Focusing manually on an LCD screen isn’t as difficult as you might think, particularly if you have the XSi mounted on a tripod, but Canon has made the process even easier by providing a magnified view. Just follow these steps to focus manually.
Make sure the focus switch on the lens is set to MF.
Use the left/right/up/down cross keys to move the white focus frame that’s superimposed on the screen to the location where you want to focus.
Press the Zoom button. The area of the image inside the focus frame will be magnified 5X. (See Figure 5.24.) Press the Zoom button again to increase the magnification to 10X. A third press will return you to the full-frame view. The enlarged area is artificially sharpened to make it easier ...