For this reason, all modern DSLRs include an autofocus micro adjustment tool built into the camera that allows users to configure their autofocus to be perfectly aligned for each lens in their collection. Phase detection requires incredible precision in order to function correctly and even the slightest misposition of the micro lenses relative to the main lens can cause the system to become inaccurate. As far as I understand it, it's impossible on the edges. Because of the nature of this system, focus points cannot reach the edge of the frame as the analysis becomes more difficult as the focus point is moved further from the center. If this point of critical focus is within the bounds of the active autofocus point, the camera is in a focused state. It then aligns those two images on top of each other. Due to the images being drawn from a different position on the main lens the pair of images will be slightly different. However, the point of critical focus will be perfectly aligned. Phase detection autofocus functions using a pair of micro lenses to compare light flowing through opposite sides of the lenses. Understanding the difference between these type of autofocus can help you make the decision of which autofocus mode is a preferable choice in each given situation as a shooter. This happens because contrast detection is not possible unless an image is actually being projected on the sensor itself while phase detection can function without the camera's sensor being part of the operation. When focusing through the viewfinder you are using a technology called phase detection while activating live view switches over to a system based on contrast detection. The first and most important aspect of autofocus in a DSLR that needs to be understood is that each DSLR camera has two distinct autofocus systems.
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