One of the most difficult areas to grasp within photography is the Depth of Field factor (DoF). Depth of Field refers to the range of distance that appears acceptably sharp. Four major factors influence the DoF - the camera model (type), focal length (lens model), aperture and distance to subject.
My subject is a withered flower placed on a table with a brick wall as a back drop. I want the brick wall to be out of focus and the complete flower to be in focus. Normally I would chose a high aperture (small f-value) like f/1.4. The distance from the camera to the flower is 0.5 meter. The distance from the flower to the wall is 3 meters. I'm using my Nikon D300 with a AF 50mm f/1.4 prime lens.
Please go to my blog to read about the DoF calculator.