Lens Shift and the Projector Sweet Spot

Lens Shift and the Projector Sweet Spot

Lens shift, or image offset, can be defined as the ability to shift a projected image vertically or horizontally in order to accommodate specific installation challenges. In most installations, lens shift can be used to optimize image geometry and scaling performance by optically landing the image on the screen without having to tilt the projector and employ keystone correction. It can also be used to position the projector such that the average viewer is placed in the illumination sweet spot – an especially important consideration in rear screen and high gain front screen applications.

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Lens Shift and the Projector Sweet Spot

Lens shift, or image offset, can be defined as the ability to shift a projected image vertically or horizontally in order to accommodate specific installation challenges. In most installations, lens shift can be used to optimize image geometry and scaling performance by optically landing the image on the screen without having to tilt the projector and employ keystone correction. It can also be used to position the projector such that the average viewer is placed in the illumination sweet spot – an especially important consideration in rear screen and high gain front screen applications.

Within Digital Projection's current product lineup, the only projectors that do not offer lens shift are the iVision 30 series products. These projectors are designed to mount in a specific position with respect to the top or bottom of the screen.

With the benefit of lens shift, our other displays can be located within a much wider area of the screen. This area is usually defined as a percentage of the image (or screen) height or width. With lens shift centered, dVision, Mercury, TITAN, HIGHlite, LIGHTNING and Reference displays are designed to be installed on-axis with respect to the screen. The lens employed in the application defines the amount of total lens shift available, from that on-axis position.

Zoom lenses generally offer the most lens shift - up to 50% of the image height (vertical) and up to 25% of the image width (horizontal). Short throw fixed lenses generally offer more limited lens shift – approx 10% of the image height and little or no horizontal shift. Fixed short throw lenses are most often utilized for on–axis rear screen applications.

As an example, when using a typical zoom lens with a TITAN HD projector on a screen that is 60” high, the installer would have as much as 50% vertical lens shift (offset), or 30 inches of up and down image movement from the screen center. This allows the projector to be installed anywhere from the top of the screen to the bottom of the screen. Given a 16 x 9 aspect ratio, the same 60” high screen is nearly 107” wide. The same zoom lens would offer 10% of that screen width in horizontal lens shift (horizontal offset), or nearly 11 inches of image shift left and right.

It is important to remember, if both horizontal and vertical lens shift are employed, the total available lens shift in either dimension will normally be less than the maximum specified for the lens.

Although lens shift provides great flexibility in projector placement, be aware that in any installation, the goal should be to install the projector to assure maximum transfer of illumination from the projector to the average viewer position. This makes the most of screen gain and optimizes luminance uniformity, peak brightness, contrast and also minimizes hot spotting. Projector placement is especially critical in rear-screen applications, as well as in front screen applications where high gain screens are utilized.