Depth of field is the fancy way of talking about how much we see in focus.
Just remember, the biggest control on the camera for affecting depth of field is your aperture. The more open your aperture, the more shallow your depth of field.
The subject’s distance from the background will help your background look more blurry, but it’s not affecting the depth of field per se; it’s just creating distance to take advantage of the current depth of field. The most common question I’ll hear, is “Wait a minute, someone said that large sensors have more shallow depth of field. Is that not so?” And the answer, technically speaking is: no, a larger sensor does not reduce depth of field. So why do two shots appear different when the DOF is the same between a MFT sensor and a full-frame sensor? To get your subject to fill the frame in the exact same way on a MFT vs a full frame camera, your camera-to-subject distance is going to differ, which does affect the depth of field. You’ll get closer to the subject at wider focal lengths. Remember, subject proximity is one of the factors mentioned above. It get’s more shallow as you get closer. Also, remember the perspective distortion thing discussed earlier? The out-of-focus background is larger relative to the foreground, so it gives the illusion of being blurrier when it’s really that the blur is just more obvious. So the more zoomed in you are the more shallow your depth of field appears to be because the out of focus background is actually larger relative to the subject.
This is the best video I’ve seen on the only two things that truly affect your depth of field: Aperture (or “entrance pupil” to be precise) and distance from camera to subject. That said, for beginners I wouldn’t even worry about it. You don’t care so much about the ‘technical’ as the ‘aesthetic’ reality of depth of field: in other words, you want the appearance of a blurry background and maximum subject separation.
Remember this: To get a blurry background, open your aperture all the way, get your subject as far away from their background as possible, and get the camera as close to the subject as possible. Use a long zoom lens or crop in during post.
There are some other awesome tools and techniques available to help us use depth of field creatively.
Click the image below for a “Depth of Field Simulator”: an interactive tool from dofsimulator.net that will teach you more about the practical application than I can.
These are available in a number of apps and will show you exactly how much of your shot is in ‘acceptable’ focus.
This is the distance you can focus at to get everything in focus. It’s especially useful in landscape photography. If you focus at the hyperfocal, the nearest point of acceptable focus will be ½ the hyperfocal distance. You usually have ⅓ in front of focus point acceptable and ⅔ behind it acceptable–that’s what makes up your DOF?
Photo Pills has a great depth of field and hyperfocal distance calculator.