Scale focusing refers to focusing which is done utilizing the distance scale and DOF markings on a lens (or distance scale and a printed DOF table, etc.) instead of using a split prism, AF sensor, or rangefinder to focus on something.
Types of photography where scale focus can be an especially valuable technique:
- Street photography, where scale focusing in advance lets you take the shot without delay
- Long-exposure night photography, where it may be extremely difficult to focus using other methods because of low light
- Landscape and architecture photography, where the goal is to have all or most of the image in focus, rather than having any one particular point in focus
- Infrared photography (esp. w/unconverted camera), where (a) you can't see what you're shooting through the filter, and (b) you have to account for the difference between the point at which IR light focuses and visible light focuses
Remember, scale focusing ahead of time is always faster than autofocus or manually focusing on the subject.
When scale focusing a lens with DOF markings, locate the DOF markers for the aperture at which you are shooting. With older lenses, these markings are color-coded, which makes it easier. The markers will match up to the feet/meter markings on the focus ring, letting you see that the DOF should extend from, say, 5' to 20'. Or, if you position the infinity marker at the farther DOF marker, you'll know that everything from the near marker to infinity will be in focus. (This is shooting at the hyperfocal distance.)
When shooting with a film/FX lens on a DX camera, use the markings for an aperture one stop wider than the one you're shooting at. (If shooting at f/11, use the markings for f/8.)
If your lens has a distance marker, but no DOF markings, you can print a DOF table out:
If your lens has no distance scale, buy one that does. The old 28mm f/3.5 lenses are cheap and good for scale shooting.
More info on hyperfocal distance: