Watch Photography 101: Part 2 – Depth of Field, Exposure, and Staging the Shot

Look at the photo below to see how f-stop impacts depth of field. From left to right, the f-stops are f/8, f/3.2, and f/1.4. At f/8, you can see that the opening is very small, and the resulting photo has a deeper depth of field with more of the background in focus. At f/1.4, the lens is wide open. Here the depth of field is much more shallow, with most of the background blurred.

One thing worth noting: as you go wider (again, a smaller f-stop number), you might encounter issues such as distortion, vignetting, and loss of detail sharpness.

There are a few other factors you should consider when playing with depth of field and focus. The first is subject-to-camera distance. If a subject is closer to the camera, you will get a more shallow depth of field. The second is the focal length of the lens itself. For example, a longer lens like a 200mm prime will achieve a shallower depth of field than a lens that is 50mm. And third, you have to consider the camera itself–or rather its sensor if you're shooting digital. Larger sensors are capable of achieving a much more shallow depth of field. For example, a f/2.0 setting on a small point-and-shoot or smartphone will not create as much background blur as a DSLR with a larger sensor will at f/2.8, despite the lower f-stop on the point-and-shoot.

Being able to manipulate depth of field is in such high-demand that smartphone companies have developed optical wizardry and software tweaks to simulate what is traditionally done through the lens. Admittedly, these high-tech gimmicks are very hit or miss, but I recommend playing with them if they're a feature on your phone or whatever camera app you use.

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Source: Watch Photography 101: Part 2 – Depth of Field, Exposure, and Staging the Shot