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lets try some troubleshooting first..
try to clean the camera lens using a lint free cloth (usually the one that comes with eye/sun glasses)
make sure there is no scratches, or if you are using a screen protector that it doesnt cover the lens
try to see to your camera settings, is the resolution set to max 13mp? are you using automode or manual?
restart your device
go to settings>apps>camera>clear data ( to reset the camera app)
if those doesnt help, maybe you can compare with another Xperia m5 at your local service center/showroom before sending it back