The one aspect that greatly enhanced my photos was when I began to understand the depth of field concept. Initially, I used the CameraSim App to understand how aperture, focal length, and distance affect depth of field. However, I feel that just by trial and error and documenting the three factors, I could determine when I wanted to capture an image with a shallow or deep field.
The two above images were both taken from a distance of 24 inches away from the subject and approximately the same focal length of 55mm. Only the aperture changed; the first image has a slightly shallower field than the second image since a wider aperture (F/5.6) was used.
The two above images were both taken with a medium wide aperture of F/11 and approximately the same focal length of 18mm. Only the distance changed; the first image has a shallower field because the subject is closer to the camera.
The two above images were both taken with a medium wide aperture of F/11 and from a distance of 48 inches. Only the focal length changed; the first image has a shallower field since the focal length is longer.
The last image displays what happens when a combination of all three factors are adjusted to produce a shallow depth of field: a wide aperture of F/5.0, focal length of 140mm, and the closest distance within focus of 48 inches.