To measure your face lenskart asks you to place your debit/credit card in front of you face with the magnetic side facing the camera. Doing this will expose your CVV no. used for card validation to lenskart.
Most probably you will be using that same credit/debit card to shop on lenskart. Doing that you give then almost all the information one needs to to make a online transaction, Hackers generally use such features to make transactions.
So please realize the potential threat. At least kindly refrain from exposing your CVV.