Depth of field

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Depth of field refers to how much of an image is in focus, specifically the distance between the nearest and farthest in-focus parts of an image. Images with a shallow depth of field exhibit a lot of out-of-focus area (blurred foreground and/or background) while images with a deep depth of field can have nearly everything in focus. Depth of field is related to both the lens' aperture and the size of the image sensor. Larger apertures (smaller f numbers) will leave less in focus - they give a shallow depth of field. Smaller apertures (larger f numbers) leave more of an image in focus - a deeper depth of field. Larger sensors also make it easier to blur a foreground and/or background at a given aperture, while a smaller sensor makes it easier to get everything in focus, at a given aperture.

For example on the image with leaves in the foreground all the background is blurred. I also took a couple of images of a table football and practised changing the aperture (f number) on it. This resulted in different players being in and out of focus.