Autofocus
As autofocus ( AF ) is the technique of a camera or other optical devices referred to, on the subject automatically sharp note . A basic distinction is made between active autofocus , which also works in complete darkness, and passive autofocus , in which only the light emitted or reflected by the subject is used.
Today, the speed and accuracy of the autofocus are usually above what can be achieved manually. Modern cameras measure different image areas in order to ultimately decide for themselves where the object is. Some analog Canon cameras use eye tracking to determine the currently relevant focus field based on the photographer's line of sight.
history
The first camera to be sold in Germany with a built-in autofocus was the Konica C35 AF in November 1977 . This was followed by a wide variety of variants from all manufacturers very quickly. The equipment feature became standard for new cameras in the amateur sector within a year. The first 35mm SLR cameras with automatic focusing of the lens through the camera housing followed very quickly: in 1981 the Pentax ME F , followed in 1983 by Nikon's Nikon F3AF, which worked on a similar principle .
In the meantime, autofocus systems are already being used in some cell phones with built-in digital cameras in order to replace the fixed focus lenses that have been used up to now and thus ensure a higher sharpness of the image.
General
Simple AF systems only have a single focus sensor, while more advanced systems now have a whole grid of sensors. Cameras located in the semi-professional sector often have more than 50 sensors, which can usually be selected individually for capturing the object to be focused, whereby the availability of the individual sensors can depend on the lens used.
Auto focus modes
In photo practice today, in addition to traditional manual focusing ( MF ), various autofocus modes are distinguished:
- AF.S ( single autofocus ): The camera sets the focus on the targeted object and then maintains this setting, even if the object moves to a closer or more distant place until the picture is taken.
- AF.C ( continuous autofocus ): The camera sets the focus on the targeted object and then adjusts this setting continuously depending on whether the object is moving to a closer or more distant location before the picture is taken.
- AF.A ( automatic autofocus ): The camera decides on a case-by-case basis whether the object is stationary, which it can best focus using AF.S , or a moving object which it can use AF. C focused.
Focus failure
If the focus is not determined in the actual image plane (with analog cameras that of the photographic film, with digital cameras that of the image sensor), but, for example, with a focusing screen or an autofocus sensor in a separate auxiliary plane, this can lead to focusing errors when setting the distance . which is then reflected in slightly blurred images or recordings. Modern digital cameras therefore now also offer the option of readjusting the position of their autofocus sensors in the camera.
Passive autofocus
The most widespread today are passive autofocus systems, and the two basic techniques of phase comparison and contrast measurement. Passive autofocusing, however, always depends on adequate lighting and sufficient object contrast. If the subject is additionally illuminated with an auxiliary light, the purely passive autofocusing can also be expanded to a quasi- active process .
Edge contrast measurement
Focusing by means of contrast measurement on contour edges basically works in the same way as the eye , i.e. the photographer, focuses without further aids: The image range of the lens is varied until the brightness curve at light / dark contour edges has a maximum steepness. The camera's processor calculates the frequency distribution in the image, and the greater the proportion of high frequencies (i.e. abrupt light-dark changes), the sharper the image.
However, the camera has to try out different foci to determine the direction of adjustment. Not only is the direction known when at least two measurements are available, but the next focus position can also be extrapolated if necessary.
The edge contrast measurement method is often used in video and compact digital cameras because of its cost efficiency .
Due to the computational effort and the necessary prior knowledge of the absolute methods ( depth from defocus ), only relative methods ( depth from focus ) are used in the practice of autofocusing by most manufacturers , whereby relative methods in this context mean that in order to improve or improve the focus To determine the deterioration of the image sharpness and the direction of the necessary focusing, always several shots with different focusing are necessary (so to speak, a "test series" for image sharpness optimization). The disadvantages of this method are computation and engine expenditure, which has a negative effect on the battery charge and the time required. For each new focus measurement (even without changing the image section), a focus change, that is to say defocusing, is first of all necessary; therefore, time is again required even if no improvement can be achieved.
Because the measurement is made with the image sensor, no misalignment is possible, the optimal focus setting should always be achieved.
Phase comparison
The older passive method is phase comparison, which is more complex and requires additional sensors. In modern digital cameras these are located directly above the sensor level (e.g. Sony Alpha 6000 ), with older SLR cameras a separate sensor chip is used. In principle, less computing power is required with phase comparison autofocus, since the focusing direction can be determined with the first measurement.
The method is derived from the purely optical cross- sectional image indicator , an aid that was used in single-lens reflex cameras from the 1960s to 1980s with mechanical focusing. In the center of the subject through the viewfinder shim while two mutually inclined unmattierte circle halves were placed, so that two fields zuschoben each other until they fit together precisely in focus during manual focusing. This technique is also the basis of phase detection autofocus. It consists of two rows of sensors that can assess the position of the two fields relative to one another on vertical edges.
The method was first implemented in 1976 by Honeywell's Visitronic chip. The first serial camera equipped with this was the Konica C35-AF in 1977 . The functional principle is based on triangulation of the object distance by (at least) two autofocus sensors ( stereo image ) looking through the same lens . The result is fast and precise focusing that can be repeated as often as required without renewed mechanical focusing and thus without wasting time. In the case of digital cameras, this process is mainly used in the more expensive SLR cameras and system cameras because of the higher costs and technical complexity , but many compact cameras have now also been equipped with this technology.
Line sensors and cross sensors
Autofocus sensors that work with the phase comparison look for comparable light intensity patterns at two points that are distant from one another. The focus can only be successfully determined if the light intensities vary between the two points (longitudinally or transversely) in the direction of image propagation; Flat or parallel lined image motifs do not allow focus determination with these line sensors. For this reason, several AF sensors are usually accommodated in a camera in such a way that their line sensitivities are oriented transversely to one another.
In order to achieve an even higher sensor density and thus a higher and dynamic focus determination, there are so-called cross sensors whose line sensitivity is aligned in two dimensions. The number of cross-type sensors often highlighted in the specification of a camera model could therefore be multiplied by two and used together with the other AF sensors as a quality parameter. With older cameras and current entry-level models, cross sensors are mainly placed in the center of the image, which is important for the subject, where not twice the number of line sensors can be accommodated in the same space. Today's flagship models like the Canon EOS 80D work exclusively with cross-type sensors.
Dual-pixel autofocus
With Canon's Dual-pixel autofocus is an evolution of the phase comparison measurement. Instead of applying a quasi-invisible layer of phase comparison sensors above the sensor plane, all effective pixels on the sensor surface have two separate photodiodes that are separated for the phase detection AF and read out together to generate the image data. In order to carry out such a phase detection on the image sensor, the left and right photodiodes are read out separately and the resulting parallax images are used to determine the phase difference. The advantage of this method is that each pixel can be used to determine the phase difference. At the same time, disadvantages of an additional phase autofocus layer are avoided, such as e.g. B. that under certain backlighting conditions the sensor lines become visible.
Hybrid autofocus
The combination of the above-described edge contrast measurement with phase comparison measurement is referred to as hybrid autofocus. The camera combines the advantage of a faster phase comparison in the first step with the more precise contrast edge measurement in the second step. This ensures that focusing can take place at any desired image point, although the phase autofocus is limited to certain points in the image due to the number of sensors.
Active autofocus
The active autofocus also works in complete darkness. A distinction is made between direct distance measurement using ultrasound and the extension of passive methods using object lighting.
Ultrasonic transit time method
An active ultrasound process ( sonar ) has been used in various Polaroid cameras , for example, since 1982 . The time it takes for the sound to travel from the camera to the object and back is measured and focused depending on the calculated distance. The advantage of this method is that it works extremely quickly, as no test focusing is necessary as is the case with contrast measurement. The disadvantage is that no precise selection of the focus on the motif is possible and that, since it is not an optical process, it does not work at all with glass panes and only to a limited extent with mirrors. In addition, the procedure only works up to a certain distance, which in some cases is well below the hyperfocal distance .
Object lighting
A phase comparison or contrast measurement can be carried out in spite of the insufficient light output of the motif if it is actively illuminated. Either an auxiliary light, which is similar to that of a flashlight , or measuring flashes are used.
AF-assist illuminator
The AF auxiliary light is usually red or - e.g. B. with the Pentax K-5 - green (visible), alternatively infrared (invisible), in the latter case, however , a bit less precise due to the longitudinal CA of the lens. As can be seen in the picture example, ideally no uniform light spot is used, but a pattern projected onto the motif. If the phase comparison is measured horizontally, a vertical line pattern is particularly suitable. The greatest advantage, however, is that such an illumination pattern can even be used to focus on areas without any contrast, which is why this method is also used when the object actually provides enough light for the measurement, but has too little contrast. If the camera does not have its own flash, the AF auxiliary light is usually built into the associated flash unit .
In addition to the (temporally) continuous illumination with an AF auxiliary light, measuring flashes are also used. Their advantage, apart from their low cost (no additional light source is required), is the fact that they can also be used to focus fast-moving objects due to their suppressed motion blur, while the disadvantage (in addition to their "conspicuousness") is that they are evenly illuminated, as with purely passive method, again only objects with sufficient contrast can be focused.
Special enhancements to the autofocus
In connection with the auto focus, some cameras offer additional functions.
Focus trap
In the case of the focal trap , trap focus , catch-in focus , the camera leaves the setting of the focus level to the photographer and only checks periodically whether an object is in the selected focus level: If this is the case, solve she from. An application example of this technology would be the observation of a feeding place in a clearing: As soon as an animal appears in the focused position, it is automatically photographed by the camera.
Predictive autofocus
With predictive autofocus , often incorrectly referred to as " predictive autofocus ", the camera can detect in series recordings whether and how fast the object is moving and use this to estimate the likely next position of the object. For this purpose, the focus is measured again between the individual images of the series exposure, extrapolated for the next exposure and adjusted accordingly. This makes it possible, for example, to depict approaching vehicles consistently sharply, even when taking a series of photos.
Cameras with predictive autofocus, first implemented by Canon in their still analog Canon EOS RT from 1989 and later christened AI Servo , are now offered by numerous manufacturers. a. from Nikon, Sony, Pentax, Olympus and Panasonic.
See also
Individual evidence
- ↑ a b Michael J. Hussmann: 20 years of EOS - Canon's SLR system. In: Photo magazine. May 1, 2007, accessed June 21, 2012.
- ↑ SLR photography guide: Nikon focus modes , accessed June 21, 2012.
- ↑ canon.de: Dual Pixel CMOS AF
