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A photo is said to be correctly exposed when it gives good reproduction of detail in both the deepest shadows and brightest highlights of the scene or subject.  Fortunately, in many cases there is more than a single exposure that will produce such a result -- there is a wide range of possible exposures within which satisfactory tone separation is possible.  The "minimum" satisfactory exposure is one in which good tone separation is just attained in the deepest shadow areas.  The "maximum" satisfactory exposure is one in which detail is just retained in the brightest highlights.  Any additional exposure will cause this highlight detail to become flattened out or "blocked up."

Exposure is the term used to define the lens aperture and shutter speed used to allow light to pass into the camera and form a latent image on the digital camera’s charged-coupled device (CCD).  Exposure is the total amount of light reaching the CCD.  Correct Exposure is the exact amount of light required to record maximum detail.

Exposure Considerations

Good separation between the light, dark and middle tones of a picture depends almost entirely on the exposure.  In digital imagery, exposure is governed by the intensity of light on the DCS’s CCD.  The digital imagery process is less forgiving.  However, within limits, an under- or over- exposed digital image can be electronically enhanced or repaired to yield a useable product.

Underexposure, or too little light, results in a loss of tone separation, contrast and detail in the darker parts of an image.  Underexposed images may produce an unusable dark blur. 

Overexposure, or too much light, produces a light image with poor tone separation in the lighter parts of the picture.  It occurs when the aperture is too large or when the shutter speed is too low. 

Most films come with a daylight exposure table which you can use to get good exposures.  In Figure 2-1 the table shows that when shooting a picture of an average subject (such as a fair-skinned person) in hazy sun you would set the aperture at f/11.  Note that you should "use shutter speed which is reciprocal of film speed (with ASA/ISO 400 you would use 1/500th shutter speed, with ASA/ISO 125 you would use 1/125th shutter speed, etc.)  Digital camera users should refer to their owner’s manual for ISO setting recommendations.

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Figure 2-1. Exposure Table

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