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A halftone effects converts an image into a series of dots. These dots can differ in size as well as spacing between them, giving the look of a complete image when viewed from a certain distance.
To achieve the look, convert the image to greyscale (Image>Mode>Grayscale) and then, with the image selected go to Filter>Pixelate>Colour Halftone. In the Colour Halftone window, enter a value in pixels for the maximum radius. The smaller the value, the smaller the circles will be. A larger value produces larger circles for a more abstract effect. This image has the Max. Radius set to 30px. You can change the angles (in degrees) of each of the four channels, and playing around with each will result in different dot formations and patterns.
In this image, all four channels are set to 45 degrees. And there you have a simple halftone effect!