In a previous post about aperture we learned about depth of field being the creative aspect of the aperture setting. We learned with a smaller aperture, small hole (less light), there is more depth of field than with a larger aperture, large hole (more light).
From the diagram above, we see there is more to consider, not only with the aperture setting but also the focal length, and distance to subject to determine the depth of field around the subject but also how much of the background is in or out of focus based on the distance to background from the point of focus.
In the Northern Gannet image the subject was about 100ft away with a blue cloudy sky in the background. With an aperture of f/8 and a focal length of 400mm you can see the Gannet head at the bottom is out of focus and the blue cloudy sky in the background also considerably out of focus.
Using a calculator, we find that with a distance to subject of 100ft, the front focus distance is at approximately 95ft 8in and the back focus distance point would be at about 104ft 9in. We can conclude that with a long focal length and a smaller aperture of f/8 the area of acceptable focus is only about 9 feet, which is fairly shallow.
The statue depicting one aspect of the life of a fisherman on the Gaspé Peninsula, QC, Canada, we see right away that the focal length is 24mm which is much wider than the focal length used for the image of the Northern Gannets. Notice also that we are using even a smaller aperture of f/11.
Using a calculator, we find out with the subject distance of 10 feet, the front focus distance is at about 3ft 7in with a back focus distance to infinity. This allows all of the image to be within acceptable focus, from 3ft 7in through to the clouds and the sky (infinity).
Notice that even though we are close to the subject, 10ft which is fairly close, we can still achieve a very large depth of field with a small aperture in combination with a shorter focal length (wide angle of view).
The most important thing to take away from this lesson, is that all three aspects; aperture, focal length, and distance to subject combined control the amount of depth of field. From the two examples above, the general rule of focusing 1/3 of the way into a scene to get everything in focus is not true for longer focal lengths and that there is a bit more science into understanding what will be or not be acceptably sharp in your image.
There is one more concept in picking the optimum point of focus that we need to understand called hyperfocal distance as it relates to landscape photography, which we will discuss in the next lesson.