Canon A3400 IS 6186B011 User Manual (en)

Shooting Range and Focusing
This mode helps avoid missing shots of subjects in motion, because the 
camera continues to focus on the subject and adjust the exposure as long as 
you press the shutter button halfway.
Configure the setting.
Press the 
n button, choose [Servo AF] 
on the 
4 tab, and then choose [On] (p. 51).
The focus and exposure are maintained 
where the blue AF frame is displayed while 
you are pressing the shutter button halfway.
Shooting with Servo AF
Still Images
Focusing may not be possible in some shooting conditions.
In low-light conditions, Servo AF may not be activated (AF frames 
may not turn blue) when you press the shutter button halfway. In this 
case, the focus and exposure are set according to the specified AF 
frame mode.
If adequate exposure cannot be obtained, shutter speeds and 
aperture values are displayed in orange. Release the shutter button, 
and then press it halfway again.
AF lock shooting is not available.
[AF-Point Zoom] on the 
4 tab is not available.
Not available when using the self-timer (p. 63).