Many people take pictures.  Some take a lot of pictures.  Most have at least one or 2 shots they consider great!  Professional photographers need to have consistently good shots.  A photographers portfolio should contain a strong opening image and a strong end image, but it is the consistency of the images between them that must be evaluated.  If the difference in quality varies, you can't be sure of what you will get.  You can almost bet on not getting the same results as the first and last picture in the portfolio.

By consistency, I do not mean the subject matter is the same.  Contrast, Brightness, Color Correction, Focus, and Timing all contribute to creating an image.  It is the manner in which the photographer shows his ability to produce results that reflect mastery of these elements that you need to see consistency.  If you are hiring a photographer to shoot a sporting event, the work should consistently show an ability to capture subjects at an optimal time (just as the football leaves the hand of the quarterback, at the moment the ball hits the bat, etc.).  Wedding photographers need to show consistent color balance over the 4 to 8 hours they shot even though the light varies.  When the work is good and shows consistency, that's a photographer worthy of consideration.

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