Understanding Lens Offset and Lens Shift
Some people don't pay much attention to lens offset or lens shift when choosing a projector. 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. 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.
Unfortunately, decoding shift and offset specs can be a challenge. 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. But the specs can often be misleading or confusing.
One major problem is that there is no standard among projector makers for describing either one. Lens offset, for example, tells you how far the lens—and therefore the image—is offset from a position that would count as 0%. 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. For example, this would be 5 inches higher for a 50-inch high image. (As a point of reference, a 100-inch diagonal, 16:9 image is 49 inches high.)
The problem you run into is that different manufacturers measure the offset from different starting positions. 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). And still other manufacturers give different numbers. Without knowing where the 0% position (i.e., the starting point) is, the offset spec doesn't tell you anything.
We address this issue in our reviews by giving all the information you need to understand how much offset or shift there actually is. However, if you're not thoroughly familiar with either or both features, or you're used to a different way to describe them, you can probably benefit from a roadmap for translating our comments in the reviews into a better understanding of where you can place the projector.
How Do Lens Shift and Lens Offset Differ?
Start with the basics: Lens shift and lens offset are different, but interrelated, features. Unfortunately, the difference isn't immediately obvious from the names, so if you're not already familiar with both they're easy to confuse with each other.
Briefly, all projectors have lens offset, even if that offset is 0%. Lens shift is available on only some projectors. What it essentially does is let you change the offset, giving the projector an offset range (although it is not usually referred to that way) instead of a single fixed offset.
One complication for talking about lens shift is that it comes in both vertical and horizontal versions, and trying to discuss both at the same time makes for convoluted sentences. Vertical shift moves the image up and down. Horizontal shift moves it left and right. Many projectors offer both, but vertical shift by itself is also common. To make this discussion easier to follow, we'll discuss offset first, then vertical shift only, and come back to horizontal shift at the end.
Offset is always vertical. More precisely, there's no reason why a projector couldn't be built with a fixed horizontal offset, but we can't recall ever seeing one that was. The same comment applies to horizontal shift appearing without vertical shift—we've never seen it.
Why Care About Lens Shift and Offset?
Most projectors provide keystone correction of +/-30 degrees or more, which allows you to square off the image if you end up tilting the projector to aim it at the screen. So, you might reasonably wonder why lens shift or offset matters. There are two key reasons.
First, using keystone correction lowers image brightness. The feature works by mapping the pixels in the image to use only part of the imaging chip. Light that would shine through (for LCD) or reflect from (for DLP and LCoS) the unused part of the chip doesn't make it to the screen, and less light means a dimmer image.
Even more of a problem is that remapping pixels in the image can create artifacts. When the video source is set to the native resolution of the projector, it gives a one-to-one match between the native resolution (the number of mirrors on a DLP chip for example) and the number of pixels in the image. Remapping the image to anything less than the entire chip loses the one-to-one match, which can show as moiré patterns in scenes with closely spaced slats in window blinds, for example, or repeating patterns in wallpaper or clothing.
Positioning the projector where it can point directly at the screen without tilting avoids both issues. To do that, you have to understand where the projector needs to be after accounting for the fixed lens offset and the ability of any lens shift features to adjust the offset. Keystone correction is best reserved for when you have no other choice.
Projectors with Offset Only
For projectors without lens shift, if you know the offset you can easily tell whether the projector can be positioned above the screen, below it, or somewhere mid screen without needing to tilt it by looking at the percentage of offset. But to do that, you need to know how the 0% offset position is defined, whether the offset shifts the image up or down, and whether the direction—up or down—is based on the projector being right side up on a table or inverted in a ceiling or wall mount.
An example, complete with graphics, should help make this clear. Offset is measured as a percentage of the image height. So if a projector is going to be right side up, with the offset shifting the image up, a 10% offset would shift the image 10% higher compared with a 0% offset, or 10 inches higher for a 100-inch tall image.
So far so good. But you also have to ask, 10 inches higher than what? That's where knowing the 0% position comes in. If 0% is defined as having the centerline of the lens at the bottom of the image, the 10% offset will position the bottom of the image 10 inches above the lens centerline, as shown in figure 1.
On the other hand, if 0% is defined as the lens centerline being at the top of the image, the same 10% offset will position the top of the image 10 inches above the lens centerline, and leave the bottom 90 inches below it, as shown in Figure 2.
The point here, once again, is that if you know the amount of the offset, its direction based on either the projector being right side up or inverted, and how the 0% offset position is defined, you can fairly easily determine where the projector has to be relative to the screen to avoid needing keystone correction.
How to Read Our Lens Offset Descriptions
In our reviews, we describe how we measured offset in enough detail so you don't have to make assumptions. We almost always describe it with the projector right side up, and say that it's based on placing the projector on a table. If we give the offset with the projector in a ceiling mount instead, we'll say so.
In most cases, we give the offset as measured from the bottom of the image, typically saying something like, "The bottom of the image is 10% of the image height higher (or lower) than the lens centerline."
In some cases, it's more straightforward to use a different reference point. For example, "The lens centerline is at the screen midline," describes the same offset as "The bottom of the image is 50% of the image height below the lens centerline." However the first version is easier to picture without having to think about it.
The verbal description will be all that some people need to picture where the image will fall relative to the projector and decide if the offset will work for the setup they want. Others may prefer to draw a simplified picture, so they can see at a glance what the setup will look like.
Another option is to go to one of the projector calculators that most manufacturers provide, many of which include interactive diagrams that show the projector and image positioning. The graphics we used above, as well similar images throughout this discussion, are image captures from the BenQ Projector Calculator, cropped to show just the relevant part of the page. (Our thanks to BenQ for permission.)
Matching Lens Offsets to Projector Positioning
Whether you prefer using numbers or images as a way to interpret our descriptions of lens offset, there are some rules of thumb about which offsets work best for common projector placements.
All of the offsets in the following examples are based on the projector sitting on a table, so positive numbers mean the image is higher than the 0% position with the projector right side up, and negative numbers mean it's lower. A 0% offset is defined in all cases as the centerline of the lens being even with the bottom of the image.
Offsets in the range of roughly -50% (that is, 50% down) or more are good choices for positioning a projector on a bookshelf in the back of a room. The specific offset you need will depend on the ceiling height, height of available bookshelves, screen size, and how high you position the screen on the wall. In general, however, you'll want an offset of at least -50% (50% down), and the higher the bookshelf, the larger the magnitude you'll need. Figure 3 shows two possible positions for a projector based on different offsets while maintaining the same position for the screen.
Offsets of 0% to a few percent (roughly 0% to 5%) and higher are most suitable when placing a projector on a table. As Figure 4 shows, for a projector sitting on a table, a zero to small offset works nicely with a screen mounted so the bottom edge is essentially the same height as the lens centerline.
A small offset can also work with the projector upside down in a ceiling mount. But, as shown in Figure 5, the mount will need an extension pole to lower it so the top of the image isn't right at the ceiling (the top gray line in the picture), and even then you may have to mount the screen higher than you might prefer to.
Larger offsets. As you can probably tell from looking at the graphics for 0% to 5% offsets, slightly larger offsets, up to about 20% or so, are also good choices for positioning a projector on a table. The main difference is that the larger the offset percentage and the bigger the image, the lower the table has to be. For a 50-inch high image (that's 102 inches diagonal at 16:9), a 20% offset translates to the lens centerline being 10 inches below the bottom of the image. For a 100-inch high image (a 204-inch diagonal at 16:9), it would be 20 inches below the bottom. You obviously need to avoid a combination of image size and offset that would require positioning the projector below floor level.
Offsets of up to 20% or a little more are also good choices for mounting a projector on the ceiling. Compared with a 0% offset, they can eliminate the need for extension poles in rooms with typical ceiling heights. Here too, the maximum usable offset depends in part on the size of the image, with the ceiling height rather than the floor putting a limit on how large an offset you can use.
Getting the Right Numbers
These rules of thumb, and the graphics that go with them, should give you a good sense of where you can position a projector with a given offset. To nail down the position a little better, you can do the math, based on details like what size screen you have and where it's positioned on the wall, or draw a picture to scale. However, as already mentioned, most manufacturers offer projector calculators that will do the work for you.
The BenQ calculator we've used for the graphics in this discussion, for example, lets you enter your room size (including ceiling height), projector model (for BenQ models only), screen size, and screen position. It then calculates the right measurements for the projector position. You can also adjust the information interactively. If you change the screen size for example, all the other numbers are recalculated. Most manufacturers' calculators are similar. (Of course, ProjectorCentral's interactive calculator can also be used for most projector models if all you need is the throw distance or range of distances for a particular screen size.)
Its important to know that even after doing the math—or letting a website do it for you—the positioning is still only approximate, so you may have to adjust it once you have a projector in hand. How closely the offset for any given unit matches its specification depends on the tolerance allowed in the assembly line. One projector we reviewed recently had a offset spec (as we've defined it here) of 16% +/-6%, which means the offset for any actual unit can be anywhere from 10% to 22%. Some manufacturers list the tolerance range in their specs. Others don't.
Projectors with Vertical Lens Shift
Armed with a solid understanding of how lens offset determines the best positioning for a projector, you also know everything you need to understand vertical lens shift. Since vertical shift simply moves the offset up or down, all the same information applies.
As with lens offset, we describe vertical shift in enough detail in our reviews so you can calculate the range of offsets if you need to or draw a picture so you can visualize it. A typical description for a modest lens shift might read, "With the projector right side up, the bottom of the image can be anywhere from even with the lens centerline to about 15% of the image height above it."
Some projectors offer a larger shift that has a defined neutral point where a motorized shift will typically pause when moving through the range. This is often—but not always—the midpoint. In this instance, we might describe the offset at that position and then give the range up and down from that point as a percentage of the image height. No matter how we describe it, you'll want to determine the offset at the top and bottom of the shift range, and then decide if any offset within the range will let you position the projector where you want it.
As with lens offsets, lens shift ranges vary from one unit to another. Unless you're counting on positioning the projector where it requires one or the other extreme end of the range, however, the variation won't matter. Note too that even small lens shifts can be particularly helpful for installing a projector in a ceiling mount. They let you be a little less precise in installing the mount, and still avoid having to tilt the projector to aim at the screen. If you're upgrading to a new projector, they can also make the difference between letting you use an already installed mount without needing keystone correction, or having to move it.
Projectors with Both Vertical and
Horizontal Lens Shift
Horizontal lens shift offers the same kind of setup flexibility as vertical lens shift. It lets you shift the image left or right without having to swivel the projector, so you can position the projector off center from the screen and not need horizontal keystone correction.
We describe horizontal shift in our reviews as a percentage of the screen width left and right of the screen centerline, typically saying something like, "the horizontal shift as measured from the centered position is +/- 20% of the image width." For a 100-inch wide image, that would translate to being able to move the image a total of 40 inches from one extreme to the other.
A notable complication with having both vertical and horizontal lens shifts is that they typically interact with each other, so the available horizontal range can get smaller as you approach the extreme of the vertical range, and vice versa. This means the area over which you can shift the lens, image, and projector position is not the rectangle you would get if you could use any setting for either shift with any setting for the other.
It's also important to know that the shape of the range is different for different projector models. Figure 6 shows some of the possibilities, from one that looks like a rectangle with the corners cut off, to one that cuts the corners so much that it looks more like a stop sign, to another that looks like a fat arrow pointer.
Unfortunately, there's no easy way to describe the full area of the combined horizontal and vertical lens shifts in a way that will let you calculate all possible positions. That limits us to giving a general description in our reviews of the how the two shifts interact with each other.
In most cases, that should be enough to tell you whether the projector's lens shift will be enough to let you position it where you want it without having to use keystone correction. If it isn't, the best strategy is to go to the manufacturer's website and look for a projector calculator that will give you the detail you need. If you can't find the answer on the site, ask a dealer before buying, and make sure you buy from a dealer who allows easy returns if the lens shift turns out to be unsuitable for your setup.
Ultimately, the lesson in all this is simple. Lens offset and lens shift matter. If you want the best image quality, you want to avoid using keystone correction. And to avoid having to use it, you need to consider lens offset, lens shift, and how you plan to position your projector as an important part of your buying decision.