From aberration to zoom
A projection lens primer
Telescopes, microscopes, binoculars, projectors,
spectacles: humanity has understood the power
of the lens for almost three millennia. The oldest
lens artifact dates back 2700 years to ancient
Assyria and is thought to have been used to start
There are many types of lenses with specialized uses, such as cylindrical lenses that
focus light into a line instead of a point and lenticular lenses used to print images
with the illusion of depth. In this paper we’re interested in “everyday” lenses, the
type used in spectacles, magnifying glasses and projectors.
In or out?
fires by concentrating rays of sunlight.
That is the essence of what a lens does: it alters
the path followed by rays of light in a process
There are several types of lens, categorized by the curvature of the two faces. A
convex lens is thicker at the center than the edges, with two surfaces that curve
outward from the center, like a lentil. This is the oldest type, and the one that gives
the lens its name: lens is Latin for lentil.
called refraction. A converging lens concentrates
light rays, focusing them to a single spot beyond
the lens called the focal point. A diverging lens
does the reverse, spreading a beam of light apart.
A concave lens has two surfaces that curve inward. If one of the two surfaces is flat,
the lens is plano-convex or plano-concave, depending on the curvature of the
other surface. Last - but definitely not least - are meniscus lenses, with one concave
face and one convex face. These are the type used in eyeglasses and contact lenses,
making them the most widely used lenses today.
Lens shapes. Note the two faces need not have the same curvature.
To concentrate or to spread: that is the question.
Concave and plano-concave lenses are always diverging; convex and plano-convex
lenses are always converging. A meniscus lens can be converging or diverging,
depending on the relative curvature of its convex and concave faces.
The axis of a lens is the line running through the lens’s center perpendicular to its
two faces (the dotted line in the image above).
To the point
The focal length of a lens is a measure of how strongly the lens
concentrates or spreads light. A lens with a shorter focal length bends
parallel light rays more strongly, bringing them to a single point in a
shorter distance.
Spherical aberration
Spherical aberration can be minimized by carefully tailoring the
curvature of the lens faces to the specific application.
The greater the curvature, the more strongly the light’s path is altered.
Coma occurs when the lens images an object that is off its axis,
so that the light rays pass through the lens at an angle to the axis,
rather than parallel to it. Rays passing through the center of the lens
are focused at a different point than those passing through its edges,
resulting in a V-shaped flare, or “comet”.
Set it here, see it there
Image
Converging lenses can be used to project a larger (or smaller) image
of an object located behind the lens onto a wall or screen in front of
the lens.
Coma
As with spherical aberration, coma can be minimized or even
eliminated by tailoring the curvature of the lens faces to the
application.
Image projection using a converging lens.
Note that the projected image is inverted - upside down - relative
to the original object. Projectors use compound lenses - multiple
lenses with different properties - to project the image in the right
orientation. Compound lenses can also be used to cancel out common
optical aberrations that produce distorted images.
Chromatic aberration occurs when the refractive index of the lens
material - the amount it bends the light - changes with the light’s
wavelength. (The wavelength determines the color; red light has
a longer wavelength than blue light.) Different wavelengths will
be focused to different points - or dispersed - producing an image
surrounded by fringes of color.
Aberrant behavior
No lens forms a perfect image; a lens always introduces some degree
of distortion, known as aberration. Let’s look at three of the most
common types.
Most lenses are spherical, with two faces that are parts of the surfaces
of spheres. These are by far the easiest type to make, but spherical
surfaces are not the ideal shape for a lens. As a result, these lenses
are subject to spherical aberration: beams of light near the edges
of the lens are focused in a slightly different place than beams close
to its center, producing a blurred image.
Chromatic aberration
Chromatic aberration can be minimized by bonding together two
materials with different dispersions or by using fluorite, a naturally
occurring crystal with very low dispersion.
These are the three most common types of lens aberration; other
types include field curvature, barrel and pincushion distortion, and
astigmatism. As noted above, carefully choosing the lens material
and curvature and combining multiple simple lenses in a compound
lens can greatly reduce or even eliminate optical aberrations.
Factors to consider when choosing a projector lens
There are three main features that affect how well a projector lens
will work for your specific venue and application. Let’s take a look at
each one.
In film terminology, throw is the distance between the movie
projector’s lens and the screen - that is, the distance over which
the image is “thrown” to land on the screen. Projector lenses are
characterized by a related term, the throw ratio, which is the ratio
of the throw distance to the screen width.
Throw Ratio
Lens Type
Throw distance
0.1:1
Ultra Short Throw
20cm
0.8:1
Short Throw
1.6m
1.3:1
Short Throw
2.6m
2.0:1
Standard
4m
4.3:1
Long Throw
8.6m
5.7:1
Long Throw
11.4m
9.0:1
Ultra Long Throw
18m
Examples of throw distance for a given throw ratio to achieve a 2-meter
wide image
Many projectors have zoom lenses, which enable you to increase
the projected image size without moving the projector. This
gives you some flexibility in where you install it, and also makes
it easier to tweak the image size - no need to shift the projector
a centimeter forward or backward to fit the image exactly to the
screen. Zoom is also useful in mobile applications, where venue size
and projector placement can vary. The most common zoom ratio is
1.2x, representing a 20% change in image size.
A projector with a zoom lens has a throw ratio range rather than a
single number, reflecting the range of image widths available at a
given throw distance.
2X Zoom
3.9 m
7.8 M
3.9 m / 12.8 feet
7.8 m / 25.7 feet
120-inch screen
Projector throw is D, the distance to the screen; throw ratio is D:W.
The throw ratio tells you where you need to place your projector given
the size of your screen. For example, a throw ratio of 1.2:1 means your
projector should be 1.2 meters away from a 1-meter-wide screen. If
you want to fill a 2-meter-wide screen, you should place the projector
2.4 meters away. Throw ratio is useful in comparing projector lenses
to find one that meets the requirements of the venue.
The standard throw ratio for a projector lens is around 2.0:1. Short
throw lenses allow you to place the projector much closer to the
screen; ultra-short throw lenses can even be installed on the wall
just above the screen. This is useful in classrooms, for example; a
throw ratio of 0.1:1 means the lens can be placed just 18 centimeters
from a 2-meter diagonal interactive whiteboard, enabling the teacher
to write on the board without blocking the projected image. Long
throw lenses are useful in large venues such as theaters and
conference rooms.
A zoom lens lets a projector display the same image size at different throw
distances, or different image sizes at the same distance.
An optical zoom lens physically changes its focal length by moving
the lens toward or away from the image sensor. A digital zoom
lens doesn’t actually zoom at all: it digitally enlarges the image by
interpolating pixels. This can affect image quality, so optical zoom is
the better choice when image quality is important.
You have to physically adjust a manual zoom lens yourself; you
can adjust a power zoom lens (or motorized zoom lens) using a
remote control.
Finally, lens shift enables you to move the projected image up and down and left and right, giving you greater freedom of choice in where
to install the projector. In general, lens shift is measured in one of two ways: either as a percentage of the total projected image height
(or width), or as a percentage of half the projected image height (or width). Most manufacturers, including Barco, use the second method.
Following this method, a vertical lens shift of 100% means you can install the projector flush with the top or bottom edge of the screen, for
example.
Horizontal position
Vertical position
Horizontal and vertical lens shift give you freedom in projector placement.
How to find the right lens for your situation
So now you’re up to speed on projection lenses! To help you determine what type of lens best suits your projection needs, we’ve created
a handy lens calculator that does all the math for you: http://www2.barco.com/en/media_entertainment/lenscalculator/. Give it a whirl!
www.barco.com