In general, blur effects sample the area around a pixel and assign to the pixel a new value that is the average of the sampled values. Increased blurriness is the result of increasing the size of the sample, whether this is expressed as a radius or a length.

Some blur effects have a Repeat Edge Pixels option. Select this option to make the blur algorithm operate as if the pixel values beyond the edge of the layer are the same as those of the edge pixels. This option keeps edges sharp, preventing them from darkening and becoming more transparent—the result of being averaged with a lot of zeroes. Deselect this option to make the blur algorithm operate as if the pixel values beyond the edge of the layer are zero.