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What Is Focus in Photography?
In any photograph, there is one two-dimensional plane in the foreground, middleground, or background where the picture is the sharpest. This is the plane of focus, and it sits parallel to the camera’s sensor). By adjusting the focus on your camera either manually or automatically, you can change where this plane of focus falls. Adjusting the focus can lead to sharper images, or, if handled in a specific way, create depth of field and emphasize just one part of an image while blurring everything behind it.
What Is Depth of Field?
The plane of focus is where a picture is at its sharpest, but it’s not the only part of a photograph a viewer perceives as in focus. Objects near the plane of focus will appear equally sharp if they lie within a certain range—this range is called the depth of field.
You can opt for a shallow depth of field, in which less of the image is in focus (like a portrait with a blurred background or bokeh), or a deeper depth of field, in which more of the image is in focus (like a landscape). You can control the depth of field by adjusting the aperture—the part of the camera that controls how much light enters the camera and hits the sensor. The diameter of the aperture is adjusted by changing the f-stop, f/number, or focal ratio. The smaller the f/number the wider the aperture, the shallower the depth of field. The larger the f/number, the narrower the aperture, the deeper the depth of field. A wider aperture equals a large f-stop and a deep depth of field.