In order to get the deepest focus possible, you need to make sure that your pictures have the foreground, mid-ground and backgrounds clear and concise, through knowledgeable use of the camera lens you are using and allowing a large amount of light using a small aperture. It is the aperture of the lens that determines the depth of field and is part of being able to get the deepest focus possible.
This means in order to get the deepest focus possible, you need a smaller aperture. Depending on your depth of field, you will want to consider using the smallest f/stop possible with your lens in order to get the clearest focus, although small apertures up close can also take some sharpness and focus away, due to the slower shutter speed.
Another thing to be aware of is that small apertures can cause diffraction and it doesn?t take much movement of the camera during exposure to create deep focus problems when photographing a small area or object, especially one that is up close.
The less depth of field available might dictate a few adjustments to get the deepest focus possible. Using a higher ISO setting can make a difference in giving more sharpness when it comes to shutter speed. Also, using a tripod helps when photographing a non-moving subject or indoors. If the wind is blowing outdoors, it may not help because your subject may be in and out of focus.
Another way to get the deepest focus possible is to shoot multiple shots with the continuous shooting setting. Chances are that one of the shots will be better than the others, which will be more out of focus due to the slower shutter speed.
Another thing to keep in mind when you want to get the deepest focus possible is to watch the plane of focus. The plane of focus is parallel to the back of your camera typically. By tilting your camera to be parallel with the main surface of your subject, it can give the appearance of deep focus when the depth of field is shallow.
You can also try using a flash, which can help freeze the camera movement or use a tilt-shift lens to move the plane of focus, which can have some different effects by moving them away from the planes of the subject.
You may also be able to get the deepest focus possible by using the mid-range f/stops, such as f/8 or f/11, if you are shooting close-ups, to allow it to focus at macro distances, when using extension tubes, especially. The best way to get the deepest focus possible is to experiment with the f/stop positions, because one may seem blurry and the next setting will be clear and concise.
On landscape pictures, you may need to set the f/stops at f/16 or f/22 to get the deepest focus possible and to get a sharper picture. Balancing a wide depth of focus with the best f/stop positions can take a little practice. It is possible to get the deepest focus possible once you learn to apply the correct settings for the light and subject matter you are photographing.