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You must first consider the mission requirements.  For each mission there are one or more factors that must be considered.  Sometimes one factor offsets the other, or one may have an opposite effect on the other.

You should choose an ISO setting that will take the desired picture.  The slower-ISOs will give you comparatively better grain and resolving power.  Range of f/stops and shutter speeds available under the lighting conditions also dictate your ISO choice.  For example, a portrait is normally shot with a wide aperture to reduce depth of field.  Choosing a high ISOs for an outdoor portrait on a sunny day would limit you to fast shutter speeds and small apertures.  Conversely, in low-light situations where wide exposure latitude is needed or on an airborne mission where fast shutter speeds and small apertures are desired, a high ISO is ideal.

Let's take a look at some examples and discuss how one would consider ISOs for different situations.  Remember that these are hypothetical situations and are not meant to be exact solutions for that example. 

Example #1:  If the subject being photographed is in the area of poor illumination, a high ISO is desirable.  A high ISO, however, has low resolution.

Example #2:  Assume your assignment calls for photographing a subject having little difference in the shades of gray.  In this case you need an ISO that can produce fairly high contrast.  Again, higher ISOs have lower resolution.  Proper lighting also can increase contrast.

Example #3:  Suppose you are asked to take a photograph of an object, and the negative will be enlarged considerably.  A low ISO would have fine grain and high resolution.  However, such an ISO will have low contrast.  You will have to compensate by using different lighting.  Another possibility is to sacrifice resolution for contrast.

For most newspaper and magazine work, the best choice is a ISO. In this situation, grain is not as important a factor since there will be a loss of print sharpness when it is reproduced in the newspaper or magazine. 

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