Lens Shift Projector lens shift (optical image position adjustment)

What lens shift is

Every projector throws its image from a fixed point relative to the lens. Lens shift lets you slide the lens itself so that point moves, which moves the image on the screen while the projector stays square to it. Epson describes the result as a straight-edged image with uniform focus and flexible placement options, with no loss of resolution, because a physical movement does the correcting and the image data is never altered. A vertical shift moves the image up and down, and a horizontal shift moves it from side to side. This is the cure keystone correction only imitates; see keystone correction for the digital alternative and its costs.

ProjectorCentral draws a distinction that spec sheets blur. Every projector has lens offset, the fixed position of the image relative to the lens, even if that offset is 0 percent. Lens shift is the ability to change the offset, which turns a single value into a range. Offset is measured as a percentage of image height, so a 10 percent offset on a 50-inch-tall image moves it 5 inches. For scale, a 100-inch 16:9 image is 49 inches tall.

How lens shift is specified

Shift is quoted as a percentage of the image dimension in each axis, usually with a plus or minus sign for movement either side of center. Read the sign and the reference carefully. ProjectorCentral warns there is no standard among projector makers for describing lens offset, and that what one maker calls 10 percent another calls 110 percent or 60 percent depending on where 0 percent is defined, so a bare percentage tells you nothing until you know the starting point and whether the figure assumes the projector is upright or inverted on a ceiling mount. Check the installation diagram in the manual before you drill.

The ranges in current products show how wide the spread is. The Epson Home Cinema 5050UB lists lens shift of plus or minus 47.1 percent horizontal and plus or minus 96.3 percent vertical, with a 2.1x zoom, a throw ratio of 1.35 to 2.84 and a 250 W lamp. The JVC DLA-NZ8 lists plus or minus 34 percent horizontal and plus or minus 80 percent vertical on its own spec sheet, with motorized zoom, focus and shift on a 65 mm all-glass lens with a 2x zoom. The Sony VPL-XW5000ES, per ProjectorCentral, has plus or minus 25 percent horizontal and plus or minus 71 percent vertical with a 1.6x manual zoom. At the other end, the BenQ HT3550 lists vertical shift of only plus 10 percent, and the Optoma UHD38x lists none.

Worked example: a 120-inch screen

A 120-inch 16:9 image is about 104.6 inches wide by 58.8 inches tall. If a spec's percentages are relative to a lens-centered image, the NZ8's plus or minus 80 percent of vertical shift moves the picture up or down 47.0 inches (0.80 times 58.8) and its plus or minus 34 percent horizontal shift moves it 35.6 inches (0.34 times 104.6). The Epson's 96.3 percent vertical shift gives 56.6 inches (0.963 times 58.8) of travel, nearly a full image height, and its 47.1 percent horizontal shift gives 49.3 inches. That is enough to place a ceiling-mounted projector above the screen top edge and still land the image level, or to sit the projector on a shelf behind the seats off to one side, with no digital warping at all. The Sony's 71 percent gives 41.7 inches vertical. The projector throw tool shows the matching distance for your screen width.

Motorized vs manual shift, and the memory feature

Shift can be manual or motorized. The Sony VPL-XW5000ES has a 1.6x manual zoom and manual focus per ProjectorCentral. Higher-priced projectors motorize shift, zoom and focus and store lens positions, so one remote button recalls a saved setup, such as a wider or narrower screen format. ProjectorCentral lists lens memory for the 5050UB and the NZ8, and JVC lists motorized shift, zoom and focus on the NZ8.

Which projector types have it

Shift is not evenly distributed across technologies. ProjectorCentral explains that the mechanics of a DLP light engine make it difficult to add long zooms or extended lens shift, and that DLP projectors usually have short zooms with little or no shift, while LCD home theater models often have 2.0x zooms and extensive vertical and horizontal shift. LCoS projectors such as the JVC and Sony models above sit at the top of the range in both shift and price. The Optoma UHD38x, a 4,000-lumen DLP model, lists no lens shift and a 1.1x zoom with a throw ratio of 1.50 to 1.66. The BenQ HT3550 has 1.3x manual zoom and 1.13 to 1.47 throw. For the DLP and LCoS trade-offs see DLP and LCoS.

Common confusions

Lens shift is not zoom, which changes image size through the throw-ratio range described under throw ratio, and it is not lens offset, which is fixed. It is not the same as tilting the projector either. If you tilt a projector, the image becomes a trapezoid and you are back to keystone. Shift also does not rescue a bad throw distance: the distance still has to fit inside the zoom range, so compute that first with the throw ratio. Use shift to fix placement, use zoom to fix size, and reserve keystone for the last degree of misalignment.