Canon EOS-1D X Mark III EOS-1DX Mark III Advanced User Guide - Page 356

AF Shooting Tips Even when focus is achieved, pressing the shutter button halfway will focus again. Image brightness may change during autofocusing. Depending on the subject and shooting conditions, it may take longer to focus, or the continuous shooting speed may decrease. If the light source changes as you shoot, the screen may flicker, and focusing may be difficult. In this case, restart the camera and resume shooting with AF under the light source you will use. If focusing is not possible with AF, focus manually ( ). For subjects at the edge of the screen that are slightly out of focus, try centering the subject (or AF point, or Zone AF frame) to bring them into focus, then recompose the shot before shooting. With certain lenses, it may take more time to achieve focus with autofocus, or accurate focusing may not be achieved. 356