The affected phones were mostly sold in a 4-month period between September 2014 and January 2015, Apple said on its website.
The company, whose shares were set to open at their lowest this year on Monday, said a small component in the affected 6 Plus's iSight back camera may fail.
Apple said it would replace the phone's camera free of charge if it takes blurry photos and falls into a particular  serial number range. Eligible serial numbers can be checked on Apple's website.