Hello kwongstatus,
Thank you for posting in the Apple Support Communities. I see you have a question about your camera.
You mentioned the possibility of lightning and focus; have you noticed the issue happens in specific lighting scenarios? This would be a good way to determine if that may be a factor.
How often would you say this happens?
Does it happen with both the front and rear cameras? Does it happen in all apps that use your camera, or just the Camera app specifically?
Which iOS version are you using under Settings > General > About?
This resource has some great suggestions that may help: If the camera on your iPhone, iPad, or iPod touch isn't working. Based on the below, it's possible that a case could cause behavior like this, so I recommend testing with the case removed:
A case or film might block the camera or flash and affect performance. A metallic or magnetic case or camera attachment might interfere with optical stabilization. Test the camera without any of these to find out if they're causing the issue."
I hope these steps help you out. Have a great day.