according to dof tables from several sources, if sony A7 full frame. lens focal length is 35mm aperture is f8 focus distance to subject is 10 feet then:- closest focus point is 6.3 -6.2 feet furthest point in focus is 18.1 ft So images taken at 10 foot are pin sharp. With out altering any thing on the camera or its position I then move the object to 6 foot and the image is soft, move it to 7 ft its still soft move it to 18 foot its sharp. why? I am wanting to pick a focus at f8 and be able to let the subject move around not need to refocus every shot but with this discrepancy it does not work what am I missing??