Real Halftone is an full-fledged app that generates a haltone dot. You can select a dot shape and adjust screen angle of CMYK colors to make a variety of halftone dot photographs. Moire pattern are seen depending on the screen angle of each colors. I recommend a retina display supported device, it makes you see more detailed halftone dots.
This app usually retails for $3.99/£2.49 but at the moment you can pick it up for free. Click here to download.