The Judging of Fieldglasses, by Roland Leinhos
Zur Beurteilung von Feldstechern.
Zeiss Werkzeitschrift, 7 (1959), p.78-83
Translated by Ilse Roberts and Peter Abrahams
Text of this translation copyright 1996
(Another translation of this article was published by Zeiss in 1960.)

When discussing field glasses, the optical data, such as magnification 
and less frequently the FOV, are the main subjects.  With these data one 
distinguishes the different field glass models and associates them with 
their purpose. But if the layman looks at the pamphlets of different 
manufacturers, he is usually surprised to find the same optical 
specifications everywhere, at least in regards to magnification & objective 
diameter.  The only differences he might find are for the F.O.V. (field of 
view).  But this is not surprising in the least since the specifications 
serve as differentiation for various models of a manufacturer and don't say 
anything about the other characteristics of the fieldglasses.  The field 
glass amateur will then usually ask if the difference in the F.O.V. is the 
only one, or if one can see further differences in field glasses between 
manufacturers. Since this is asked again and again,and there are indeed a 
number of characteristics to differentiate, we shall go into more detail. 
But we shall stay away from the characteristic distinctions of the Zeiss 
fieldglass, as for example the smaller construction and therefore more 
comfortable use, or the models with greater eye relief, for wearers of 
spectacles.

At first we shall talk about the concept of size of F.O.V. This is one 
of the specifications peculiar to an individual field glass, since the 
F.O.V. does not depend on magnification and objective diameter, but on the 
field stop installed in the focal plane. This is valid with the exception 
that for reasons of optical design, the diameter of the f.o.v. becomes 
smaller, with higher magnification. But also for fieldglasses with the same 
magnification, and the same objective diameter, different F.O.V.s are found. 
For example, there are 10 x 50 field glasses, which in a standard 
construction have a F.O.V. of 130m at 1000 m distance, corresponding to an 
angle of 7.4 degrees on the objective side. But one also finds field glasses 
with about 100m (5.8 degrees) or even those that have a F.O.V. of only 88 m 
and 112 at 1000m (5 degrees). For certain glasses the F.O.V. is thus 1 1/2 
times larger than that of other makes. There is not as much difference with 
8 x 30 binoculars, where one can find models with F.O.V. between 150 m (8.5 
degrees) and 112 m (6.4 degrees). These are then values which the 
manufacturer prints in his pamphlets; and for a quality product it is to be 
expected that these values correspond to the experimentally established 
ones.

One can easily discern differences in the F.O.V. by mounting the field 
glasses on a tripod and pointing them so that at the left rim of the F.O.V., 
a detail of an object with a grid (tile roof, windowpanes, or something like 
it) is visible and then counting the details (tiles or panes) towards the 
right rim. Of course one can also do such a measurement of F.O.V. in a 
larger room ( 7 by 10 m) with a measuring stick. Also, a measuring stick or 
similar markings can be attached directly onto a test slide with Foucault 
line figures, as in fig. 1.

But the information about the F.O.V. is not enough; as one can easily 
see, there are also differences in regards to the sharpness of the image in 
the center, in the mid zones and at the rim of the F.O.V.  For example, 
there are field glasses which, when the center is in focus, have a 
distinctly noticeable sharpness decline towards the edge of the field, and 
also those which produce a focused image up to the edge.  It is obvious, 
that the fieldglass with the better edge sharpness does not only produce an 
aesthetically more satisfying image, but also facilitates the early 
recognition of details at the edge of the F.O.V. when the field glass is 
traversed across a view. The edge sharpness does indeed have practical 
importance. 

(Fig 1. Test plate with Foucault line system.  The numbers give a quantity 
to the focus, an arbitrary unit of length, that depends on observation 
distance.  In the depicted size the test plate is appropriate for an 
observation distance of about lOm.)

Visual tests of the image quality are not so simple and require some 
experience.Such tests are much easier when a testplate with the Foucault 
line system can be used (fig. 1).  With it, the resolving power can be found 
for the center, as well as at the edge or the zone in between.  Astigmatism 
can be recognized when the vertical and horizontal lines appear sharp at 
different dioptre settings of the field glass. Such a procedure is not 
totally objective, since the aberrations of the eye are not eliminated and 
furthermore the eye movements and adjustments have to be considered.  A 
secure position of the fieldglass and careful observation are matters of 
course.

If the quality test is to be made more exact, a star test instrument is 
required as shown in fig 3.  A collimator with a pinhole in its focal length 
produces an artificial star.  The field glass is set in front of the 
collimator so it can be rotated around the objective opening.  Behind the 
fieldglass, instead of the eye is a camera with long focal length (500 mm) 
lens.  Swiveling and shifting adjustments allow the placement of the image 
of the star in the middle of the field glass F.O.V., as well as at the rim 
or in the mid zones, and from there to the photo plate.

(Fig. 2.  Star tests of various field glasses.  A, nearly perfect.  B, 
Spherical aberration.  C, Lens is screwed in too tight.  D through F, 
centering mistakes of various sizes.  G & H, astigmatism.  I, mistake from 
roof ridge.)

Fig 2 shows some of those star images, as photographed through various 
field glasses.  In a, the image of a star through a perfect field glass is 
reproduced nearly without fault as a round circle.  The next image, b, shows 
a wide ring around the image proper, which may be caused by incomplete 
correction of spherical aberration, or by manufacturing mistakes like 
incorrect air spacing or wrong radius of curvature of a lens.  The 
triangular star image in c is caused by physical distortion of an optical 
part (objective or prism).  D through f show centering mistakes of various 
sizes, which means, the center of curvature of the separate lenses do not 
lie on a common line, the optical axis.  The images in g and h show 
astigmatism, as it appears when the optical planes are not exactly of the 
same form, for example the evenness of the reflection planes of the reversal 
prisms.  I shows the faults that appear through an imperfectly worked roof 
prism.  

It is obvious that field glasses, which have mistakes in the axis do not 
have the resolving power of a good field glass.  If one wants to test the 
system for resolution with the naked eye after the Foucault test, an 
auxiliary telescope should be used.

(Fig. 3.  Schematic of an instrument for testing image quality. 1, 
collimator. 2, collimator objective. 3, artificial star. 4, light source. 5, 
object of examination. 6, entrance pupil of object. 7, exit pupil of object. 
8, camera tube. 9, camera objective. 10, film plane.)

In fig 4 are star test photos of an artificial star, at a distance from 
the center of the image which is 2/3 of the F.O.V. radius, as shown by 
different field glasses.  In this figure,too, it can be seen how different 
the image sharpness can be.  Even the best field glass cannot image a star 
at the rim of the F.O.V., or in the mid zone, as a dot, as it does in the 
center.  The optical designer can minimize this remaining error by a careful 
choice of the lenses, radii of curvature, thickness, air spacing, and types 
of glass.  Fig. 4 gives a few examples and shows qualitative differences.  
The two figures to the far right show that because of construction mistakes 
the figures can become unsymmetrical, while the size generally is dependent 
upon the geometric optical errors.  Of course, it is very important for 
these figures that the fieldglass in the center has been focused on 
infinity.

(Fig 4. Star test photos outside of the center of F.O.V. of different field 
glasses.  Above left: especially well corrected and perfectly constructed 
system.; below left and center: fairly significant remaining mistakes; 
right, above and below, distorted figures caused by construction mistakes.)

One can lessen the expansion of the figures, when the fieldglass is 
defocused.  This adjustment, to the smallest possible number, generally 
between minus 0.5 and minus 4 dioptres, gives the F.O.V. a curvature in this 
zone; and is also an indicator of the quality of the image. 

Now to another matter, concerning the throughput of the field glass, and 
consequently the brightness of the glass. We know today that the brightness 
of the field glass image is not the only factor in the performance of the 
glass at dusk and night, as thought previously.  Nevertheless it is 
understood that a field glass with reflection diminishing T coating, is 
better than one without this coating, since it delivers a brighter image.

According to a law of nature, at each meeting of air and an optical 
medium, a certain amount of light is reflected (fig 5); 4% of the incoming 
light for a glass with refractive index of 1.5, 7% for a glass with 
refractive index of 1.7.  By steaming on a thin coating (1/4 wavelength of 
light), these losses can be diminished to 1%.  The refractive index of the 
coating must be between that of the glass and air.  A modern field glass has 
up to 16 such glass-air surfaces, and without T coating only about half of 
the incoming light could exit, while the actual loss for modern coated field 
glasses is less than 20%.

If the light loss in itself is undesirable, it is more important that 
the light stopped at the surfaces reflects around in the housing and finally 
appears in the image as a grey veil; so that a field glass which does not 
have an antireflection coating cannot deliver a high contrast, brilliant 
image, like one that is properly coated.

(Fig 5. Path of a ray in an uncoated, and a coated lens.)
(Fig 6. Path of a ray in a prism with a small index of refraction.)

Not every coating can be called a compensating coat.  A blue coating or 
other attractive coating, does not necessarily have the above mentioned 
effects.  There are also field glasses where only a few of these surfaces 
are coated.  This is not enough to significantly improve the light 
throughput and contrast; for that, it is necessary to coat all surfaces, and 
an exception should be made at most for the exterior surfaces of of the 
objective and ocular.  To measure the effect of the antireflection coating, 
exact measurements of the throughput and the decrease in contrast are 
necessary, a complicated task.

A further cause of light loss and additional scattering of light can be 
the prisms, which serve to reverse the image and shorten the instrument. If 
these prisms are made of a glass of an insufficient index of refraction, as 
is done quite frequently nowadays, the critical angle for total reflection 
of the edge rays is surpassed at the reflecting surfaces of the prisms.  The 
edge rays from the rim of the F.O.V. strike the reflecting surface of the 
prism at a larger angle than the central rays.  At these surfaces, light 
exits the prism, as the arrows in fig 6 show.  At an index of refraction of 
1.52, the largest angle of approach for ray which will be totally reflected 
by the prism is about 6 degrees.  Here, too, brightness and reduction of 
contrast are the main problems.  If one could perhaps tolerate the loss of 
light, so that the effective objective diameter and therefore the 
performance at dusk do not correspond to the indicated objective diameter; 
the worsening of the contrast is perhaps much more unpleasant.

(Fig. 7. Exit pupil for field glass with prism of glass with too small a 
refractive index.)

Even the amateur can easily find out if the critical angle of total 
reflection was surpassed at the prisms, by observing the exit pupils.  The 
exit pupils usually are visible as light round little disks, if one holds 
the field glass at about 30 cm distance from the eye against a light 
background and looks towards the oculars.  Since the ray bundle is being 
reflected four times in the prisms, when lower quality glass is used, at 
four places the periphery of the pupils are cut.  The exit pupil is then not 
seen as a round disk, but as a square which is inscribed in the circle.  The 
cut off segments appear a dim, pale blue, as is indicated in Fig 7.

At this point a word about fastening of the prisms may be allowed.  A 
secure fastening is especially important for binocular instruments.  Even a 
tiny shifting of a prism causes misalignment of the light rays, so that 
double images appear to the observer.  This can happen when bumped and 
shaken, which is to be considered normal for field glass usage.  For 
example, the tipping over of a glass on a table causes forces on the field 
glass which correspond to 40 to 80 times the acceleration due to the earth's 
gravity (9.81m/s).  The allowable tolerance for parallelism of the exit axes 
is about 20 arc minutes.  Since a manufacturer cannot fit the the prisms 
into the housing as exactly as is required, the possibility for adjustment 
at the fieldglass is required.  For quality field glasses, adjustment with 
eccentric rings at the objective mountings is the accepted practice.  These 
two eccentric rings allow the shifting of the objective within certain 
limits, perpendicular to the optical axis in any direction; and consequently 
permit the adjustment of the axes of the two field glass halves. 

Another means of performing this adjustment, using the prisms, is to 
adjust the prisms after the assembly of the whole field glass, mostly from 
the outside, by screws through the housing to the edges of the prisms.  But 
one never has the guarantee that the prism is really fixed in its position, 
or is not under such pressure that the prism is warped, thus worsening the 
image quality.

A secure prism mounting is the fundamental prerequisite for maintenance 
of the binocular alignment over a long period of time and with rough 
handling.  In early Zeiss field glasses, this fixation of prism position was 
achieved by making the housing a little larger than the prisms, which had 
two flat grooves on the sides, into which some material from the housing was 
pushed with a center punch.  In later times, Zeiss switched to retaining 
rims, which are placed around the prism and then screwed in.  Thus the 
prisms are either fastened into the housings individually, or both prisms 
are mounted onto a plate, the prism seat, and then put into the housing.  In 
addition, the prisms are held in position by a spring which presses on the 
upper edge, so that they cannot pop out of their seated position when 
bumped.

The simplest method of prism fastening is the use of a more or less 
suitable cement.  Here the main difficulty is the stability of the cement.  
It can change because of temperature influences or in a leaking 
fieldglasses, weather can loosen the prism.

That brings us to a further point, weather tightness.  To protect a 
field glass against weather influences, especially the entrance of humidity, 
all openings, slits, and pores have to be carefully closed.  Humidity can 
condense on the optical surfaces, and the fieldglass becomes at least 
temporarily unusable.  Formerly, wax was used, today, rubber seals are used 
for Zeiss field glasses.  Special difficulties with center focus models made 
the sealing of these field glasses impossible in earlier times.  These 
glasses now show the effects of their sliding oculars.  But this problem was 
solved by the introduction of the inverted sleeve for the Zeiss center focus 
field glass, fig 8.  Here a tube like piece of rubber, folded like a cuff, 
was fastened to the ocular seat on the housing and also to the movable 
ocular.

(Fig. 8.  Cuff seal for Zeiss field glass with center focus.  All seals are 
drawn in heavy black line.)

In the absence of such a solid seal for the field glass, other methods 
of avoiding humidity and condensation are used.  These are usually 
containers of a drying substance which loses its effectiveness after a short 
time, and in the best of cases only protects the field glass for transport 
across the ocean.

In conclusion it may be noted that even in the outer appearance of field 
glasses from different manufacturers are noticeable differences.  These 
could be the coverings of the housings; or the engravings at the center 
screw, the ocular, or at the trade mark.  But even the amateur will be able 
to detect the conscientiousness which was used at the factory by examining 
the smooth and silky glistening lacquer.  Summing up, here are the 
characteristics one should keep in mind when comparing field glasses of the 
same type.
--the size of the F.O.V.
--the image quality in the middle and at the rim of the F.O.V., as well as 
the curvature of the F.O.V.
--the throughput and therefore the brightness of the image, as well as the 
contrast, in regards to the antireflection coating.
--the additional loss of light from inferior glass in the prisms (squared 
instead of round exit pupils).
--the provision for adjusting collimation, with double eccentric rings at 
the objectives. or by positioning the prisms.
--the prism fastening itself, not visible from the outside.
--the sealing of the field glass housing against atmospheric influences; a 
well sealed field glass does not need containers of drying substances.


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