The backscatter is a resource of utmost importance in highlighting the main subject, or even to save a shot behind when inconvenient. Let c UATRE details that are part of the procedure required to do so.
Opening wide . One of the basic lessons in handling the exhibition is that the degree of opening of the shutter controls the depth of field. The more open you are, the lower portion of the picture comes out clear, and focused - that is, shallower the depth of field. So choose low values of openness, as f4 to f2 (smaller f-stop number, the greater openness) if you want a well-marked effect.
Distance from the subject . Approaching the subject is always good advice, but charges primary importance in these cases. For example, it is much easier to create a shallow depth of field 3 meters away from being subject to 15.
Optical Zoom . When you zoom in, compress the elements within the frame, so that everything may be out of focus behind. For example, a wide angle lens will not create very shallow depths of field, given that it is designed to capture details from afar, but if you zoom with a telephoto lens of 100 mm to 200 mm, the effect will be evident.
Sensor size . The sensor will interfere in the quality of the final outcome in many ways - make the background blur is just one of them. For example, it would be impossible to obtain a shallow depth of field with a mobile camera, its sensor is so small that the effect was not appreciated. In the case of a compact camera, the depth can be manipulated to some degree, but with difficulty. And finally, with a DSLR be truly appreciated by the results that your sensor is aa much larger. Sensor uses a full frame format and the results are amazing.
Consider these factors and experiences with each one of them or in terms of the other three, as they often interact to affect the outcome. Once mastered, surely you do use this versatile resource in more than one occasion.