Long focal lengths and large apertures allow you to isolate your subject from the chaos that surrounds it. I utilize the Av mode for the majority of my shooting. I also like to use a longer focal length to shrink the depth of field to a very narrow area. The blurred background and foreground force the viewer’s eye toward the sharper, in-focus areas, which gives greater emphasis to the subject (Figure 9.1).
Occasionally a greater depth of field is required to maintain a sharp focus across a greater distance. This might be due to the sheer depth of your subject, where you have objects that are near the camera, but sharpness is desired at a greater distance as well (Figure 9.2).
Or perhaps you are photographing a reflection in a puddle. With a narrow depth of field, you could only get the reflected object or the puddle in focus. By making the aperture smaller, you will be able to maintain acceptable sharpness in both areas (Figure 9.3).
A mirror is a two-dimensional surface, so why do you have to focus at a different distance for the image in the mirror? The answer is pretty simple, and it has to do with light. When you focus your lens, you are focusing the light being reflected off a surface onto your camera sensor. So if you wanted to focus on the mirror itself, it would be at one distance, but if you wanted to focus on the subject being reflected, you would have to take into account the distance that the object is from the mirror and then to you. Remember that the light from the subject has to travel all the way to the mirror and then to your lens. This is why a smaller aperture can be required when shooting reflected subjects. Sit in your car and take a few shots of objects in the side-view mirrors to see what I mean.