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Projection Lenses

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track

A lens is an optical device which transmits and refracts light, converging or diverging the beam. A Projector use a variety of lenses and prisms to first split  the light into the 3 primary colours (3 Panel LCD and DLP projectors) Red, Green and Blue light is then reflected by (or passes through) the projection panels. Lenses then recombined the primary images into a full colour image, before directing the light through the main projector Lens and onto your screen.

Zoom lens

 

If you are using your projector for mobile applications, a zoom lens can be very handy. It gives you the ability to set the projector at a convenient location, and adjust the projected image size. Without a zoom lens, the only way to adjust the picture size is move the projector forward or back. A unit with a zoom factor of 1.2x will only let you adjust picture size by 20%. You can often move the projector a foot or two either way and accomplish the same adjustment. Nevertheless, if you have a fixed screen size you are trying to fill, even a limited range zoom will make it easier to fine tune the image size to the screen.

 

Short Throw Lenses

 

When the throw distance from the projector to the screen is restricted a Short Throw Lens may be necessary to allow focusing of the image. Short throw lenses increase the possibility of hot spotting.

Long Throw Lenses

 

Ideal when the projector can not be positioned close to the screen. Often used in auditoriums, Churches and conference rooms to allow positioning of the projector behind the audience.

Micro Lens Array (MLA)

You may have noticed on the projector specifications under projection system "with MLA". Micro Lenses are small lenses, generally with diameters less than a millimeter. In projectors the main aim of the Micro Lens Array is to redirect the light that would otherwise be lost inside the projector through the prism and out of the lens. This has two benifits one is an increase in brightness and the other is a reduction in the heat this light would have created inside the projector.

Lens Shift

 

A growing number of projectors now come with Lens shift allowing you to position your projector off center to the screen. Horizontal lens shift permits placement either to the left or right of the room, Vertical gives more flexability above or below the screen. Lens shift changes the projected image inside the lens maintaining the maximum resolution of your image. Keystone Correction changes the image projected at the projection panel (lcd or dlp etc.) and reduces the resolution of  the image.

 

Lens Shift example:

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Lens Shift center 
Lens Shift + 50% 
Lens Shift minus 50% 

 

Throw Ratio

 

Every projector has a different set of lens characteristics. One of the more important characteristics of a lens is its throw ratio. Throw ratio is the distance from the screen that a projector needs to be in order to create a specified size image.

Throw ratio is usually defined by the width of the image. For a fixed lens with no zoom the Throw ratio is quoted as 2:1 where 2 refers to the Throw Distance and 1 refers to the screen width at that distance. So for a throw ratio of 2:1, the projector would need to be 2 meters away from the screen to give a 1 meter image. Zoom lenses have an adjustable throw ratio between the minimum zoom and the maximum zoom. An example of the throw ratio for a zoom lens is 2-2.5:1, the 2-2.5 refers again to the Throw Distance. At a distance between 2 meters and 2.5 meters an image size of 1 meter is possible by adjusting the zoom setting.

 

Calculating the Throw Ratio from the Focal Length of a Lens

 

The throw ratio of a projector can be calculated from the focal length of the lens and the diagonal size of the projectors internal panel.

A projector with a zoom lens will have two focal lengths, one for each extreme position of the lens - Wide-angle and Tele.

A constant factor has to be used to calculate the throw ratio, this factor is dependant on the native aspect ratio of the projection panel. Approximately these factors are:

 

4:3 = 1.25
16: 9 = 1.15
16:10 = 1.17

 

Throw Ratio =  Factor x Focal Length
Panel Diagonal

 

Note:The focal length and panel diagonal must be expressed in the same unit (e.g. mm). If you are using the projector close to the extreme position of the lens - remember this is an approximation. Check with the manufacturer to confirm your calculation.

 

Using the Throw Ratio

 

The Throw Ratio can be used when either the Throw distance is known to calculate the image size at that distance or when the screen size is known and the throw distance is variable. Both use the formula below.

 

Width (W) x Throw ratio = Throw Distance (D)

 

Example 1:

 

Calculating the Throw distance from the screen size - if we consider a projector with the 2-2.5:1 throw ratio and  a screen 3 Meters wide.

 

Throw Distance (D)  = Width (W) x Throw ratio

 

Minimum Throw Distance = 2 x 3 = 6 Meters

 

Maximum Throw Distance = 2.5 x 3 = 7.5 Meters

 

Example 2:

 

Calculating the Screen size from the Throw distance - if we have to place the projector at 5 Meters from the screen, what would the Image size be?

 

Width (W) = Throw Distance (D) / Throw ratio

 

Largest image = 5 / 2 = 2.5 Meters

 

Smallest Image = 5 / 2.5 = 2 Meters


One thing to note when using the Throw Ratio is that it will give you an answer for any throw distance and any image size, remember the image still needs to fall in the focus range of the projector.

 

Costs

 

If your projector position is not flexible and you require either a Short or Long throw lens, Look for a projector that has optional lenses available in our specifications, details of the lenses can normally be found in the PDF brochure. If possible look for a projector that comes with the lens you require as standard, Purchasing lenses separately can be very expensive.

 

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