Depth of Field Blur
Blurring of images in the background and foreground is both an artefact of lenses and an artistic technique. It is not based on any real atmospheric effects. Camera lenses (and the lens in our eyes) all have some focal length where the image focus is sharp. As you move away from the focal length (both closer and further away) the lens will progressively blur the image. The performance of the lens will also vary depending on the wavelength (color) of the light which can lead to color fringing in the out-of-focus areas.
Until recently, macro lenses used for close-up shots of small objects have had a very narrow focal length. Viewers have been trained to associate large amounts of out of focus blurring with macro shots. This is not to be confused with light-scattering which gives rise to distance hazing. Lenses with extremely wide depth of field have become more popular recently. However, the images they produce are often viewed as “trick photography” and can be confusing if there are no other strong depth cues.
Deliberate blurring (sometimes extreme) of the foreground and background is a technique that both photographers and artists often employ to draw attention to the main subject. Combined with selective lighting this can result in images that are characterized by terms like: warm, romantic, or atmospheric.