Q - How and where should I focus when I'm taking a picture of my family or friends where I want both the models in the foreground and the scene in the background to be sharp?
A - The trick is to have the people in the foreground close enough so you can tell who they are and the background view shown fully so you can tell what it looks like. First compose your background scene in the viewfinder as though it were a separate picture. Find a distance and angle from which it look best.
Next, introduce your models into the scene by posing them in front so they're not obscuring the view, but are big enough so they're recognizable.
Usually the background scene will be a monument or a mountain or a building, which will rise over the heads of your models. If this is not the case, then simply find a higher viewpoint for your camera by stepping up on steps, a wall or a rise of ground, so the lens looks over the top of their heads.
To focus so the people in front and the background are sharp, use your depth-of-field focusing scale rather than drawing a bead on the people or the back view. This is simply a matter of using the built-in sharpness of your lens by setting your far-distance scale (this is the marking on your lens between the distance numbers and the f/stop indications) on infinity, and checking the near end of the scale to see how close you are in focus.
When you look through the lens on an SLR camera with this setting, you'll notice that nothing seems sharp: Neither the people in front nor the scene in back. Don't worry about it. The entire picture will be sharp if you ask your models to stand just a little farther back than the near-sharp distance shown on your depth-of-field scale when you have the far distance set on infinity.
The depth of sharpness will vary not only with the f/stop you use but also with the focal length of the lens. The wider the angle of the lens, the more depth you will have.
Here's an example of how this works with three different focal-length lenses.
If your scene is in bright sunlight and you're using an ASA 64 outdoor color film, the exposure would be f/11 at 1/125th of a second.
On a 50-mm lens, the depth would be from about 15 feet to infinity; with a 35-mm (moderate wide-angle) it would increase to six feet to infinity, and with a 24-mm you would be sharp from three feet to the horizon.