There is a common misnomer that image stacking and focus stacking refer to the same technique. Image stacking is the process of combining multiple images of the same frame together for varied effects. It is used for making HDRs, photographing star trails and even reducing noise in an image. Focus stacking, however, is just another application of this principle.
So, What is Focus Stacking?
Focus stacking involves taking multiple images while keeping exposure settings constant. In each image, the photographer focuses on different points in the scene. These images are later combined in postprocessing to get a single image with significantly greater DOF. The most common application of the technique is either while shooting in low light when you are forced to shoot wide open or when you need an extremely narrow aperture but don’t want the image affected by diffraction, like in macro photography.
What to Keep in Mind
Choose the exposure settings you want and then switch to the Manual mode so that the settings don’t get affected. Use a tripod or a sturdy surface with a remote trigger so that the composition remains the same across the frame. Unless you need to shoot wide open for low-light photos, shoot at the sweet spot of your lens. It will help you get the sharpest images. If there is a fast-moving subject that you need in the frame, shoot that first. Once the subject has left the frame you can concentrate on shooting stationary objects.