When we are talking about the true meaning of depth of field there is a need to first understand what focus is. Focus is at the heart of the depth of field because it is the purpose of this field. The depth of field is really the area in which an object can remain in focus. This area is measured in terms of distance in relation to the camera that is capturing the image. To really understand how to use the depth of field to your advantage you have to first understand exactly what focus is in terms of digital video.
An object is in focus when it has acceptable sharpness. The over all image sharpness that determines the focus can be affected by a number of different elements. These things can include lens type, camera specifics, the overall resolution, and other factors. For example, it can be estimated that High Definition cameras actually have a smaller depth of field because the image itself is actually being produced at a very high resolution. Standard Definition cameras actually get nothing in a comparable manor of clarity or sharpness, so the depth of field for an acceptable Standard Definition video camera is actually longer.
There are ways to help ensure that an object is in focus, or at least to head in that direction. A general rule seems to be that the more an object is magnified the less "in focus" it will end up appearing. This is why digital zoom is not a good measure of a digital camera's abilities, mainly because it just amplifies the image.
Optical zoom, on the other hand, actually changes the focal length. It is also true that you should try to get the highest resolution digital video camera as possible to help ensure the focus of an object. If you use any corrective eye wear it is important that you be wearing them when setting up your depth of field, otherwise you really will have no idea how the image is turning out.
It is best to check both the LCD display and the viewfinder, but if you have the option to plug into an actual monitor this is going to be the best way to check. Always make sure that you zoom into the farthest possible image past the main object and then calibrate the focus, This will make everything else in front of that distant object in focus. This should always be done at each set up.