Introduction to Cameras for Nature Photography Part 3: Depth of Field

These photos are of the same plant, from the same position, and with the same lens, yet they are very different. The flowers in the center of each image are basically the same, but there is a significant difference in the appearance of the backgrounds. (You may click on the photos for a larger image.) Most people would probably agree that the photo on the left is “better” than the one on the right, because it is easier to see the flowers. The background plants are a distraction in the photo on the right.
So how did the camera with the same lens, same focus point, and same position produce such different images? The answer is that it was done by changing the aperture of the lens. The photo on the left was shot at an aperture of f5.6 and the one on the right was exposed at f36. (Remember the smaller the “f” number the larger the opening in the lens.) Why does changing the aperture change the focus of the image? It is a phenomena known as depth of field.

You may recall from the first post in this series that a lens can be focused to a very specific point. Everything on the same plane as that point will be in sharp focus. However, there is also an area in front of the point and behind it that are in acceptable focus.  Objects in this acceptable focus area will appear to be in focus more or less. The distance of the closest object in acceptable focus to the most distant object from the camera that is also in acceptable focus is the depth of field.

Depth of field varies with the size of the aperture. The smaller the aperture the greater the depth of field and vice versa. Depth of field also varies with focal length. A wide angle lens will have a greater depth of field than a telephoto set to the same aperture. The depth of field will also vary with the size of the film or digital sensor and the distance between the lens and the subject. There is a lot of heavy duty math involved in calculating the depth of field, but fortunately there are tables and “apps for that” that make it easy to determine.

I’m sure that you can see how understanding how to control depth of field can be an important tool in creating images that convey your vision. At some point you may want to observe and be concerned about the quality of the out of focus or blurry area that is known as bokeh.  This is a Japanese word that describes the aesthetic quality of the blur which is largely controlled by the design and construction of the lens. (That’s right there are some of us who really care about that!)

I hope that as you have read this post a question has occurred to you that is something like “If the the aperture controls the amount of light that enters the camera, how can you have such a wide range of aperture in the photos above and still have the same apparent exposure?” In other words why isn’t the f5.6 photo not over exposed and the f36 image grossly under exposed? Obviously there are some other variables in play. Those variables are controlled by the camera which we will start discussing in the next post.

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