The Mechanical Tube Length – A Short Introduction - Microscopy-UK

The Mechanical Tube Length – A Short Introduction - Microscopy-UK
The Mechanical Tube Length – A Short Introduction
Gregor Overney
Micscape Magazine, November 2015
The Mechanical Tube Length – A Short Introduction
Gregor Overney, California 2015
The mechanical tube length is the distance between the flange of the objective lens (also known as the
shoulder of the objective) and the seating surface on which the eyepiece rests (see Figure 1). In the case
of a simple monocular microscope (e.g. the Leitz LL from 1950), the mechanical tube length is equal to
the actual physical length of the tube. But for most research microscopes, the effect of the tube lens (or
tube lenses) must be taken into account when measuring the effective, mechanical tube length.
Provisions to add certain accessories force
microscope makers to build microscopes with a
longer finite tube length than the mechanical
tube length required for correcting their optical
system (e.g. longer than 170 mm). As a result,
for each such accessory in a finite system,
optical elements (known as a tube lens) must
be added to bring the tube length precisely
back to its proper value (e.g. 170 mm).
Using an objective turret for transmitted light
observations, the Leitz Ortholux research
microscope uses one of several different tube
lenses to adjust the actual physical length of its
223 mm long tube to a desired norm (see
image on cover page). Hereby, the actual
physical length of the tube is defined as the
measured length without considering any
optical components inside the tube. To adjust
to a mechanical tube length of 170 mm, the
Ortholux uses the 170/223 tube lens. This
concave tube lens leads to an unwanted extra
magnification factor that commonly plagues
microscopes designed for the finite mechanical
tube length. To provide the so-called infinity
tube length, the tube lens ∞/223 must be used.
Figure 1: Leitz LL student microscope from 1950.
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The following table (see Table 1) lists tube length parameters published by various microscope
manufactures.
Table 1: Tube length parameters. R. P. Loveland, p. 56 from Photomicrography – A Comprehensive Treatise, Volume 1, John
Wiley & Sons, Inc. (1970) [1].
Make
Mechanical
Tube
Length
∆ in mm
Remarks
160
(location of
intermediate
image plane in
mm)
11
Biological
215
11
(in mm)
Bausch & Lomb
Inc.
Metallographic
(e.g. Dynoptic Labroscope Model DMETR)
American
Optical Co.
Ernst Leitz
GmbH
160
11.3
Biological
180
11.3
Metallographic
∞
11.3
Series 10 and 11 microscopes
170
18
Biological
(e.g. Ortholux, Orthoplan)
185
18
Ultropak
215
18
Ore microscope
∞
18
Metallographic
(e.g. Metalloplan-HL)
Zeiss
(Oberkochen)
160
10
Old value of ∆ = 13 mm
DIN 58887
160
10
Since 1976 also Leitz with the Dialux 20
The symbol ∞ indicates an “infinite tube length”. In this case a special relay lens (or telescopic objective)
located inside the body tube of the microscope focuses the collimated beam from the objective to form
the intermediate image (or primary image). The infinite tube length design allows for more flexibility in
microscopy design.
In the following, we limit our observations to microscopes using objectives for the finite tube length
(such as 170mm or 160mm). Microscopes using infinity-corrected objectives (e.g. Nikon Eclipse series
microscopes or the Leitz objectives for episcopic illumination) are not discussed.
The combination of tube lens and objective is responsible to generate an intermediate or primary image
of the specimen at the fixed diaphragm of the eyepiece or photo relay lens. It is interesting to note that
for an optical system that provides all necessary corrections inside the objective/tube lens combination,
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the intermediate image can directly be projected onto the image sensor without any loss of image
quality (e.g. Nikon Eclipse series microscopes using the CFI60 optical system).
For many microscope designs (e.g. Leitz Orthoplan and Leitz Ortholux II) the optical tube length, which
defines the exact location of the intermediate image below the shoulder of the ocular, is proprietary.
While optical tube length is defined as the distance between the objective rear focal plane and the
intermediate image plane at the fixed diaphragm of the eyepiece, it is often measured as the distance
between the shoulder of the objective and the intermediate image plane.
The following information relates to Leitz microscopes. – In 1976, with the introduction of the Leitz
Dialux 20, Leitz adopted German Standard Commission DIN 58887. DIN 58887 recommends a
mechanical tube length (TL) of 160 mm. The Leitz Inter-Office Memorandum No. 84 [2] clearly states
that in most cases objectives designed for a mechanical tube length of 160 mm can successfully be used
on the older Leitz microscopes, which use a 170 mm mechanical tube length. Of course, the oculars
designed for a 170 mm tube length must be used in all cases. In a nutshell, the reason for this backward
compatibility is the fact that the older Leitz microscopes use an optical tube length (or image distance)
of 152 mm, which is just 2 mm longer than the one proposed by DIN 58887 (150 mm). (See Figure 2 for
details.)
Figure 2: Schematic drawing showing mechanical tube length.
For objectives with a magnification larger than 1:16, the 2 mm displacement of the intermediary image
has no noticeable effect on image quality. Roger P. Loveland plotted the tolerance to tube length change
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versus objective NA in his excellent book about photomicrography [1]. Loveland concludes that the
tolerance to change of tube length is affected only by the numerical aperture NA of the objective. Every
dry lens with an NA of 0.80 or less should cope very well with a 2 mm difference in image distance. Even
better, a difference of just 2 mm does not impact image formation when oil immersion objectives are
used. The reason that Leitz does not recommend using objectives with a magnification of less than 1:16
is most likely due to the fact that one cannot maintain parfocality. If parfocality is not desired, even
lower power objectives designed for a 160 mm TL could be used.
From the same Inter-Office Memorandum we can learn that newer oculars should not be used on older
Leitz microscopes. Of course, when using an adjustable monocular viewing port, one may still be able to
use newer Leitz oculars by appropriately reducing the mechanical tube length.
Already many years prior to 1976, during the "glory days" of the black
microscope stands, Leitz offered objectives with two different parfocal
distances. The older objectives use a 37 mm parfocal distance ("short
barrel") while the newer ones were designed for a 45 mm parfocal
distance. In order to combine the two different types on the same nose
piece (objective turret), Leitz offered special adapters called PLEZY (see
Figure 3) and FLU-PLEZY (also referred to as PLEZY-FLU).The PLEZY and
FLU-PLEZY contain a lens that corrects for the 8 mm extension. Without
such a lens, the optical tube length will be increased by 8 mm
(significantly more than just 2 mm). Such a large increase in optical tube
Figure 3: Leitz PLEZY adapter.
length would most certainly degrade image quality. The higher the NA
of the objective the more would be the impact on image quality (see [1]).
The difference between the FLU-PLEZY (Leitz number 519 382) and the PLEZY (Leitz number 519 164) is
that the FLU-PLEZY offers better transmission for shorter wavelengths, making it more suitable for
fluorescence microscopy using episcopic illumination. An additional designation ‘P’ indicates that the
glass inside the adapter is free of stress birefringence, which makes such an adapter suitable for
polarized light microscopy.
References
[1] R. P. Loveland, p. 59, Figure 2-2 "Tolerance to tube length change versus objective NA." in
Photomicrography – A Comprehensive Treatise, Volume 1, John Wiley & Sons, Inc. (1970).
[2] Leitz Inter-Office Memorandum (available at http://www.science-info.net/docs/leitz/Leitz-160mmMemo.pdf) from September 30, 1976 entitled "160 mm Mechanical Tube Length".
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