Focusing system

Principles

Focusing

Focusing means setting the distance of the subject to have a sharp picture.

The simpler cameras (for example disposable cameras) have no focus setting at all: this is fixed focusing.

On most cameras, though, you focus by setting the distance on a ring around the lens. How do you know that distance, and how will you focus the image? On the simpler old cameras, you had no indication and had to guess it: this is scale focusing or guess focusing. The more advanced cameras had some kind of focusing help, the main types are the ground glass back, the rangefinder and the reflex finder. (Chronologically they appeared in that order.)

From the beginning of the 1980s the cameras began to incorporate an auto focusing mechanism: the camera determines the distance to the subject automatically. There are two main types of autofocus: active autofocus, where an infrared light is emitted by the camera and reflected by the subject, and passive autofocus, where the image transmitted by the lens is analysed for its sharpness by a detector inside the camera. Active autofocus is used by the simpler point and shoot cameras, and passive autofocus is used by the autofocus SLR cameras.

The different focusing systems are detailed below.

Framing

You want to know with enough precision what will appear on your picture. This is the role of the viewfinder. There are different kinds of viewfinder:

• Frame finders: No longer in use. This was the simpler design, with no optics at all, consisting merely of framing devices that the user looks through to frame the scene they wanted to take. These types of viewers were present even on cameras which had conventional viewfinders, such as press cameras; they were called "sportsfinders" since they could be used to quickly frame action on a field. Typically, such a finder had two parts: a large wire frame that extended above or to the side of the camera, and a smaller frame that the user put their eye up to.

• Simple optical viewfinder: This is used on cameras with no zoom lens. It's basically a tiny telescope that you peer through at the scene. It only helps you to frame the image as you want. This is the type found on most low cost cameras.

• Zoom optical viewfinder: Similar to the above, but with a zoom feature, so that the view through the viewfinder changes as you zoom the lens. This is used on low cost cameras with a zoom lens and autofocus.

• Optical viewfinder with rangefinder: This is the combination of a simple optical viewfinder and a rangefinder. See rangefinder below.

• Bright finder: No longer in use. This was a small finder with a mirror inside. You looked through it from above, with the camera held on your chest or belly. The image was inverted left to right, and it was not really comfortable. It was used on inexpensive cameras until the 1950s.

• Reflex finder: See SLR and TLR below.

Fixed focusing

A camera with fixed focusing has no focus control. The focus is preset once and for all, at a distance for which sharpness will be correct enough for most subjects, usually from 1 meter (3 feet) to the infinity (far away). These cameras have a lens with a very small aperture (f/5.6 or f/8 for example). If they had a larger aperture they could not be sharp for both medium and long distance.

Scale and guess focusing

Guess focusing was the most common way of focusing during many years. You had to estimate the distance to the subject yourself, and set the focusing ring or focusing lever accordingly. You were somewhat assisted by the depth of field scale. Usually the lens was fixed and its aperture was not too big, typically f/2.8 as a maximum. It is not possible to guess the distance with enough precision for a larger aperture.

These cameras were usually more expensive than the fixed focusing ones, but cheaper than the rangefinder or reflex type.

A variant was scale focusing, used on cheaper cameras, targeted to people who would not take the trouble to guess a distance in meters or feet. Symbols on the focusing ring, (usually "one person", "a group of people" and "a mountain") indicated the most probable setting for your picture depending on the subject.

This range of camera has evolved to become the point and shoot cameras, equipped with an autofocus device, consisting of an infrared light emitted by the camera and reflected by the subject (note it might differ on the digital ones?). You do not have to guess the distance yourself, and of course the operation is faster.

Rangefinder focusing

The rangefinder is an optical device to set the distance. There are two superimposed images (superimposed image rangefinder), or one image split in two parts (split image rangefinder). As you turn the focus control one of them is moving, left and right if the camera is hold horizontally, up and down if it is hold vertically. The focus is set once the two images coincide.

Usually the rangefinder is combined with the viewfinder, the moving image appearing as a central bright patch. So you can focus and frame at the same time. On some older cameras it was independent from the viewfinder, for example on the Leica screw cameras. You had to focus first by looking through the rangefinder, then look through the viewfinder to frame your picture.

Only one digital camera so far uses this method: the Epson RD-1. Many famous film cameras use it however, most notably the Leica M series. There are many rangefinder cameras available on the used market, and these can be a very cheap yet high quality first camera.

(See rangefinder for more info)

Reflex focusing

SLR

SLR means Single Lens Reflex. Inside the camera is a mirror that reflects the image onto a focusing screen. You view this focusing screen through the viewfinder. This means that you are seeing pretty much exactly what the camera will capture. Note that in most SLRs, there will be some cropping, meaning that the camera will capture slightly more than what shows in the viewfinder. This is usually expressed as a percentage, e.g. 95% meaning that the viewfinder shows 95% of what the camera will capture. Some professional cameras have 100% viewfinders. (more info)

TLR

A TLR or Twin Lens Reflex camera uses two lenses, one for viewing and one for taking. The light passing through the viewing lens is refected on an internal mirror and projected onto the ground glass at the top for viewing, focusing, and composing. Due to the use of a single mirror, the viewed image is reversed. (more info)

Ground Glass focusing

Some cameras have no viewfinder per se, but instead use a piece of ground glass at the focal plane to focus the scene. This type of focusing and viewing system is typically used on so-called large-format cameras (cameras which use film 4x5 inches or larger). In such a camera, before the picture is taken, the ground-glass viewer is inserted in the back of the camera, and the lens opened to its widest aperture. This projects the scene on the ground glass upside down and backwards. The photographer focuses and composes using this projected image, sometimes with the aid of a magnifying glass or loupe. In order to see the image better, a dark cloth is used to block out light, hence the image of the old-time photographer with his head stuck under a large black cloth.