Focal Distance and Depth Of Field
We understand Depth of Field and the focal distance. Remember, the depth of field is how much near-to-far we have in sharp focus, and the focal distance is literally what we adjust when we adjust the focus. Where, near-to-far, the camera is focusing, from as-close-as-possible to infinity.
There is an interesting and important relationship between these two properties. Let's first lay it out on the table:
The closer you are focused, the shallower the depth of field. The further you are focused, the deeper the depth of field.
This can be hard to discover without paying attention to, because the relative size of things shrinks as you get further away, (things far away = small. Perspective.) Thus, how much depth of field you have in terms of image-impact may not change all that much. Let's look at two images taken with the exact same image settings. These images were taken in Sacksville waterfowl park, in Canada.
Both of these images were shot at 100ISO, f/8 aperture, 1/500th shutter speed, on a 40mm prime lens, and only about 2 minutes apart. Notice the depth of field in both images. In the top, it's what we call 'deep focus'. Everything from the foreground to the background is in focus. (Okay, not quite everything, I lose a little bit on the railing close to the camera). In the second flower photo, notice how the background and other flowers are out of focus.
The only thing that changes was the focal distance. But focal distance affects the depth of field. The further away the lens is focused, the deeper the depth of field. This is how I can take both types of images without changing my aperture, my lens width, or any other camera setting.
In the following diagram, the blue range is the depth of field. As we focus further from the camera, the depth of field increases exponentially.