Clearly it matters where in your image you choose to focus. With faster apertures and longer lenses, focus becomes extremely precise — to the point where it can be very difficult to be accurate about where you want the critical focus point.
Here are two series, one with the absurdly fast Canon 85mm 1.2, another with the Canon 180 3.5 macro lens. Both sequences were shot wide open, and the point of focus varied. Although both setups are pretty similar, the look of the 85 and 180 is quite different.
Each setup was done specifically to create a series of layers of points of focus.
Bottom Line: You can choose what is in focus, and what is not, and that is a primary tool for shaping the image.
Left
Right
Focusing on the flower, the closest thing to the camera.
85mm, f1.2, studio flash
Focus on the eye and the cheek.
85mm, f1.2, studio flash
Focus on the lipstick.
85mm, f1.2, studio flash
Focus (more or less) on the background. That 85mm 1.2 lens is not easy to focus!
85mm, f1.2, studio flash
Focused on the flower.
85mm, f1.2, studio flash
Focused on Nicole, with the foreground and background out of focus. Here the woman in the painting is adding something to the image, but it is not the main subject, as it is in panel 8, where she is the only thing in focus.
180mm, f13.5, studio flash
Focused on the wine bottle. If this was a wine advertisement, we would have needed to space everything differently, so that the wine could be in focus, and everything else MORE out of focus.
180mm, f13.5, studio flash
Here we are focused on the painting in the background, which shifts the story of the image to one about the attitude of the woman in the painting towards to woman putting on her makeup.
180mm, f13.5, studio flash
Focusing on the flower, the closest thing to the camera.
85mm, f1.2, studio flash
Focus on the eye and the cheek.
85mm, f1.2, studio flash
Focus on the lipstick.
85mm, f1.2, studio flash
Focus (more or less) on the background. That 85mm 1.2 lens is not easy to focus!
85mm, f1.2, studio flash
Focused on the flower.
85mm, f1.2, studio flash
Focused on Nicole, with the foreground and background out of focus. Here the woman in the painting is adding something to the image, but it is not the main subject, as it is in panel 8, where she is the only thing in focus.
180mm, f13.5, studio flash
Focused on the wine bottle. If this was a wine advertisement, we would have needed to space everything differently, so that the wine could be in focus, and everything else MORE out of focus.
180mm, f13.5, studio flash
Here we are focused on the painting in the background, which shifts the story of the image to one about the attitude of the woman in the painting towards to woman putting on her makeup.