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V1.0 – CMAD 04/15
Lens Optical Performance Characteristics Explained
Explained
GENERAL RELEASE APPROVED
Lens Optical Performance Characteristics Explained
Introduction
Optical assemblies are one of the main performance differentiators within a projector. Components in
brief comprise integrating rods, mirrors, lens arrays, prisms and of course the projection lens. For the
purpose of this document we will concentrate on the projection lens and the terminology used to
describe its performance characteristics in simplistic terms.
Throw Ratio
Is a factor that describes the relationship between image size and distance to the display surface
(screen).
Lens Shift
Allows the lens position to be shifted up, down, left and right within the projector body. Shift provides
image adjustment within pre-determined parameters without introducing image distortion (tilting a
projector chassis to shift an image introduces Keystone errors).
Working F#
Describes the lens aperture size (i.e. the light path opening through the lens / how much light will come
passed):-
A low F# number gives higher brightness.
A high F# number gives reduced brightness, but increased contrast.
Depth of Focus performance characteristics can be associated with the F# rating.
Zoom Ratio
Describes how much a lens can zoom. It is the ratio between the shortest and longest range of a lens.
Focal Length
A measure of a lens converging ability, expressed as the distance between the DMD and the focus point
within the lens:-
A long Focal Length gives a telephoto angle (i.e. a small image).
A short Focal Length gives a wide angle (i.e. a large image).
Focus Range
The distance at which the lens is designed to give optimum performance.
MTF
Modulation Transfer Function describes lens performance:-
Optical resolution.
Details in an image, etc.
The higher the number the better.
Lateral Colour
Describes a lens ability to converge light of differing wavelengths. How much backsliding (error) of
each colour is acceptable. Less than 1 pixel is good:-
For DLP products 1 pixel equals the size of one DMD mirror. This equates to 7.5 micron @
WQXGA resolution on a 0.9” DMD.
Optical Distortion
Image deflection. Typically pincushion and barrel (curvature in the extreme top/bottom of an image):-
Artefacts are typically worse on a Wide Angle lens.
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