By: Cody McCaw, MCH Media Systems Consultant
When placing a projector in a room, there are multiple variables to consider: How far from the screen should it be positioned? How high/low should it be placed in front of the screen? Are there any obstacles that need to be avoided by the projector light? Luckily, there have been several advancements in projection technology over the years which aid in making these decisions. Two examples of which include replaceable lenses and lens shifting.
For years now, higher end projectors have had the option for replaceable lenses. Gone are the days where a projector could only be placed in one location determined by a stock lens. Today, projectors come with a suite of lens options that allow flexible projector placement from a few feet to hundreds of feet away from the screen.
The “lens ratio” or “throw ratio” is a formula that indicates where the projector, with a given lens, should be placed in relation to the projection screen in order to fill the image area:
The higher the ratio number, the further the projector must be placed away from the screen in order to correctly fill the image area. Certain lenses are still “fixed,” meaning they can only be placed at a specific distance from the projection screen (ultra-short throw lenses are an example of this), while most other lenses (in combination with the projector’s zoom function) can be placed within a given ratio range. This range is especially handy for ceiling projector installations when other ceiling elements such as HVAC or sprinkler heads must be avoided. Additionally, if a presenter is in front of the target screen, then putting the projector up high and closer to the screen (lower ratio lens) prevents the projection light from being blocked by the presenter’s head.
Lens shifting is a technology that physically moves the image around in the projector which allows for a larger area which to locate the projector in relation to the screen (and still retain the actual image resolution) while not having to resort to image transformation techniques such as keystoning, which negatively impacts image quality. Understanding a projector’s capabilities requires a dive into the user manual. Sure, the simple data sheet will typically give the horizontal/vertical shift, but it only tells part of the story.
Projector lenses are typically aligned with the center of the projection screen, presuming the projector housing is centered to the screen. The lens shifting capabilities are based on this center point being the origin, and are represented as percentages. 0% typically means no shift, and percentages above 0% could mean several different things, as described later. The projector’s shifting ability in the vertical direction is usually greater than in the horizontal direction. This allows projectors to be located close to the ceiling, or close to the ground (e.g. sitting on a table)). As an example, shifting the image center in the vertical direction, to the top/bottom of the screen, is typically represented as a 50% lens shift (50% of the total image height). Lens shifting in the horizontal direction is usually only around 20 to 30% depending on the unit.
One must use caution when interpreting shift range metrics. First, always read the manual to determine how the lens shift range is defined. The 50% shift example, from above, could be represented as 100% from a different manufacturer, or the origin point for the projector might not be the center of the screen. Second, different lenses will have different shift ranges, with lenses below a ratio of 1 typically having reduced ranges when compared to higher ratio lenses. Lastly, when a lens has both a horizontal shift range and a vertical shift range, using one shift range limits the other (i.e. a shift in the horizontal axis would lose some of the shifting range in the vertical axis). This reduced shift capability has resulted in the general recommendation that projectors should be located centered (or as near as possible) to the projection screen to retain the larger vertical shifting range.
Replaceable lenses and lens shifting are both excellent technologies which aid designers of AV systems. These tools, when properly employed, allow multiple location options, and helps designers more efficiently navigate design challenges. Consult your Media Systems designers to determine appropriate projector locations.