Macro lens
You don't need macro lens for iPhone 3GS because it has auto focus with its camera.
But you need macro lens for iPhone 3G. We strongly recommend to use 'Gliffin Clarifi' to take photograph of barcode with iPhone 3G.
Tap this link to get information of the Griffin Clarifi with your ItemShelf.
Or you can tap following link:
Scan barcode with Gliffin Clarifi
Barcode scanning tips for Gliffin Clarifi.
Take a photo of barcode with around 1/3 width of display width. Do not too close to barcode to avoid out of focus.
After take a photo, magnify it and set position like this screen shot.