Short Throw Projectors

Short throw projectors

Projector throw distance

When doing your research on which DLP or LCD projector you want to buy, you will most likely have seen the “throw distance” of each projector quoted on the projector specification sheet. A projector’s throw distance basically refers to the length between the projector’s lens and the projection screen – i.e. from how far away the image can be projected while still staying in focus. You may also have seen projectors referred to specifically as “long throw projectors” or “short throw projectors”.

Throw distance can be worked out by simply measuring the distance between the projector and the screen it is projecting on to. A typical projector throw distance – to ensure an optimal image size – is one foot of screen width for every two feet between the projection screen and the projector.

However, the lens that the projector uses can alter this ratio quite substantially as projector lenses can be tailored to render small images over long distances, or big image projections over a comparatively small distance. This Bamboo AV Advice article considers the second of these two examples – projecting an image on to a screen or other surface from a very short distance away. This is what is known as a short throw projector lens. In this Bamboo AV Advice article we will explain how short throw projectors work, and give you an idea of how they can be used to add flexibility to your multimedia presentations.

What is a short throw projector?

A short throw projector is a special type of projector which has been specifically designed to project larger images when there is only a very small distance between the projector and the projection screen. This can be for cosmetic reasons – i.e. that the projector needs to be hidden, or the reason can simply be a room size constraint.

While standard projectors can be adapted for short throw purposes by installing an external short throw lens, specific short throw projectors are manufactured with the short throw lenses already installed. These are generally considered to be the best option for short throw projection.

There are several examples of dedicated short throw projectors on the market, including the Hitachi CP-X275, the BenQ MP771, and the NEC WT610 which are all popular short throw projector models. These projectors have been specifically designed for projecting images from close to the projection screen. By using an extremely short throw lens and a special short throw optical system, these short throw projectors can project a 60-inch wide (diagonal) image from only five feet away.

The image below shows the difference in positioning between a regular projector in the top image, and a short throw projector in the bottom image. Note than for the regular projector, the projector has to be placed back in the audience, which can cause problems with shadows being cast on the screen. The short throw projector is able to project the same size image, but it can be placed in front of the entire audience.

Example of a short throw projector

How does a short throw projector work?

Short throw projectors actually function in much the same way as any other projector does. The way in which a short throw projector produces an image internally is basically the same as a regular projector, although the optical system in a short throw projector may be tweaked slightly in some models – for example in the Hitachi CP-X275 which has an ultra short throw optical system and a free shaped lens and mirror.

The big difference, and what allows a short throw projector to project high quality images from an extremely short distance away from the screen, is the magnifying qualities of the short throw lens being used. These magnifying qualities are determined by the short throw lens’s shape.

What is the difference between short throw and long throw lenses?

The physical shape of the projector lens is what differentiates short throw and long throw lenses. The lens’s shape radically alters the size of the image projected. A long throw lens can keep the projected image small and tight when the projector is placed far away from the screen. This type of lens is applicable in situations like a church where the projector needs to be placed hidden away at the back of the building. If this type of lens was not used then the projected image would appear too large and too unclear. Similarly, if a short throw lens was not implemented in situations where the projector is placed close the screen then the image would appear too small to be read clearly. In a short throw projector, the lens is slightly bent to allow an image of a greater size to be projected.

Hitachi, in their short throw projectors, now use a brand new type of lens which has many more variables on its surface than a regular projector lens, which allows for a far greater adjustment of the image which in turn enables a focus that is much improved, better uniformity and better geometry when compared to standard lenses. Using this free shaped lens technology, Hitachi have also included an offset or lens shift, meaning that the projector can be positioned from completely outside of the display area. Hitachi also uses a free shaped mirror in addition to the free shaped lens, which is used to correct distortion without adding chromatic aberration, and to provide keystone correction.

Limitations of regular projectors

Standard projectors are impeded by their ability to adjust to varying room specifications and changing presentation situations. Problems present themselves if the projector is not positioned at a set distance and a set angle to the screen. While it is sometimes possible to use lens shift and zoom lens capabilities to try to remedy these problems, when using a projector at extremely close or far away distances then a dedicated lens is preferable to ensure that the optimal image is projected on to the screen.

Where are short throw projectors beneficial?

There are numerous benefits to be gained from using a short throw projector in the right situation. The primary advantage is the flexibility it gives to users when placing projectors in a small room since a short throw projector can project a large image from a short distance. They are particularly suitable for schools, small businesses or any other fairly intimate environment:

  • Smaller sized classrooms
  • Home theatres with limited space
  • Small conference rooms, such as those often found in hotels
  • Advertising stands and booths at exhibitions
  • Small home theaters

A particular advantage for those people that frequently use projectors is that short throw projectors help eliminate any eyestrain as the light shining from the projector does not shine directly into the presenter’s eyes. This is a big plus for teachers who might use projectors all day long!

A further benefit is that since the image has to travel a shorter distance, the effect of light ambience is lessened, thus resulting in a much brighter image.

Short throw projector pricing

It is possible to turn any standard projector into a short throw projector by purchasing and installing an external short throw lens. The price of these lenses can vary from $1,000 to $6,000 depending on the quality of the short throw lens and the projector model that the lens is made for. Generally, a more expensive lens will output a larger and sharper image that can be projected from a shorter distance from the projection screen.

The problem is that installing an external lens can potentially void the warranty on your projector, and that not every projector can accept external short throw lenses. This means that often purchasing a dedicated short throw projector can be more advisable.

The price of these projectors very ranges from $500 to $5000. The more expensive short throw projectors usually offer a better quality lenses and thus a bigger, better quality image when closer to the screen. A good guideline is that as price increases, you can move the projector closer to the screen without reducing the quality of the projected image.

We hope that this Bamboo AV Advice article has been useful to you!

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