Tests possible
with this setup
On-axis and off-axis tests
 The
first step is to examine the performance of the lens
on (or near) the optical axis. However, most practical
applications require that the lens also image well
over a specified field-of-view. Sometimes a lens achieves
good performance over a surprisingly small FOV, so it
is important to check operation across the full field
that will be used in the application.
The best method for off-axis testing
depends on the pupil location of the lens. In many cases
the lens may be rotated on the nodal stand as shown
below:
In other situations it may be desirable
to rotate the telescope with the optional off-axis mount.
Plane of best focus
For a well corrected lens this is simple—adjust
the microscope axial position until the image is sharpest.
The location of the plane of best focus may be read
directly from the digital micrometer.
If the lens has aberrations then the
concept of a simple "plane of best focus"
may not apply. For example, if the lens has astigmatism,
there will be two distinctly different points of optimum
focus, one for X features and one for Y features.
Image quality measurements
There are many ways to evaluate image quality, including
MTF, 3-bar contrast, and visual examination of the PSF,
just to name a few. These measurements are common to
many different OpticStudio setups, so they are discussed
in a separate chapter on measuring image quality.
Measuring EFL
"EFL" is the effective focal length of the
lens. EFL can be calculated by measuring the size of
a known feature. (Recall that short focal length lenses
form smaller images.) The EFL wizard in PixelScope makes
it easy to do this with a few clicks of the mouse. For
more information see measuring
EFL.
Measuring BFL
"BFL" or Back Focal Length is the distance
to the plane of best focus, measured from the mounting
surface of the lens. While this is obviously related
to EFL it is not identical. BFL is read directly from
the digital micrometer. For more information see measuring
BFL.
Measuring Magnification
When a lens operates at infinity linear magnification
is not defined. Instead, magnification must be specified
in angular terms, with units of [mm of image height
per radian of object height]. Stated in those terms
the angular magnification is exactly equal to the EFL
in mm. For more information see measuring
magnification.
Axial color
"Axial color" means that the best focus plane
is not the same for all colors. In a perfectly corrected
lens, the plane of best focus remains constant over
a range of wavelengths. However, in most real-world
lenses, changing the narrow band filter in the collimator
will cause a shift in image location. This shift may
be read directly from the digital micrometer. For more
information see measuring
axial color.
Lateral color
"Lateral color" means that the location of
the image is not the same for all colors. Because of
symmetry, lateral color is not a problem on-axis. However
at off-axis field points the image location may be slightly
different for different wavelengths. Any shift is due
to color aberration in the lens, and may be measured
directly with PixelScope. For more information see measuring
lateral color.
Distortion
"Distortion" means that the magnification
is not the same at all field points. PixelScope does
not have a tool for direct distortion measurement. However,
distortion may be inferred from measurements made in
PixelScope. For more information see measuring
distortion.
Field Curvature
"Field curvature" means that the plane of
best focus is not the same at all field angles. PixelScope
does not have a tool for direct measurement of field
curvature. However, field curvature may be inferred
by plotting the plane of best focus vs. field angle.
For more information see measuring
field curvature.
The
setup with a pivoting collimator makes it particularly
easy to measure field curvature and distortion.
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