First a bit about Depth of Field
To answer the question of where to focus, we must first understand Depth of Field, or DOF. A quick overview of DOF: the aperture setting affects how much of the scene will be in focus. A common misconception (held surprisingly often by clients on our workshops) is that some apertures are used for close focusing and others are used for focusing on distant objects. This is incorrect! Any aperture can be used to focus at any distance. Instead, what the aperture affects is how ‘deep’ the focus is.
Essentially, a wider aperture (smaller f number, such as f/2) will result in a shallow DOF. A narrower aperture (bigger f number, such as f/16) will result in a deeper DOF. Keep in mind that ‘deeper’ does not mean farther ‘away from the camera.’ Rather, it means farther away from the point of focus.
The second thing that affects DOF is the focus distance that the lens is set to. For example, if I focus on an object 30 centimeters in front of the camera, the DOF might be from 28 centimeters to 36 centimeters. This means the focus area is only about 8 centimeters in depth. However, if I focus on an object 5 meters away, the DOF might be between 2 meters and 8 meters, resulting in a DOF of 6 meters. Keep in mind, this is regardless of the aperture setting—the closer you focus, the narrower the DOF will be.
Of course, these two effects operate simultaneously. Meaning that if we focus close and use a wide aperture then then the DOF will be razor thin—only a few centimeters. However, if we focus far away and set a narrow aperture, then the DOF might be many tens of meters in depth. For a landscape photograph (urban or otherwise), this second circumstance is usually ideal!
So, with all that, we can deduce where to focus if we want to maximize DOF for urban landscapes—on an object rather far from the camera. However, there is a bit more to it, and for that we should think about how the camera focuses.