Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 145)

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Selecting the AF Method (Live View Shooting)
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 fl icker, and
focusing may be diffi cult. 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 ( = 147).

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.