Depth of Field refers to the range of area in a picture that has things in sharp focus. For example, if you take a photo of your daughter in a field of flowers, you may want both her and the flowers behind her ALL IN SHARP FOCUS (wide depth of field).
OR You may want to take the same picture with HER IN FOCUS AND THE BACKGROUND BLURRY (narrow depth of field). Many people like this option for portraits because it makes the subject stand out, while minimizing a distracting background.
See my wonderful homemade diagram below. This is a bird's eye view, where the X is the camera position. The depth of field is the range of the picture that is in focus.
Ok, enough of the "technical stuff". How do you use this to your advantage?
If you want a narrow depth of field (with an out of focus and blurry background), ZOOM your lens all the way in (getting in close), and try for low light situations. If you have a camera with these options available, choose a small f-stop number. The smaller the f-stop (example f4), the narrower the depth of field. Set your camera to the lowest number ISO you can (like ISO 100). All of these things will open up your aperture - the size of the opening in your lens, which will narrow your depth of field. Remember to hold the camera steady!
If you want a wide depth of field (everything in sharp focus), ZOOM OUT, go for bright light situations, and choose a high f-stop (f11 - f22) number. A higher ISO (like ISO 400 or 800) will also help. This will make your aperture small, widening your depth of field.
Don't worry about the technical terms. If you are a novice and want to try this out, just pay attention to the tips I've written in red. :-)
This picture has a wide depth of field (not great pics but they make the point):
This picture has a narrow depth of field:
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Have a great day friends!