Sometimes you may need to blur out the background of your images. Let’s say you are shooting portraiture outdoors and there is a cluttered background which you don’t want to incorporate into the final image. Well you could possibly move the subject away from that spot and place her somewhere with a nicer background. But, sometimes, moving the subject may not be feasible as the lighting may change or the location being a bit cluttered overall may not allow you too much flexibility, like in the image below. So what do you do in a situation like this? There is an easier and more practical method.
Blurring the background.
As you can imagine, background blurring is primarily used to isolate the subject from the background, either as a necessity, as has been described above, or sometimes because of aesthetic reasons. There are other uses as well such as to control the depth of field. You can blur the background of your compositions using one of the following methods, or combinations of them:
Using a Small F-Number
Background blurring can be achieved through a number of ways. The easiest and the most widely used method is by using a small f-number (big aperture). Wider the aperture that you use, smaller the area in focus there’ll be and thus more of the background blur. Actually anything beyond and up to the point of focus will be blurred in various degrees. As you progressively move from smaller to bigger apertures you will notice more and more of the image is going to be out of focus. In other words except for the small area where you are focusing and the immediate vicinity, everything else is out of focus, and thus blurred.
Kit lenses such as the 18-55mm opens up only to f/3.5. But that is only at its widest end, which is unsuitable for portrait photography. You can try to buy lenses that have a slightly wider maximum aperture. This is why manufacturers produce dedicated portrait lenses such as the Nikkor 135mm f/2 DC. It has just about the perfect focal length for full-frame cameras to shoot portraits with, a wide aperture of f/2, plus the defocus control feature which allows you to selectively control the degree of background blur.
Keeping sufficient distance between the subject and the background can help
Sometimes we are guilty of framing an image with no space between the subject and the background. With no space between them both the subject and the background are almost on the same plane of focus. If you are shooting with a long lens, and that means shooting from a distance to keep the subject properly framed, you are going to notice this problem a lot more than when shooting with a smaller lens. Simply ask your subject to take a few steps forward and retake the shot. Better still, ask him to stand at a place where the background is further away.
Using a longer lens
Notwithstanding what has been mentioned in the previous paragraph, longer lenses can help you to get a lot of background blur provided you are not placing the subject and the background close to each other and you are framing tightly.
If you are using a zoom lens try zooming in and framing the subject tightly. But before that take a few steps back, if necessary, to ensure that your framing is correct. It helps if your lens has at least 2x zoom. Smaller Point & Shoots have an incredible advantage here as they offer a fantastic zoom range to work in.
Smaller sensors vs larger sensors
For the same lens a larger sensor will offer slightly more blur than a smaller sensor. A larger sensor captures more of the frame compared to a smaller sensor. Thus with the same lens you will need to step back on a smaller sensor camera to ensure that you retain the same composition. When you do that you push your subject closer to the background because in effect the ratio of distance from the camera to the subject and the subject to the background becomes smaller. This reduces the background blur (and also the depth of field).