Today we are going to talk a little bit about how to get that blurry background you often see in portraits or lifestyle product images. That blurry background really makes the subject stand out, and it is one of the first things aspiring photographers are keen to find out how to do.
Image by Ria Mishaal FRPS; aperture: f/2.2, shutter speed: 1/200s, ISO: 500
The aspect that influences how much of your image is in focus (and so how blurry the background is) is the depth of field. Technically put, the depth of field in an image is the area of acceptable sharpness that appears in focus. It extends in front and behind your focal point (where you focus). If your depth of field is large, a lot of the scene in front of your lens (front to back) will be in focus; if the depth of field is small, then only a small amount will be in focus, giving you a blurry background and a sharply in-focus subject.
Let’s use an analogy to help: if you imagine the scene in front of you is made up of slices like a loaf of bread, the more of those slices you want in focus, the larger your depth of field needs to be.
How do we influence the depth of field in our image? By changing the aperture.
Aperture is the setting that will give you a lot of creative control because, as well as influencing the amount of light that reaches the sensor, it influences the area of an image that is in acceptable focus (the depth of field).
Aperture is the variable opening of the lens. By changing the aperture to be large (small f-number e.g. f/2.0), you will let a lot of light in (good if you are working in low light) and you will get a shallow depth of field. The opposite is true of a small aperture.
There are other things that influence the depth of field, like the focal length of your lens and your distance from your subject, but at the beginning, let’s concentrate on this: the smaller the f-number, the shallower your depth of field becomes (relative to a subject at a fixed distance when taken with the same lens).
Recap on how to get a blurry background by adjusting your f number:
Blurry background = small depth of field
Small depth of field = large aperture opening
Large aperture opening = small f number e.g. f2
Below we can see this clearly: note that the flowers at the back and the book are less in focus in a than in b.
a: aperture: f/2.0, shutter speed: 1/500s, ISO: 1250; Image by Ria Mishaal FRPS
b: aperture: f/8.0, shutter speed: 1/30s, ISO: 1250; Image by Ria Mishaal FRPS
Give it a go and please do let me know how you get on below!