iPhone 6 rear camera problems

Hi there,
I bought iphone 6 16gb two months ago with ios 8.3 and everything was alright. But after I updated ios to 8.4 rear camera didn't focus, it was blurred (just right corner of the photo was focused) it lasted 4-5 days, then it was okay. Unfortunately it's happening every week, I tried every advice I found, I tried to reboot the phone or reset settings but it didn't help. I thought it was caused by my laptop, that it somehow "affected" my iphone so I tried to keep it from it, but it happened again.

My iphone is second hand so I thought maybe this wasn't caused by the ios 8.4, but the problem was with my iphone.

I asked stuff at istore about this problem and they didn't want to take it to repair (free of charge), because I don't have the certificate of warranty from seller, but iphone is under limited warranty from apple.

I really need to solve this issue, I don't want to give some more extra money for repair and it's annoying. Thanks for any help.