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.
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
Lens Shift example:
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:
16: 9 =
Throw Ratio =
Factor x Focal
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)
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
Minimum Throw Distance = 2 x 3 = 6 Meters
Maximum Throw Distance = 2.5 x 3 = 7.5 Meters
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.
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.