In a scenery photograph, or a picture of any subject matter on more than one plane, we are representing three-dimensional space in a two-dimensional image. In order to reproduce the feeling of depth as we saw it in the original scene - depth perspective as photo people call it - we need a strong foreground object. It makes no difference what the foreground object is. It might be a tree or a bush, a rock or a person, a child's toy or a puppy dog, a car or a boat; it might be the subject of the photograph, or anything that just happens to be there. The important thing is, the closer the foreground, the greater the feeling of depth.
We sometimes take informal portraits, or photos of individual subjects in which the background or foreground is incidental. If nothing but the subject is sharp, that's okay; if everything is in focus, that's fine too. But in scenery and landscape photography, maximum sharpness is important in every part of the scene, from the nearest object to the distant horizon. In order to get maximum overall sharpness, we want to select the point of focus very carefully.
The term depth-of-field refers to the distance between the nearest and farthest objects that are in sharp focus in a photograph. Typically, one-third of the total depth-of-field is in front of the point of focus, and two-thirds is behind the focus point. When taking a scenery shot, many of us point the camera at the scene and shoot. This usually results in the camera focusing on infinity. but this wastes a lot of depth perspective. Think about it: what's beyond infinity? More infinity. So if we focus on infinity, we lose one-third of the potential depth-of-field.
There are several ways of increasing depth-of-field in a photograph. One way is to use the wide-angle lens. Another option is to use a small aperture. If you use an SLR, or any 35mm or APS camera that has aperture controls, you could select f/11 or f/16 for maximum depth-of-field. Digital cameras, however, require different aperture mechanisms than 35mm or APS cameras. Few of them have apertures smaller than f/8. It would seem that this would reduce the chances for getting adequate depth-of-field, but that's not the case. Because of other factors in digital cameras, f/8 would provide great depth-of-field.
The use of a wide angle lens or a small aperture are great techniques when you can use them, but there are situations in which you must use other means of getting the optimum depth-of-field, such as when photographing a large group of people, or when shooting from the sidelines of a night or indoor sporting event. With any camera - compact or SLR - you can increase depth-of-field by focusing about one-third of the way into the scene. The best way of doing this is to select the near point and far point that you want to be sharp, then focus approximately one-third of the distance between the two points. Remember to lock in the focus before recomposing the shot. Some sophisticated SLRs will do the work for you. All you do is set the camera on the depth-of-field mode, dial in the near distance that you want to be sharp, dial in the far distance, then compose the image and shoot.
Back to scenery photography in which the furthest point is infinity. If you own an SLR but are not blessed with the above feature, you can use a simple formula (even simpler with a hand-held calculator) to determine the Optimum Focus Distance. The OFD is the focus distance that will give the greatest range of sharpness in a scenery shot.
To find the OFD, compose the shot as desired, then square the select lens Focal Length (FL) and divide by 100; that would be (FL X FL)/100; round up to the next whole number. Multiply the result by 11 and divide by the selected f/stop (if f/11 is used, forget the last two operations). The result is the OFD. Set the lens on this distance.
On most modern SLRs, determining the exact distance on the distance scale is not easy, so just estimate. But if in doubt, it's better to estimate slightly toward the far distance.
It doesn't matter how you do it. The important thing is to make sharp in a picture everything that's important in the scene.