Autofocus lenses as used, for example, on digital cameras mostly use a feedback loop to focus. Optical sensors are used to determine if the image scene is properly focused. Several principles, such as contrast based autofocus or phase autofocus, are employed. Contrast based autofocus is based upon calculation of the contrast in a certain area of the scene, around the object to be focused. The focus of the lens is adjusted with a control loop until a maximum contrast is achieved. The contrast measurement does not indicate the direction of the focus. Therefore it can take some time before the correct focus setting is achieved. Phase detect autofocus measures separately the projected spot of the object to be focused as it is seen from distinct parts of the lens, on separate optical detectors. The focus is adjusted until the same response is achieved on both detectors. This ensures a correct focus. From the peak response of the different phase autofocus detectors, the focus direction and the amount of focus adjustment can be calculated. This results in much faster focusing. In many single lens reflex cameras this is done with a dedicated phase detect autofocus sensor, which is illuminated via the mirror in the camera or via a beam splitter or prism. However, in a live view mode or video mode, this secondary phase autofocus sensor cannot be used. Also in cameras without a mirror or prism, a faster phase autofocus is being implemented.
FIG. 1 shows a camera composed of a simple lens and an image sensor. When the object is focused correctly by the lens, all optical rays from the object are correctly projected onto the same location on the image sensor. When the object is incorrectly focused, different rays of the object are projected on different locations on the image sensor, as shown in the Figure. This results in a blurred image.
FIG. 2 shows more detail on how the lens actually focuses the object. An object at the principal axis of the lens is shown. A pixel array is shown, indicated by microlenses and photodiodes. The principal ray goes directly from the object to the pixel array. The centre pixel sees the object through this principal ray. The centre pixel also sees this object through rays from the edge of the lens, from all sides of the lens. If part of the pixel is made insensitive, by any means, only rays from one side of the lens will project the object on to the pixel. This is shown at the bottom of FIG. 2. On the left drawing, the right part of the pixel is made insensitive. The lens still focuses the object correctly onto the pixel but only the left part of the lens is used. On the right drawing, the left part of each pixel is made insensitive. In that case, only the right part of the lens is used. The object is still correctly focused in this drawing, and all the rays are projected on the same pixel.
The situation when the object is not in focus, is shown in FIG. 3. In the case shown in the drawing, the object is closer to the lens, and the object is focused in front of the image sensor. When now the right part of the pixel is made insensitive, the rays of the object are create a response at the pixels at the right of the principal ray. When the left part of the pixels is made insensitive, the rays of the object create a response at the pixels to the left of the principal ray. The response of the different rays for different focus positions is also shown in FIG. 1. A cross-section of two phase detect autofocus pixel with a light shield and two different exit pupils are shown in FIG. 4, together with a cross-section of a normal pixel. The unused area of a pixel is covered by a light collecting charge drain. This light shield is implemented using a metal layer on top of the photodiode. This can be an extension of the metal routing that is present in between pixels towards one side, as shown in FIG. 4. It can also be a dedicated metal layer, or another non-transparent material, that is deposited on top of the photodiode.
Several different techniques have been proposed to use phase autofocus pixels as part of a pixel array. All use partially masked phase pixels with a light shield. These pixels employ a light shield above the pixel, so that the phase autofocus pixel only receives the light from a part of the main lens. Pairs or multiple of such autofocus pixels are then used in order to calculate the focus distance and adjust the lens focus. Implementations of such can be found for example in the following patents and patent applications: Japanese Patent Application JP2011250325A (Fujifilm), International Patent Application WO 2010/101096A1 (Canon), International Patent Application WO 2011/055617 (Canon), EP 2 330 449 A1 (Sony) and U.S. Pat. No. 8,098,321 (Sony).
The above referenced documents all describe the use of a light shield to cover part of the pixels or part of the pixel pairs. The light shield is used to form an exit pupil for the phase autofocus pixel. This exit pupil selects only rays that come from a part of the camera lens. The focus is then adjusted until the rays from different parts of the lens are projected on the same pixel. The response of the pixels with light shield depends on the distance between the light shield and the photodiode and on the distance between the light shield and the microlens.
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