Peripheral
darkening (vignetting) in microscopy and macroscopy photography
Vignetting in micro or macrophotography is peripheral darkening at
the edge of the image. This effect can be either desirable or
undesirable depending on the field of application.
Vignetting
is categorised
according to different degrees of severity:
In addition,
the symmetry of the vignetting is also judged.
An
asymmetrical appearance to the vignetting ring is the first indicator
that the optical system is not correctly centred.
Mostly
the camera unit is not positioned perfectly over the microscope’s
optical axis.
In
rare cases, the camera's sensor chip is not correctly positioned
inside the camera body.In
normal photography,
these production tolerances mostly go unnoticed. However,
when used in conjunction with a microscope, all mechanical
irregularities and image distortions in the parallel light path are
very much intensified.
Centric and non-centric position
of the vignetting ring:
 |
 |
|
centric position |
non-centric
position |
Workaround:
If
it is not possible to adjust the optical system so that it is
precisely centred, then with cameras with in-built zoom lenses it is
possible to zoom in to the image until the vignetting and
misalignment disappear.
The
image quality is not disadvantaged by this approach, as in the
process any image section within the vignetting is enlarged to a full
format image.
Depending
on the field of application, vignetting may be acceptable. In
the case of gastroscopic or endoscopic photography in medical use,
100% of the round image must be reproduced on the rectangular camera
sensor.
The
doctor needs to view 100% of the information on the image transmitted
through the fibre optics, and will therefore accept vignetting and
blurring around the edges of the ring.
In
microscopy, more importance is placed on achieving a full format,
rectangular reproduction of the field of view than on achieving a
circular reproduction of the field of view that is true to the
original.
The
camera sensor should be illuminated in such a way that there is no
possibility of vignetting.
With this optical adjustment, the camera’s pixel count can be
utilised to the optimum.
What
actually causes vignetting?
The majority
of optical elements such as lens elements, apertures, tubes and lens
units are manufactured as round elements.
Only
the camera sensor is made as a rectangular electronic component.
Vignetting
is therefore the reproduction of the optical edge of the microscope’s
light path.
By
reproducing the round microscope image on the rectangular camera
sensor, all of the areas of the image lying outside of the round
microscope image remain unlit and thus black.
Only with greater magnification
of the round microscope image is the rectangular camera sensor
exposed to light in such a way that the image is illuminated over the
entire sensor area without peripheral darkening (vignetting).
If
high image quality is important to you, then optical adjustment
between the microscope and the camera is indispensable. The LM
digital adapter ensures that the image is projected onto the camera
chip at the optimum size. Furthermore,
the microscope’s chromatic image distortions must be compensated
for.
As
a result of the interaction between the optical limit and the light,
there are considerably more serious image distortions on the edges
than in the middle of the optical axis.
If
the adapter does not fulfil its function to the optimum, then there
will be flaws in the reproduction (chromatic and spherical
aberrations) and vignetting.
An
optimum adjustment between the microscope and the camera is only
possible if the camera’s lens can be removed.
The
camera lens unit with its many optical elements (up to 19 lens
elements and more in the case of zoom lens units) only has the effect
of reducing quality in the microscopic light path and is not needed
in microphotography. For
this reason, digital SLR cameras and professional camcorders are
equipped with interchangeable lens mountings.
In this case,
the microscope adapter is directly connected to the camera body via
the bayonet mount.
The
most common causes of vignetting in microphotography and
macrophotography:
-
Optical
axis of camera and microscope not in line
-
Camera
lens limits the light path
-
No
optical adjustment between the microscope and the camera
-
An
unsuitable projection lens is already integrated in the microscope’s
photo tube or C-mount connection
-
The
optical system is off-centre
Related subjects
Caution! This manual is protected by copyright. Unauthorised reproduction thereof, in whole or in part, is punishable by law. Only use with MICRO TECH LAB’s approval.New LM Digital Adapter for: Canon EOS 1D X / Nikon D4 / Canon EOS 1D Mark IV / Canon EOS 60D / Canon EOS 600D / Sony NEX-5N / Nikon D3S / Canon EOS 5D Mark II / Canon EOS 7D / Canon EOS 550D / Rebel T2i / Kiss X4 Digital / Pentax K-5 / Sony Alpha 55 / Sony Alpha 580 / Sony NEX-C3 / Sony NEX-7 / Sony Alpha 65V / Sony Alpha 77V / Nikon D3x / Canon EOS 1100D / Nikon D7000 / Sony Alpha 33 / Sony NEX-5 / Sony NEX-3 / Sony Alpha 35 / Olympus E-5 / Pentax K-r / Canon EOS 1D Mark III / Nikon D5100 / Nikon D700 / Nikon D3 / Panasonic Lumix DMC-GX1 / Olympus E-30 / Olympus E-3 / Olympus E-620 / Canon EOS 1Ds Mark III / Canon EOS 500D / Digital Rebel T1i / Sony NEX VG10 / Olympus E-600 / Nikon D300S / Panasonic Lumix DMC-G2 / Pentax K-x /
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