Projector Lenses Demystified

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How much should the average installer know about projection optics, beyond specifying a certain focal length for a specific projection throw and image size? The answer is simple, the more you understand how the projector’s lens affects the quality of the projected image, the better you will be at choosing the right projector.

In most cases, the lens that comes with a given projector is the only one it can use, so you might think: “so what? The lens it has is the lens it has.” Even though you can’t change the optics in these projectors, you should still understand how the performance of this lens can be improved by understanding its capabilities. The projector's lenses and the quality, and features of those lenses can significantly impact the quality of the projected image.

Throw Ratio and Zoom Range

When choosing a lens, its throw ratio is an important factor. Throw ratio is the ratio of the distance from the lens to the screen (throw) to the screen width. Lower throw ratio results in a shorter throw distance. A variable-zoom lens' throw ratio can be adjusted for example from 1.4:1 (wide angle) to 2.73:1 (telephoto) which means it has a zoom range of about 2.1.

The lens on many 3LCD and LCoS home theater projectors have a zoom range of 2:1. Depending on the throw ratio of the lens, for a 100″ diagonal 16:9 screen, that means the projector could be mounted between 10.5 feet to 21 feet away.

How Zoom Range Affects Brightness

When shopping for home theater projectors, brightness is one of the most obvious specs you see, defined here as Lumens. The interesting thing is that depending on how/where you set up your projector, brightness can vary by over 30 percent.

Where a projector is placed, along with the throw range and quality of the lens, can have a noticeable impact on light output. When a lens is in wide-angle mode (the largest image it is able to produce from a specific distance), it will produce more lumens. When it is in telephoto mode, it produces less.

A lens with a 2:1 zoom can provide tons of placement flexibility, but remember if you mount it 10.5 feet from the screen, the projector will output more light (Lumens) than if it is mounted 21 feet back.

There is no straightforward formula, because a lens' design impacts the actual amount of brightness change, but with a 2:1 lens, it could be as much as 30%.

If, on the other hand, the projector has a far more limited lens, say with a zoom ratio of 1.2:1, then the change in brightness from one “extreme” to the other, becomes minimal, and not a serious consideration.

Most of the DLP projectors have limited zoom lens (1.1:1 to 1.3:1), so there isn’t much to concern ourselves with there. But, with the 3LCD, and LCoS projectors, where all of them seem to have at least 1.5:1, and most are around 2:1, where you place the projector can matter a lot.

Ultra Short Throw Lens

An ultra short-throw projector has a lot of benefits, but for the sake of this article we're just going to focus on the benefit of the ultra-short throw lenses.

With ultra-short throw projectors, there is no worry about someone interrupting projected content by walking in front of and breaking the projector's light path, casting shadows on the screen. You can also use ultra-short throw lenses to create a cleaner and more seamless visual area, with no obstruction to seating, maximizing the space.

No more projectors over your head to maintain line-of-sight when space is tight, giving you a range of options for installation. Whether you’re outfitting a confined space, a typical ultra-short throw projector lens can display an image up to 120” from less than two feet away. A UST lens combined with an ambient light-rejecting screen is an excellent solution for an environment with a large amount of ambient light.

Ultra-short throw lenses are available for many interchangeable lens projectors but are usually more expensive than a standard lens due to their complex optics required to project a non-distorted image at an extreme angle. This is also why a UST projector or "Laser TV" usually costs more than a standard projector with the same performance. Ultra-short throw lenses typically offer little to no zoom range and have a throw ratio of 0.37:1 or less.

Short Throw Lenses

Short throw lenses can project larger images from a shorter distance than a standard lens, for example about 120 inches from less than 8 feet away. If a large image is desired in a shallow room or the projector needs to be placed in front of the viewers, this type of lens is a great option. A lens with a throw ratio between 0.38:1 to 1.4:1 is considered short throw.

Standard Throw Lenses

Projectors equipped with standard throw lenses are often less expensive than their short-throw counterparts. They are the most common solution utilized in classrooms, conference rooms, as well as home theaters. Unlike the short throws, if someone or something breaks the projected light, it will block the image, obviously depending on where the projector is mounted, but something to take into account.

Consider the reason you get a 2:1 zoom on most of these projectors is to allow flexibility not just to ceiling mount the projector, but also to give you the option of mounting the projector on the rear wall of your room. Projectors with wide-range zoom lenses mean that probably 90% of people can rear shelf mount the unit.

Rear shelf mounting can save money, regarding the length of cables as well as installation costs – as it’s harder to get power to most ceilings, and if you have fairly high ceilings, working up there means ladders or scaffolding. For most, shelf mounting is just plain simpler and less expensive. A lens with a throw ratio between 1.4:1 to 4:1 is considered standard throw.

Long Throw Lens

Sometimes the projector or projectors need to be mounted very far away from the screen. This often occurs in large lecture halls, ballrooms, concert venues, and houses of worship. A lens with a throw ratio above 4:1 is considered a long throw.

The Benefits of Lens Shift

Achieving optimal image positioning, size, and shape can be a critical part of setting up the best possible solution for your installation. Having a large amount of zoom and lens shift capability makes installation a breeze, especially when you are trying to replace an older unit that was previously fixed mounted.

Lens shift is a feature that assists in properly aligning the projector’s lens with the screen. It is far easier to correctly mount or position a projector for optimal image quality with lens shift than without. Projectors with vertical and horizontal lens shift have the ability to move the projected image up and down and left to right, to more easily accommodate placement/installation at a wider variety of installation locations.

Lens shift enables users to shift the projected image along the horizontal and vertical axis without changing the angle relationship between the projector and the screen – eliminating the need to physically move the entire projector and allowing successful installation in spaces with limited placement options, such as low 8-foot ceilings, whether ceiling mounted or placed on a rear shelf. You can preserve headroom in spaces that would otherwise require an extender pole for accurate positioning and achieve correct image positioning without the need for digital keystoning.

Conversely, projectors without lens shift must be positioned with 100% accuracy to project an image with the proper shape and distribution across the screen. This is usually quite difficult so many installers may resort to digital keystoning which decreases onscreen resolution and adds unwanted artifacts.

Anamorphic Lenses

Most movies that have been made in the past several decades have been filmed using anamorphic lenses so it makes sense to use the same type of lens to project and get the best performance from the projector when watching movies. 

Anamorphic lenses dramatically improve the brightness and visual performance of home theater movie experiences by projecting all the pixels from your image onto your screen, not above and below your screen as letterboxing does, read below...

Letterboxing refers to the practice of projecting a film that was shot using a widescreen aspect ratio (using an anamorphic lens) to a standard-width format which ends up with black bars above and below the image, setting the projector’s lens to zoom up a letterbox movie, which is a movie with black bars on each side to fill a cinema-format screen. 

Letterbox zooming uses the projector’s lens setting to zoom the letterboxed movie making it fill a full cinema-formatted screen. This is an easy way to keep those black bars off of the screen by actually projecting them on the wall above and below the screen. After all, a 4K projector should be using the entire 4K for a full screen movie experience, not the 3K performance that happens when you use letterbox zooming.  

Now, even though there are several benefits to using anamorphic lenses, there are a few things to consider. For example, some anamorphic lenses can shed between 5-8% light, depending on the design and quality making for a slightly less bright picture. 

Also, depending on the quality of the lens, the image can be slightly softened which is caused by the additional optics in the light path which can also slightly decrease the black levels and dynamic range due to the light scatter of the lens. 

Typically both of these scenarios can be avoided if using a higher quality lens, but that will of course, cost a bit more as well.

Summary

I began this article by asking the question, how much knowledge about projection optics is enough knowledge? The more you understand about how a projectors lens ultimately impacts the resulting on-screen image quality, the better chance you have of improving the quality of that very same image. Understanding the difference between ultra short throw, short throw and long throw will help you strike the appropriate balance when deciding where to place your projectors. Taking advantage of any lens shift capabilities of your projectors lens will limit, or completely eliminate, the need for the use of keystone. And finally how anamorphic lenses can both positively and, depending on quality, negatively impact viewable resolution and picture quality.

I hope this information helps you make the best possible decisions when choosing the components for your projection solution.