So, how does a SLR camera work or what makes it so complex? The key parts for the light to move through the camera are: lens, mirror, focusing screen, prism and eye piece. Probably you already know that the lens is made of several optical elements and it is for focusing the light on the film. The mirror is small and light, capable of moving up and down. Sometimes the focusing screen also has some features that will help you focus the picture, but these are not important at the moment.
The prism its shape may vary, but the idea remains the same is made of glass and it reflects the image from the focusing screen to the eye piece, which itself is just a piece of glass or simple lens, that you are looking through. The green rays simulate light coming through the camera lens. The main mirror in a DSLR is semi-transparent, and some of this light passes through the mirror, and is reflected downward in the camera. It ultimately ends up as pairs of sharply-focused rays of light, hitting the extremely precisely-positioned AF sensor.
How can an AF sensor tell when an image is in-focus? Every line of pixels is actually a pair of lines, arranged in the same direction. We just mentioned that incoming light is split into two separate and sharply-focused beams, which shine upon these tiny rows of individual pixels. Phase-detection for autofocus is able to work because it can compare two incoming beams of light. Unlike an imaging sensor, which has light-sensitive pixels covering its entire area, the AF sensor uses lines or rows of pixels.
A single incoming beam of light is precisely split in two, and focused upon each row, illuminating either the pixel at the center of each row, or pixels further inward or away. Engineers discovered early-on a very interesting natural phenomenon of these light beams: when the camera lens is focused sharply at a given AF point, these beams would split evenly, and strike the pixel s in the middle of both rows on the AF sensor.
By reading-out each light-gathering pixel independently, when those known to be in the exact center register brightness, the AF system knows that the lens is now in-focus.
In continuous-servo AF AF-C , the camera will continue to focus if the shutter-release button is kept pressed halfway after the camera focuses.
Because the camera continues to focus up to the moment the shutter-release button is pressed all the way down, this mode is a good choice for subjects that are in motion. Because focus locks while the shutter-release button is pressed halfway, AF-S is not suited to subjects that are constantly moving toward or away from the camera. If you change the composition after focusing in AF-C mode, the camera will refocus on the object that is now in the focus point, making it impossible to lock focus using the shutter-release button.
While holding that button down, use the rotating dial shown below to move between the numerous focal points. Note: If you find this does not work, it may be due to you having the shutter speed or aperture screens displayed in your LCD. You might have to hop out of that particular screen first, then set your focal point before going back into the shutter speed and aperture if need be.
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