the 50% size has twice as much dpi, if i zoom in i won't notice the pixelation. the 100% size has equal the dpi, if i zoom in i wil notice the pixelation. both will have the same output dimentions. Instead of sampling one pixel from a 150 dpi images (to produce a colour) it samples four pixels from a 300 dpi image. People will be viweing this image from 14 inches away more or less. if you held a ruler in front of your face, about 14 inches away and measured a bill board , you can imagine that it has a higher dpi from the rulers point of view.