Learning how to blur backgrounds is better said: Learning how to manage my “DOF – Depth of Field”.
Have a look at the picture of the text above. This image has a very shallow depth of field because only one line of text is in focus. There are three factors that contribute to this effect, and it’s important to know each one.
- Aperture – Recall that aperture is one of the three light management devices in our cameras, and that it refers to the adjustable opening inside our lenses. The size of the opening is expressed as “f-stop”, and the numbers start from a low of 1.8 and rise to usually about 22. The important thing to know and remember about these f-stop numbers for this tutorial is that the lower the numbers, the wider the opening, and the shallower the depth of field (hence the greater the amount of blurring both before and after the DOF).
- Lens Length – Lens length is expressed in millimeters, and is the way magnification is expressed in camera lenses. The rule to know in the context of blurring backgrounds is that the longer the lens, the shallower the DOF and the greater the amount of background blurring. Note that when we say lens length, we’re not talking about how long the actual lens body is, but how many millimeters of length has been designed into the lens. That is, the effective length of a zoom lens for instance, is the number you have the lens set to, not how long the lens actually is.
- Focus distance – Finally, the farther you are away from your subject when you zoom in on it, the greater the DOF compression, and therefore the greater the blurring effect you’ll realize. This is often convenient, as the farther away you are, the more you’re likely to zoom your lens to magnify the subject, and together they’re working toward the same end.
The bottom line then is, for the greatest DOF compression and the greatest background blurring, Choose the lowest aperture number, and the longest lens, at the greatest distance practical.
Parenthetically, often new camera owners are disappointed to find that they’re not able to blur backgrounds at all. Most kits include a very short zoom lens that tops at 55mm. Can you see what their problem is?