A projection apparatus includes a light valve, a lens module, a distance measuring module, a lens position detection module and a processor. The light valve forms an image beam. The lens module projects the image beam onto a display surface. The distance measuring module detects a distance between the lens module and the display surface to obtain a first distance value. The lens position detection module detects a position of the lens module to obtain a second distance value. The processor receives the first distance value from the distance measuring module, receives the second distance value from the lens position detection module and obtains an effective focus range according to the second distance value.