Without knowing where the 0% position (i.e., the starting point) is, the offset spec doesn't tell you anything. And still other manufacturers give different numbers. What some manufacturers call 10% offset, others call 110% (measuring from the top of the image) or 60% (measuring from the horizontal midline of the image). The problem you run into is that different manufacturers measure the offset from different starting positions. (As a point of reference, a 100-inch diagonal, 16:9 image is 49 inches high.) For example, this would be 5 inches higher for a 50-inch high image. So, if 0% is defined as the point where the lens centerline is even with the bottom of the image, a 10% offset would put the bottom of the image an amount that's 10% of the image height higher than the lens centerline. Lens offset, for example, tells you how far the lens-and therefore the image-is offset from a position that would count as 0%. One major problem is that there is no standard among projector makers for describing either one. But the specs can often be misleading or confusing. These are supposed to tell you where you can, or have to, position your projector-whether up, down, left, or right-relative to the screen to avoid keystone correction. Unfortunately, decoding shift and offset specs can be a challenge. However, there are good reasons to avoid keystone correction (more on that later), and the better strategy is to pick a projector in the first place with an offset or lens shift that won't need keystone adjustment from your planned mounting position. They either don't think about them at all, or they just figure they'll rely on keystone correction to square off the image if they have to tilt the projector to point at the screen. Some people don't pay much attention to lens offset or lens shift when choosing a projector.
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