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After some further investigations, I've found that there were two things that can caused the zoom function of the Volume key didn't work, the camera mode and the Home screen mode.
How to fix can't zoom in or out using the Volume key in the Camera app?
If you're also experiencing this problem, you can check the following steps to make the zoom function of the Volume key works again:
- Check the camera mode
Some camera modes (e.g: panorama mode) will automatically disable the zoom function. So, to make the zoom function works, you need to change the camera mode to another mode. You'll be informed with the error message "cannot zoom in or out in this mode" if you're using a camera mode where the zoom function is not allowed to use.
- Check the home screen mode
If your phone is using the easy mode as its home screen mode then it will automatically disable the zoom function. So, to make the zoom function works, you need to set the home screen mode to the standard mode. You can do so by using the following steps:
- Go to Settings.
- Tap My device tab.
- Tap Home screen mode and then select Standard.
If the step #1 and #2 didn't work, you still be able to zoom in or out by pinching the screen with two fingers.
For the record, I've tested these steps in the Samsung Galaxy SIV with Android 4.4 Kitkat and it worked. So, I can conclude that these steps should be worked in another Samsung Galaxy phones with Android 4.3 Jelly Bean or above.