Getting the depth setting right is important. When one has acquired the desired image, it is a good idea, to remove the distal parts of the image that are not necessary to maximise the image. Place the area of most interest within the middle of the screen. Start with maximal depth and then reduce it as much as you need.
The site of the depth button will vary, but all machines have one.
There is usually a depth measure on the side of the image that tells you what depth the image is at.