Learn some of the basics of how depth of field works and how you can work it.
Getting Into Depth of Field
Depth of field can be somewhat vague for those new to digital video production. The definition of depth of field really relates to an acceptable area of clarity that is being captured by the film or video camera. Over all the depth of field is a measure of distance, both of area and of the distance from the camera to the object.
Video Image Definition
Depth of field really determines where an image gets proper definition. This is where the lens gets the right focus for a distance that has been set for the object in question. When you are trying to estimate what the depth of field is it is simply just the depth or area where the object retains its definition. This is going to be different for every situation, object, lighting situation, camera, and lens. You cannot estimate the depth of field ahead of time for any given situation.
Lens Aperture Diameter
Usually the depth of field is in direct subservience to the lens aperture diameter. When you reduce the aperture diameter you can usually extend the depth of field, while at the same time reducing light. At the same time you can usually affect the depth of field with lens type and how you calibrated the zoom.
Calibrate Zoom
The practicum of the depth of field is a lot more simple than the details lead it to sound. The reality is that you are going to want to find the image that looks best for you. If you are using a digital video camera you will have an LCD display, or be using a monitor, so you can see what the image looks like.
Here you can begin to use the different digital mechanisms to get the image to the place you want. You can adjust the camera position in concert with the digital video camera settings so that all aspects of the image end up being in focus. If you want certain things to remain out of focus, such as the background, you can adjust those things to shorten the depth of field and render some objects soft. All aspects of this are adjustable.
The best way to really get your depth of field to the position you want is to calibrate zoom to infinite zoom by zooming in to the farthest object you want then focusing the image to perfect definition. Then zoom out to the position you want and everything from the base point to the shooting spot will be in focus. That will be your constructed depth of field.