Autofocus Apk ##HOT## Download
Fusberta Loparo
Automatically focusing a form control can confuse visually-impaired people using screen-reading technology and people with cognitive impairments. When autofocus is assigned, screen-readers "teleport" their user to the form control without warning them beforehand.
Use careful consideration for accessibility when applying the autofocus attribute. Automatically focusing on a control can cause the page to scroll on load. The focus can also cause dynamic keyboards to display on some touch devices. While a screen reader will announce the label of the form control receiving focus, the screen reader will not announce anything before the label, and the sighted user on a small device will equally miss the context created by the preceding content.
Automatically focusing a form control can confuse visually-impaired people using screen-reading technology and people with cognitive impairments. When autofocus is assigned, screen-readers \"teleport\" their user to the form control without warning them beforehand.
An autofocus (or AF) optical system uses a sensor, a control system and a motor to focus on an automatically or manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication. Autofocus methods are distinguished as active, passive or hybrid types.
Through-the-lens optical autofocusing is usually speedier and more precise than manual focus with an ordinary viewfinder, although more precise manual focus can be achieved with special accessories such as focusing magnifiers. Autofocus accuracy within 1/3 of the depth of field (DOF) at the widest aperture of the lens is common in professional AF SLR cameras.
The data collected from AF sensors is used to control an electromechanical system that adjusts the focus of the optical system. A variation of autofocus is an electronic rangefinder, in which focus data are provided to the operator, but adjustment of the optical system is still performed manually.
The speed of the AF system is highly dependent on the widest aperture offered by the lens at the current focal length. F-stops of around f/2 to f/2.8 are generally considered best for focusing speed and accuracy. Faster lenses than this (e.g.: f/1.4 or f/1.8) typically have very low depth of field, meaning that it takes longer to achieve correct focus, despite the increased amount of light. Most consumer camera systems will only autofocus reliably with lenses that have a widest aperture of at least f/5.6, whilst professional models can often cope with a widest aperture of f/8, which is particularly useful for lenses used in conjunction with teleconverters.[citation needed]
Between 1960 and 1973, Leitz (Leica)[1] patented an array of autofocus and corresponding sensor technologies. At photokina 1976, Leica had presented a camera based on their previous development, named Correfot, and in 1978 they displayed an SLR camera with fully operational autofocus.
The first mass-produced autofocus camera was the Konica C35 AF, a simple point and shoot model released in 1977. The Polaroid SX-70 Sonar OneStep was the first autofocus single-lens reflex camera, released in 1978.
There are various ways to measure distance, including ultrasonic sound waves and infrared light. In the first case, sound waves are emitted from the camera, and by measuring the delay in their reflection, distance to the subject is calculated. Polaroid cameras including the Spectra and SX-70 were known for successfully applying this system. In the latter case, infrared light is usually used to triangulate the distance to the subject. Compact cameras including the Nikon 35TiQD and 28TiQD, the Canon AF35M, and the Contax T2 and T3, as well as early video cameras, used this system. A newer approach included in some consumer electronic devices, like mobile phones, is based on the time-of-flight principle, which involves shining a laser or LED light to the subject and calculating the distance based on the time it takes for the light to travel to the subject and back. This technique is sometimes called laser autofocus, and is present in many mobile phone models from several vendors. It is also present in industrial and medical[3] devices.
Passive AF systems determine correct focus by performing passive analysis of the image that is entering the optical system. They generally do not direct any energy, such as ultrasonic sound or infrared light waves, toward the subject. (However, an autofocus assist beam of usually infrared light is required when there is not enough light to take passive measurements.) Passive autofocusing can be achieved by phase detection or contrast measurement.
Contrast-detection autofocus is achieved by measuring contrast (vision) within a sensor field through the lens. The intensity difference between adjacent pixels of the sensor naturally increases with correct image focus. The optical system can thereby be adjusted until the maximal contrast is detected. In this method, AF does not involve actual distance measurement at all. This creates significant challenges when tracking moving subjects, since a loss of contrast gives no indication of the direction of motion towards or away from the camera.
Contrast-detect autofocus is a common method in digital cameras that lack shutters and reflex mirrors. Most DSLRs use this method (or a hybrid of both contrast and phase-detection autofocus) when focusing in their live-view modes. A notable exception is Canon digital cameras with Dual Pixel CMOS AF. Mirrorless interchangeable-lens cameras typically used contrast-measurement autofocus, although phase detection has become the norm on most mirrorless cameras giving them significantly better AF tracking performance compared to contrast detection.
Contrast detection places different constraints on lens design when compared with phase detection. While phase detection requires the lens to move its focus point quickly and directly to a new position, contrast-detection autofocus instead employs lenses that can quickly sweep through the focal range, stopping precisely at the point where maximal contrast is detected. This means that lenses designed for phase detection often perform poorly on camera bodies that use contrast detection.
The assist light (also known as AF illuminator) "activates" passive autofocus systems in low-light and low-contrast situations in some cameras. The lamp projects visible or IR light onto the subject, which the camera's autofocus system uses to achieve focus.
Many cameras and nearly all camera phones[a] lack a dedicated autofocus assist lamp. Instead, they use their built-in flash, illuminating the subject with bursts of light. This aids the autofocus system in the same fashion as a dedicated assist light, but has the disadvantage of startling or annoying people. Another disadvantage is that if the camera uses flash focus assist and is set to an operation mode that overrides the flash, it may also disable the focus assist. Thus, autofocus may fail to acquire the subject.
Some external flash guns have integrated autofocus assist lamps that replace the stroboscopic on-camera flash. Many of them are red and less obtrusive. Another way to assist contrast based AF systems in low light is to beam a laser pattern on to the subject. The laser method is commercially called Hologram AF Laser and was used in Sony CyberShot cameras around the year 2003, including Sony's F707, F717 and F828 models.
A rare example of an early hybrid system is the combination of an active IR or ultrasonic auto-focus system with a passive phase-detection system. An IR or ultrasonic system based on reflection will work regardless of the light conditions, but can be easily fooled by obstacles like window glasses, and the accuracy is typically restricted to a rather limited number of steps. Phase-detection autofocus "sees" through window glasses without problems and is much more accurate, but it does not work in low-light conditions or on surfaces without contrasts or with repeating patterns.
A newer form of a hybrid system is the combination of passive phase-detection auto-focus and passive contrast auto-focus, sometimes assisted by active methods, as both methods need some visible contrast to work with. Under their operational conditions, phase-detection auto-focusing is very fast, since the measurement method provides both information, the amount of offset and the direction, so that the focusing motor can move the lens right into (or close to) focus without additional measurements. Additional measurements on the fly, however, can improve accuracy or help keep track of moving objects. However, the accuracy of phase-detection auto-focus depends on its effective measurement basis. If the measurement basis is large, measurements are very accurate, but can only work with lenses with a large geometrical aperture (e.g. 1:2.8 or larger). Even with high contrasty objects, phase-detection AF cannot work at all with lenses slower than its effective measurement basis. In order to work with most lenses, the effective measurement basis is typically set to between 1:5.6 and 1:6.7, so that AF continues to work with slow lenses (at least for as long as they are not stopped down). This, however, reduces the intrinsical accuracy of the autofocus system, even if fast lenses are used. Since the effective measurement basis is an optical property of the actual implementation, it cannot be changed easily. Very few cameras provide multi-PD-AF systems with several switchable measurement bases depending on the lens used in order to allow normal auto-focusing with most lenses, and more accurate focusing with fast lenses.Contrast AF does not have this inherent design limitation on accuracy as it only needs a minimal object contrast to work with. Once this is available, it can work with high accuracy regardless of the speed of a lens; in fact, for as long as this condition is met, it can even work with the lens stopped down. Also, since contrast AF continues to work in stopped-down mode rather than only in open-aperture mode, it is immune to aperture-based focus shift errors phase-detection AF systems suffer since they cannot work in stopped-down mode. Thereby, contrast AF makes arbitrary fine-focus adjustments by the user unnecessary. Also, contrast AF is immune to focusing errors due to surfaces with repeating patterns and they can work over the whole frame, not just near the center of the frame, as phase-detection AF does. The down-side, however, is that contrast AF is a closed-loop iterative process of shifting the focus back and forth in rapid succession. Compared to phase-detection AF, contrast AF is slow, since the speed of the focus iteration process is mechanically limited and this measurement method does not provide any directional information. Combining both measurement methods, the phase-detection AF can assist a contrast AF system to be fast and accurate at the same time, to compensate aperture-based focus-shift errors, and to continue to work with lenses stopped down, as, for example, in stopped-down measuring or video mode.
df19127ead