Iphone 4s camera and using a screen protector

I have put a screen protector on the back of my Iphone that covers my camera lens. I have read though that this can cause the quality of the image to be degraded. I would like to know if this is true? I am thinking it could cause distortion with the reflective surface of the screen protector. Also, I am using a clear cover on the back of my white Iphone which has a hole for the camera but is it likely this can cause extra glare when taking photos? I recall reading something about there being more glare in the white iphone vs the black or vice versa and then Apple had to do a bit of messing around to sort out the problem. I can't remember which exact phone model it was. and finally is there any real point about worrying about scratching the lens? I am not familar with how scratch resistant or strong Iphone camera lens's are. The camera on my nokia was exposed and never got a scratch.