In photography, focus refers to the sharpest area of the image, and is often used in an artistic way to highlight a certain subject. A well focused image can lead viewer's attention to an intended motive almost like a story, whereas a poorly focused image can rarely be used at all. In optical terms, focus is the point where light rays from a single object point converge after passing through the lens. Focusing a lens at a certain object means to adjust the lens position so that the image is precisely formed at the image plane.
Every lens – either an individual lens element or a composite optical system – has a focal length. If an object A is infinitely far away from the lens, its light rays are focused at a single spot A' at the focal point F. Note that in photography, infinity is usually defined to be sufficiently far away so that incident rays of light are nearly parallel. The image A' is only a single spot due to the fact that no particular object features can be seen at this infinite distance. As soon as an image sensor is placed at that position, the image dot can be recorded by the camera.
As the object is brought closer to the lens, the image increases in size, and at the same time moves further away from the lens. If the image plane is kept in the initial position, the approaching object is increasingly getting blurred on the image sensor.
Every photographic lens has the ability to adjust its focusing distance. The basic principle for the lens to form a correctly focused image at the image plane is that
The relation between the object distance O (length between A and P) and the image distance I (length between P and A') is expressed via the formula
1⁄O + 1⁄I = 1⁄f .
Intra-focus and extra-focus are terms that describe defocused situations. More precisely, if an image is out of focus they indicate whether rays of light focus too early (in front of the image plane) or too late (behind the image plane). Although these descriptions are less relevant in photographic practice, they can be helpful to clarify some optical concepts. Both terms intra-focus and extra-focus are also used in the chapters about depth of field and bokeh.
Photographic lenses consist of numerous groups of lens elements. Even with very complex optical designs, the principle is always the same: In order to correctly focus different objects at the image plane, lens elements have to change their position along the optical axis. These moving lens elements are called the focusing group, and it depends on the optical design which elements are part of this focusing group. The direction in which the focusing group moves is – in most cases – towards the image plane for focusing at distant objects, and away from the image plane for focusing at close objects. Here is an overview of the most common focusing implementations.
This type of focusing is also known as all-lens-group focusing. All lens elements of the complete lens assembly are moved together along the optical axis. This type of focus is sometimes slower than others due to the combined weight of all lens elements that need to be moved. However, one significant advantage is that unit focusing introduces very little change in aberration with respect to change in focusing position. This leads to ultra-constant imaging performance across the entire focus range. Unit focusing is the most commonly used focusing implementation in prime lenses (single focal length lenses). With larger telephoto and super-telephoto lenses, however, this type of focusing becomes less feasible due to the increased size and weight of these optical systems.
This focusing type is also called front cell focusing. As the name suggests, only the front lens group is moved when focus is adjusted. This slightly changes the physical length of the lens as the barrel extends. Front-group focusing is often used in zoom lenses and can be realized via relatively simple lens constructions. Unfortunately, the range of movement that is required by the front group can restrict the zoom ratio of the lens to a certain extent.
Lenses with front-group focusing are available in two different types: If the front unit does not rotate while being focused, the form of implementation is called rectilinear focusing. Conversely, if the front unit rotates around the optical axis while being focused, the lens uses helical focusing. The rotation of the front section is particularly undesirable when photographing with direction-sensitive attachments such as polarizing filters.
This focusing system is typically used in telephoto and super-telephoto lenses. The focusing group of lens elements is located somewhere inside the lens. A common definition is that with internal focusing, the moving lens elements are between the front group and the aperture stop of the lens. As all moving lens elements are inside the lens, the physical length of the barrel doesn't change while focusing. The front element neither extends nor rotates, and therefore internal focusing lenses are ideal for attaching directional filters to the front element. Lens groups used for internal focusing are comparatively lightweight. For that reason, manual focusing typically has an extremely light feel, and autofocus is often faster than unit or front group focusing. Another feature is that lenses with internal focusing sometimes allow for shorter minimum focus distances.
This type is moving only one or more rear lens groups while focusing. According to the common definition, a lens has implemented rear focusing when its focusing group of optical elements is located between the aperture stop of the lens and the image plane. This focusing method has all the advantages of internal focusing.
Note that in the diagram showing the rear focusing system, the focusing lens group is shifted towards the image plane when it focuses at a close object. This is an exception to the general rule. The reason lies in the special design of the focus lens group. In the Canon EF 24-70mm F2.8 L USM lens, the focusing group of lenses has negative refractive power, and therefore the direction of focusing is inversed.
Photographic lenses can focus at subjects within an entire range of distances, from the lens-specific minimum focusing distance to infinity. Unfortunately, the lenses imaging performance is not perfectly constant across all of these possible focusing distances. Conventional photographic lenses are designed with an optimum balance of aberration compensation at only one or two focusing distances. At other focusing distances, however, lenses may show a certain deterioration in image quality, especially towards closer focusing distances. Wide-angle lenses which employ retrofocus designs are particularly affected by this image degradation when focusing at very close subjects. At close range, lenses can then exhibit high levels of field curvature and spherical aberration.
To optimize the correction of aberrations throughout the entire range of focusing distances, Canon has developed the floating system. Floating lens elements are separate optical elements outside of the primary focusing group that are designed to counter field curvature and spherical aberration. While focusing, the primary focusing group of lens elements shifts along the optical axis to focus the lens. At the same time, the floating group shifts along the optical axis via an independent movement. Using the floating group to fine-tune the overall focusing configuration significantly improves the close-focus imaging performance of the optical system.
Canon's floating system is implemented in their true macro lenses such as the Canon RF 100mm F2.8 L MACRO IS USM and others. Many other non-macro lenses are also being equipped with floating systems. The illustration shows the block diagram of the Canon EF 24mm F1.4 L USM lens with a floating system.
Manual focus describes the mechanical implementation of a focusing ring on the lens barrel that can be turned manually by the photographer so that the lens is focused. Autofocus, by contrast, means the implementation of an electronic motor or actuator that drives the focusing lens group, so that the lens focuses automatically.
Early photographic lenses have only had manual focusing options. With manual focus lenses, using the focusing ring on their barrel is the only option to adjust focus. It was 1981 when Canon introduced the New FD 35-70mm F4 AF, the world's first autofocus zoom lens. This lens had Canon's solid state triangulation (SST) autofocus detection system integrated directly into the lens barrel, and therefore AF was working independently from the camera but made the lens very bulky. In 1985, the release of the Canon T80 camera brought another significant improvement to the autofocus concept: Instead of having every interchangeable lens perform the autofocus detection itself, the T80 camera had an autofocus detection system integrated in the camera body. This allowed the camera to detect autofocus through the lens (TTL) which was another advantage as the system used the actual field of view of the attached lens. However, the camera was discontinued in the following year, making the T80 the only autofocus camera that has ever been produced for the FD mount. In 1987, Canon introduced the Electro-Optical System (EOS). The EOS camera system was designed with the latest technology including a microprocessor, an electronic interface to control the attached lens, and TTL autofocus detection that is integrated in all EOS cameras by default. Together with the launch of their first EOS camera (EOS 650), Canon presented three Electro-Focus (EF) lenses. Canon's EF lenses have autofocus motors installed but no AF detection system, which makes them rather compact. (EF-S lenses are a derivative of EF lenses, and have been released much later in 2003.)
Autofocus lenses offer both types of focusing: They have kept their manual focusing ring but have the convenient autofocus motor installed. It is up to the photographer which option he wants to choose. Therefore, an autofocus lens always has a switch on its barrel that lets the user select between MF and AF. Once that switch is set to MF, the autofocus drive inside the lens turns off and leaves the photographer in control of focusing the lens.
Although the EOS camera system was developed with autofocus in mind, there are also some manual focus lenses for the EOS system. However, these are clearly designated as such by not carrying the EF name. Some examples of manual focus lenses for the EOS system are the Canon MACRO PHOTO LENS MP-E 65mm F2.8 1-5x, and the Canon TS-E series of tilt-shift lenses.
In recent years, lenses with focus-by-wire have become more popular. Focus-by-wire describes that the manual focusing ring is not mechanically connected to the focusing lens group but electronically controls the autofocus drive of the lens. The manual focusing ring has an encoder pattern attached to its inner surface, and an infrared sensor on the lens barrel is placed so that it directly scans that surface. This setup allows the lens electronics to detect movement of the manual focusing ring, and to issue an instruction to the autofocus drive accordingly. This feature is also referred to as fly-by-wire or electronic manual focus. The challenge with the design of a focus-by-wire system is to achieve instantaneous reaction of the autofocus system when the manual focusing ring is turned. One disadvantage of the technology is its complete dependence on the autofocus motor: If the autofocus system should fail, lenses with focus-by-wire no longer even be used manually.
Full-Time Manual (FTM) focusing is the ability to manually turn the lens barrel's focusing ring while the lens remains in autofocus mode. The concept originates from the early days of EOS cameras that had extremely limited numbers of AF points. With FTM lenses it was possible to focus on a subject using the central AF point, and then to slightly adjust the focus for the actual subject by turning the manual focusing ring.
Even today, there are situations in photography where the user might want to take control of the focus setting even when autofocus has been selected. This might be the case for low-contrast subjects where autofocus can become challenging to use and start hunting for the correct focus position. Another possible case is where the camera has already completed autofocusing at a selected AF point, and the photographer decides to focus at a slightly different portion of the scene without having the time to reposition the AF point. In these situations, FTM allows the user to simply grab the manual focusing ring on the lens barrel and turn it until the desired subject area is in focus. When using the FTM function, the camera's autofocus system should be set to ONE SHOT AF mode, and the shutter release button of the camera needs to be kept halfway pressed while the manual focusing ring is turned. This prevents the camera from autofocusing again after the manual intervention. FTM focusing can not be used with the autofocus system set to AI SERVO AF mode.
This convenient FTM focusing feature is generally limited to all Canon lenses that either use a ring-type USM autofocus motor, or focus-by-wire. There is only one lens with a Micro USM autofocus motor that also features FTM focusing. All other lenses use reduction gears to connect the AF motors with the focusing lenses, and these can get damaged when manually turned while in AF mode. These lenses should always be switched to MF mode before turning the focusing ring. If you want to learn more about the engineering behind Canon's autofocus lenses, the chapter about autofocus systems explains every autofocus technology used in Canon's interchangeable EF, EF-S, RF, and RF-S lenses from 1987 until today.
While the photographic lens performs the autofocus movement of the focusing lens elements, it is the camera that analyzes the subject and determines the required position to which the focusing group must move so that the desired subject is perfectly focused. Canon's EOS and EOS R cameras use various types of autofocus detection systems. You can read more about the most common autofocus technologies in another e-book explaining how DSLR cameras work.
The focusing distance – also known as focused distance or shooting distance – describes the distance between the camera and the object at which the lens is currently focused. The focusing distance is measured between the image plane (the location of the camera's image sensor) and the object. Note that every Canon DSLR or mirrorless camera has a so-called image plane indicator engraved into the body. This indicator resembles the Greek letter Phi (φ), and it shows the exact position of the camera's image sensor. The focusing distance ranges from this indicator to the object.
The minimum focusing distance is also referred to as the closest focusing distance or close-focus distance. Every lens has a limit to how close an object can be so that it can still form a properly focused image. If the object is brought closer to the lens than the minimum focusing distance, the resulting image of that object is always going to be out of focus. Again, this distance is measured between the image plane and the object. The minimum focusing distance is usually printed on the lens barrel, but is also listed in the lens specifications. Here are some examples of Canon lenses and their minimum focusing distances:
Working distance describes the actual space between the object and the first surface of the front optical element. In some lenses, the front optical element is located inwards in the lens barrel, and therefore the term effective working distance is the practically useable space between the object and the lens barrel (or a lens hood, if attached). Although working distance and effective working distance are useful terms to know, they are more relevant in microscopy and less important in photography.
Deep focus is a photographic and videographic technique that attempts to bring all elements of an image into focus. Although technically it is not possible to focus objects at different distances in a single image plane, deep focus can be regarded as a guide to maximize the depth of field. This is often used by landscape and street photographers.
