Auto focus cameras dominate the world of photography. Every camera type from point-and-shoot to the advanced digital single-lens-reflex (DSLR) makes use of auto focus lens technology. Is it possible to still focus a lens the old-fashioned manual way? Yes, the possibility still exists. Essentially all 35mm camera lenses offer manual focus options. Today's advanced features in auto focus technology like selective and multizone focus came about because of the way manual focus technicians worked many years ago.
The lens
The technology lies in the lens, not in the camera. Manual focus of a lens, either an auto focus lens or a manual focus lens, relies on the lens' ability to move its internal glass elements forward and backward until the path of light directed through the elements recreates the original scene as accurately as possible. Lenses come in various shapes and forms, but for 35mm cameras they all have the following elements: focusing ring, zoom ring (on zoom lenses) focal length, zoom range, aperture and distance indicators and lens mount. The lens works in tandem with the camera's sensors or, in the case of much older lenses, switches, to allow the user to accurately focus an image.
In practice
Mount the lens on the camera body and look through the camera's viewfinder. 35mm cameras have a mirror and focusing screen inside the body just behind the lens. The mirror reflects the image up through the viewfinder while the focusing screen acts as an aid to refine image focus. The focusing screen typically has an additional element to aid with critical focus sometimes called a split image. A split-image screen has a circle split in two parts in the center of the viewfinder. For an out-of-focus subject, the split image does not align. Turning the lens barrel until the two semi-circles align into a circle achieves sharp focus. If the viewfinder does not use a split image, then turning the lens barrel makes the image slowly come into focus against the matte surface of the focusing screen. Turn the lens barrel counterclockwise for images farther away and clockwise for images closer to the camera.
Some cameras, like the Nikon N90s, have additional focus confirmation LEDs in the camera. Used in manual focus mode, these LEDs light up to inform the user which direction to turn the lens to achieve sharp focus. When accurately focused, a green LED lights up as confirmation.
Tips
A few tips on manual focus lenses will help you achieve consistently good results. When photographing people, the eyes become the focal point. If needed, focus on the eyes, recompose the image and then shoot. When shooting elements with graphic structures (trees, buildings, bricks, etc.) try to use vertical lines within your composition to aid with focus. Telephoto lenses have a higher susceptibility to bad focus techniques than wide-angle lenses; therefore, use a tripod with telephoto lenses whenever possible. Unlike single-touch zoom lenses that use a single ring for focus and zoom action, two-touch zooms use separate rings for each action, making them slightly slower in actual use.
Tags: focus lens, manual focus, focusing screen, auto focus, lens barrel, split image, 35mm cameras
