One of my on-line photography students asked why a particular photo had so much depth of field, yet it was taken with a large lens aperture -- f/5. The photo in question was similar to the one you see above. It was taken with a wide angle and the foreground was relatively close to the camera position. Here is the answer to the question.
Depth of field is controlled not just by lens aperture. There are a total of four factors that influence depth of field (dof):
1. Lens aperture (the smaller the aperture, the more dof)
2. Focal length of lens (the wider the angle lens, the more dof)
3. Distance from camera to subject (the further away the camera is to the subject, the more dof)
4. Distance from subject to background (the closer the subject is to the background, the more focused the subject and the and background will be, thus the more dof you'll have)
These 4 factors interact with each other constantly, and often one will override the other. In the example of the New England boat harbor, the fact that a wide angle lens was used eclipsed the influence of the lens aperture. Wide angle lenses offer tremendous depth of field at ANY aperture. However ... and this is where the plot thickens ... if the camera is positioned very close to the foreground subject, like 2 or 3 feet away, that factor will eclipse the influence of the wide angle focal length. Thus, the background will be defined but not tack sharp. In other words, some (but not that much) depth of field will be lost because of the close proximity to the foreground. Enter lens aperture. By closing the aperture down to, say, f/22, you get that depth of field back and the distant background becomes sharp again.
By moving further away from the foreground, the depth of field in the background also comes back, but then the composition changes.
So, all of these factors come into play in each picture you take.
Let's take another example. If you want the background behind a subject to be completely out of focus such that there is no definition at all, as in the picture of the Canadian lynx kitten, and you wanted to use a 50mm lens. That wouldn't be possible. Assuming you were several feet away, even at f/1.4 the background would be blurred but we would be able to see a significant amount of definition in the trees behind the cat. Why? Because the 50mm focal length offers too much depth of field, and that overrides the large aperture. The only way you could override the influence of the focal length would be to shoot the kitten from about 12 inches away -- not a likely scenario in this particular case.
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