The human eye sees a scene in very small sections. That angle is about 3 radial degrees of acute(sharp) vision. The eye scans the entire field of view and then assembles those parts within the mind to construct an entire image. The eye moves randomly, back and forth selecting information rapidly and transmits it to the mind. It will stop momentarily at points-of-interest with precision when scanning a scene while leaving other areas unclear. Consequently, does not visualize an entire scene with complete "sharpness."
A photographer can do the same thing and direct a viewer's eye through an a photograph. A great composition can hold a persons attention in specific areas that are sharper than others. When composed well, the photographer can take a viewer on a "journey" throughout a picture directing their eyes to primary elements and then secondary elements. The successful photograph will have elements organized in a specific manner.The difference between a snapshot and a great photograph is what distinguishes an artist from ordinary people.
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