I came across this wonderful idea (http://www.instructables.com/id/10-Smartphone-to-digital-microscope-conversion/) which uses laser pointer lens to take closeup pictures.

This attachment uses standard laser pointer lens and was tested on iPhone 4S. Screw is used to adjust the focus. Sample picture is a crumpled paper towel.

Although design included a plastic screw as well, the PLA printed part has very narrow hole because of filament shrinkage. Lens hole is oversized by 0.4mm and fits lens snugly.

Best to use a standard machine screw instead.