How To Blur Your Background in Portraits

One common & helpful technique to apply to portrait shots is to "downplay" the background so as to make your subject stand out. This is especially desirable if the background is not aesthetically pleasing and is distracting, but it can also be used even if the background is fine but you still wish to bring to bring emphasis to your subject. The "bokeh" which results can be very pleasing and really add a pleasing aesthetic to your portraits.

A portrait without a blurry background. Shot at f/8,
18-55mm lens at 45mm, with considerable distance from the subject

This is very easily done if you're using a larger-sensored camera such as a DSLR, especially a "full-frame" one, but can still be easily done with a "cropped" sensor camera, such as my Nikon D5100. (It's more difficult to do with cameras with sensors smaller than this.)

The trick is understanding depth-of-field. Depth-of-field refers to how much of your photograph, front-to-back, is in focus. If you have shallow depth-of-field, your subject will be sharp, but your background blurry--in extreme cases, the depth-of-field may be so shallow that your subject's eyes (in the case of portraits) will be sharp but their nose may not be. On the other hand, if you have deep depth-of-field, then most everything front-to-back will be in sharp (or acceptable) focus.

Depth-of-field is affected by many factors. The 3 main ones I will elaborate on in this article: distance between photographer & object focused on, focal length of lens, and f-stop setting.

With the first: the closer you are to your subject (assuming that's what you focus on), the more shallow the depth-of-field is. On the other hand, if you put some distance between you & the subject your lens is focused on, there is more depth-of-field.

With the second: lenses with shorter focal lengths (say, 18-24mm) have more depth-of-field than lenses, say, with a longer focal length of 135mm or more, say. Lastly, and the one most easily & perhaps most commonly varied, especially at the time of the shoot: assuming all else is equal, a large f-stop of, say, f/2.5 will provide very little depth-of-field, whereas a smaller f-stop of, say, f/11, will provide much more. (On the other hand, even at f/11, if your subject is close & you are using a longer-focal length lens, depth-of-field will still be shallow.)

A portrait with a blurry background. Shot at f/2.5, closer
to subject, with a 50mm f/1.8 lens

To achieve a portrait with a blurry background, with what I call the "bokeh effect," photographers will typically get rather close to their subject, often-times enough to only show the head & shoulders, use a lens with a focal length of 50mm or longer (50mm is especially common because "primes" with larger maximum f-stops are readily available, compact, and less expensive), and a "wide" (or large) f-stop of, say, f/2.8 or so. 

Typical "kit" lenses found on DSLRs, commonly 18-55mm, don't work well because their maximum f-stop is only f/5.6 at the 55mm setting, not usually wide (or large) enough to achieve the desired effect. On the other hand, the "supplemental" 55-200mm lenses that may also be included can achieve this, if you zoom in around 135mm or so and shoot at the maximum f/5.6 that's common for such lenses.

However, typically, a photographer will use "prime" (non-zoom) lenses such as the 50mm f/1.8 (or f/1.4), 85mm f/1.8 (or, again, f/1.4), or they will use "constant aperture" zoom lenses such as a 70-200m f/2.8 or 50-135mm f/2.8. Of these, by far the most inexpensive (and typically also compact in size) tends to be the 50mm 1.8, which is why many photographers use that lens.

The effect can be varied, too. The 2 photos shown illustrate somewhat extreme variances of depth-of-field, but an effect somewhere in-between these 2 extremes can also be achieved. This may work if the background is not necessarily distracting or tacky-looking, but you nonetheless wish to "downplay" it just a bit, versus throwing it completely out-of-focus. Varying your length from the subject you're focusing or, perhaps even better, the f-stop you're using, can do this. If you're shooting at, say, f/2.5, try shooting around (say) f/4.5 or so and see how this works.

One tip: be careful not to overdo this effect. Overdone, it can render parts of your subject blurry you don't intend to. For instance, you may render sharp focus on a subject's eyes, only to have their hair or nose etc be blurry rather than sharply rendered. Experiment with the lens' focal length (if you have that option), distance from subject you're focusing on, and (most of all) your lens' f-stop to achieve the effect you desire.