Home Theater Hacks

Hack #70. Reduce Focus Problems on RPTVs

Most rear projection TVs have issues with focus; if you get the right side in focus, the left side of the screen blooms, and vice versa. By adjusting the lenses, you can greatly reduce this effect.

Many RPTVs exhibit focus problems. When you calibrate the picture using a good calibration DVD [Hack #62] , you can't get the entire screen to focus correctly. If you get the right side of the screen focused, the focus will hold for about 2/3 of the screen, but the left side blooms out; in other words, it starts to blur on the remaining 1/3 of the screen. If you try and focus this side, the same effect occurs on the right side of the screen.

This is what is called scheimpfluge, the angle at which the CRT hits the lens, and then how that lens hits the screen. In an RPTV, those angles will never be parallel, due to the nature of how RPTVs are designed. So, the lens is angled to compensate (not always correctly, by the way) when your TV is manufactured. However, these angles are set in stone for mass production, including the angles of the lens mounts, rather than for your specific set. As a result, even TVs from the same production line demonstrate this focusing issue to different degrees.

Tip

High-end front projectors have high-precision, spring-loaded adjustment setting screws for this purpose, providing a high-precision mechanism for changing these lens angles.

Using washers under the mounts of the screws that hold your RPTV lenses in place can ...