Frame Within A Frame
You can use elements of a scene to provide an additional frame within the edges of the image. This could be a full frame, on all four sides of the image, or a partial frame, where only one, two or three sides of the image are affected.
Potential framing elements include doorways, foliage, tree trunks or even other people. Frames can either be an obvious element in their own right, or merely blurred parts of the image. Frames can be in the foreground or background of an image, although the former is more commonly seen (and easier to achieve) in portrait photography.
Frames are used as a compositional device because they can add interest, context, depth and balance to an image. For example, a shot of a subject in front of a plain background tells you very little about the place it was taken, and has only two elements – the subject and the negative space around them. In contrast, a shot of a subject framed by an ancient archway has added detail (interest), a greater sense of place (context), greater depth (particularly if the archway is nearer to or further from the camera than the subject) and increased visual balance (particularly if the archway is centrally positioned in the overall image).
When using frames in your image, consider what will complement the tone you want to convey. For example, photographing a cuddling couple through the leaves of nearby foliage gives the impression of an intimate moment captured, and the leaves and flowers in the foreground will add to the sense of romance.
You will also need to decide how much of the frame you want to be visible. This will depend on what is physically possible (sometimes the location will dictate), whether you want the frame to be a key part of the image or an edge detail and where the frame is best positioned in order to lead the eye towards the main subject of the image.
If you want the frame within a frame to be blurred, you will need to consider your aperture settings and also the distances between the foreground element and your subject. The wider the aperture and the further apart the frame is from your subject, the more blurred it will become.