An f/1.4 lens is 2/3 of an f/stop faster than f/1.8. Each f/stop lets in 2x as much light, so a f/1.4 lens lets in 1.66x as much light as a f/1.8 lens. The wider maximum aperture of the f/1.4 makes the viewfinder image brighter and clearer. F/1.4 has slightly more shallow depth of field than f/1.8.