Following megapixels, which every modern camera is equipped with in abundance nowadays, background blur, also known as bokeh, is currently on everyone’s lips. People talk of shallow depth of field (DoF) and of how large aperture resembles engine size/cubic capacity – in other words something you can never get enough of – and how you need to switch to 35mm if you want to be part of the cool gang. Undeniably, a large aperture and an oversized sensor won’t hurt, but the knowledge of the factors which particularly favour background blur shouldn’t be underestimated either. It plays as much a part, if not more, in contributing to good bokeh.
Focal length
The longer the focal length, the easier it is to blur the background. When using a zoom lens, one should zoom in, even if most lenses will automatically stop down at least one aperture. The loss of a stop in terms of light gathering capability is compensated by means of the shallower DoF of a longer focal length. Additionally, even though this largely pertains to prime lenses, tele-lenses are easier to adjust (from a design point-of-view) and distort space to a lesser extent or even not at all. This might be especially important in the realm of portrait photography, where wrong proportions (nose too big, ears too small…) instantly catch one’s eye.
Aperture
One can buy a fast lens, but one isn’t obliged to do so. Even slower ones allow for good bokeh. To do so, aperture needs to be opened and attention needs to be paid to distance to the motif, the background as well as its structure and illumination. If photographing with a fast lens, one should pay attention that the aperture is not opened too far. Bokeh should be used, just as any other effect in the photographic bag of tricks, to enhance the picture. It should not be used as an end in itself. In times of portraits where nothing in the picture is sharp but one eye, I think, this should be duly emphasised. I have nothing against highly dosed bokeh, but the picture should be left with a certain artistic intention, which is, in my humble opinion, too scarcely the case.
Distance to the motif
The shorter the distance between camera and motif, the blurrier the background is portrayed. As a matter of fact, one should not shy away from moving closer to the motif, while simultaneously keeping the composition in mind. Parts of the motif (the head, for instance) should not be cropped randomly. This step should be greatly considered as the viewer needs to be able to relate to it.
Distance to the background
Other things being equal, the background is displayed the blurrier, the farther away it is from the camera and the motif. If the distance doesn’t suffice to let a disturbing background disappear in bokeh with the given aperture, focal length and distance between camera and motif, one should, if possible, further clear away from the background. Internalising this thought, one soon realises that all of a sudden, creamy bokehs with medium fast lenses are possible, where one has previously been wishing for light artists of the f1.4 kind.
Structure of the background
Background is not background. It is much easier to blur a homogenous background such as a smooth wall, than one with chaotic, contrasty details such as shrubs and branches. Here, also, one soon comes to realise that once starting to deal with this topic, good results can be achieved with comparably moderate technical means.
Light quantity in the background
The significance of bokeh usually lies in turning the viewer’s attention to the motif. To a certain extent, the information in the background is destroyed, in order that one is less easily inclined to avert one’s eyes from the motif. This only succeeds, however, if the bokeh is indeed inconspicuous, and huge overexposed patches are all but inconspicuous. For this reason, one has to take heed of the fact that the background, in the worst case, may be just as light as the motif. Ideally, however, darker by a 1/3 to 2/3 stop.
All of these are no rules that need to be 100% complied with. But as the saying goes, one has to know the rules in order to break them. 
