The camera's AF sensors require some details in the image to determine the phase difference. It's harder for the camera to find focus when the light is dim or there is little subject detail.

相机的自动对焦感应点需要一些影像细节来确定相位差，光线不足或者物体细节不足都会导致聚焦困难。

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An optical apparatus is disclosed which can reliably form AF images and split the AF images to provide excellent focus detection performance in the phase difference detection method.

公开了一种光学装置，该光学装置在相位差检测方法中能可靠地形成AF像并分离该AF像以提供优良的焦点检测性能。

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Even the advanced two-stage focusing system has made its way from the T4i, which utilizes both phase-difference and contrast AF in order to achieve focus more efficiently when capturing video.

ENGADGET: Canon EOS M mirrorless camera hands-on (video) Hands-on