure. You can adjust this threshold by changing your ISO; the sensitivity of the digital chip. Raising the threshold is the same as lowering your ISO, more light will be required to produce a properly exposed image. Lowering the threshold requires less photos to attain correct exposure.

The amount of light allowed to reach the sensor is determined by how long the shutter stays open, and how wide the aperture is.

Most digital cameras have a way of adjusting the Exposure Value. Adjusting this value lets you overexpose or underexpose above and below the correct exposure threshold. On most cameras this is set by rotating the EV+/- dial in either direction. For example lets say that due to a complex composition of high contrast your camera produces an image that contains a large amount of overexposed (blown) areas. The reason this happens it that the imaging processor is trying to achieve a correct exposure for all the pixels at once, and due to the high contrast of the scene this is not possible so it takes a best average. An average however, results in overexposure of the bright sections of the image. To correct this, you would dial the EV+/- to the negative side until you get the desired image with no overexposure (underexposure is a lot easier to fix in post processing than overexposure, blown pixels contain no data so they cannot be "fixed").

Another way to correct exposure is to adjust the actual shutter, aperture, or ISO settings yourself. If you want a certain depth of field, you would change your shutter speed and/or ISO to achieve correct exposure. For a sport shot where shutter speed has to be as fast as possible, you would lower your F-number, increasing the amount of light hitting the sensor or you could increase your ISO value. When you make these changes yourself, keep in mind these rules.

Each F-stop indicates a halving or a doubling of light reaching the sensor. IE going from F8 to F5.6 doubles the light, the reverse cuts available light in half. The same is true for shutter speeds. Changing your shutter speed from 1/250 to 1/500 reduces the light by half.

Lowering your shutter speed by one stop and increasing your F-stop by one stop will result in exactly the same exposure. The following chart contain combinations of shutter speed and aperture that will result in the same exposure.

Aperture Shutter Speed (fractions of seconds)
F22 1/30
F16 1/60
F11 1/125
F8 1/250
F5.6 1/500
F4 1/1000
F2.8 1/2000
F2 1/4000

Bracketing:
Most cameras today will allow you to use a feature called bracketing. When you turn on bracketing, your camera will take a series of images one after the other while varying the exposure. In Aperture priority mode the shutter speed will be varied. In Shutter priority mode, the aperture will be varied.  You can adjust the bracketing feature to take a varied number of shots, each separated by an adjustable amount of EV+/-.
 
Auto-exposure (AE):
Auto-exposure is simply the exposure value as calculated by the camera processor based on light readings coming from the imaging chip.
 
Auto-exposure types:
Averaging. This method uses the entire image chip to read the light values captured by each pixel and averages them to produce and exposure.

Matrix. When you use matrix metering, all the chip pixels are divided into zones. Depending on the light information of the pixels in a particular zone, the camera essentially guesses at what type of scene the image is by comparing it to a preprogrammed internal image database and adjusts the image exposure accordingly. This method works quite well and generally produces correct exposures while preserving highlight details.

Center-weighted. This method is similar to the matrix method but it places more importance on the light readings in the middle of the of image. This is usually a circle area measured in mm in the middle of the frame that can be changed by the user. This setting works well for portraits and other subjects that are centered in the frame because their exposure is the most important. If there are a lot of bright or dark areas around your subject they may be over or underexposed but your centered subject will have perfect exposure.

Spot metering. This setting limits the light reading to a limited area in the middle of the frame. On some cameras this are can be moved around and its size adjusted. It's useful for a relatively small subjects where the background would confuse the light meter.

Cheers.

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