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I have a problem very similar to this /t5/Xperia-Z3-Z3-Dual/White-stripe-on-photos/td-p/909906 but i am not using flash. White stripe only shows when light is coming direct to camera from a light source like fluorecent lamp and from a particular angle.
Stripe shows in both portrait and landscape mode and always at same place. I am attaching 2 photos 1 taken in landscape and other in portrait.
I have a protecting cover but it is not covering the camera lens.
I think other z3 devices do not have this issue as i have searched a lot but i only found 1 other person facing this issue and i have shared the link above.
Can i know why this stripe shows on photos? Is it a known issue? Should i claim warranty?
landscape
Portrait
Have you tried taking pictures with the cover off......just to totally rule it out as it may reflect light into the lens even tho its not covering it 
Check if the lens its dirty or if its scratched
Few other things to check/try
settings > apps > all > camera > clear data > restart your phone
also you could repair your phone with PCC which I doubt it will make any difference
Bridge (for Mac)
Alternatives on How to backup Xperias
http://talk.sonymobile.com/thread/36355
if nothing work contact your local Xperia Care
It has nothing to do with the cover as i noticed this issue before i put any cover on it.
@shaktimaan it doesn't hurt to remove the cover and check again w/o the cover; follow my advice
I have removed cover and also cleared data as suggested but problem still persists.
Lens is clean but there is a minor scratch on half the lens and it is only visilbe when i put direct light on it. I don't think scratch is causing white stripe as you can see stripe is at the same location both in portrait and ladscape. I have not used this phone much and it is only one month old.
Depending how steady you are with your hands......make a small dot with a felt tip pen on both ends of the scratch & in the middle, take a couple of pictures...if the dots match exactly on the 'white flare' then you know for certain it is the scratch......