What is "depth of field"?
The area of a photograph's frame that is in focus (desribed as being from a minimum distance to a maximum distance from the camera).
"Instamatic" and disposable type cameras often claim to have "auto focus". What they usually have is a very large depth of field. They do this by having a small aperture. This is often why they state that they work best outdoors in sunny conditions or with the flash switched on (if equipped).
More to come....
Also, you will need to be accurate with your focus. If your camera does not have auto-focus you will need to precisely measure and/or estimate the distance from the film (not the lens) to the item you want in focus and set the focus ring on your camera accordingly.