Improvement in Photographic Objectives.
Year: 1890

page_text: Date of Application, 14th Aug., 1890-Accepted, 4th Oct., 1890
COMPLETE
SPECIFICATION.
Improvement in Photographic Objectives.
I, ERNST GUNDLACH, Optician, of No. 112, South Avenue, Rochester, in the State
of New York, of the United States of America, do hereby declare the nature of this
invention, and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement :-
My improvement relates especially to photographic objectives, or combinations of
optical lenses used for the purpose of taking photographic portraits, views and other
subjects.
All the best photographic objectives of modern type consist of two achromatic or
aplanatic meniscuses this form of lenses being the best adapted for the purpose of
obtaining a wide angular field with a minimum of distortion at or near the edges of
the picture. But while, for said purpose, the meniscal form offers great advantages
over any other kind of lenses, a disadvantage of the same, however, lies in the difficulty
of correcting its optical aberrations, both the spherical and the chromatic, at the same
time, without leaving too great amounts of the so called "aberrations of second
order."
The object of my improvement is the reduction of said secondary errors to an
imperceptible minimum.
I describe my invention as follows:
Owing to the fact that the ratio of the colour-dispersive power of flint glass to its
medium refractive power (line D)-commonly called "dispersive power
or "dis-
persion" is greater than that of crown glass, an achromatic lens can be produced by
the combination of a (positive) crown-glass lens with a negative flint-glass lens, the
dispersive power of which is equal in amount to that of the crown-glass lens; for,
with equal amounts of dispersive power (which neutralize each other), the (medium)
refractive power of the crown-glass lens is greater than that of the flint-glass lens, and
the preponderant part of that of the crown-glass lens forms the requisite positive
focal power or focal length of the combination, thus being the true constituent of the
achromatic lens. ("Focal power" I term the amplifying or reducing power of a lens
in relation to, or expressed by, its focal length, the focal power being inversely as the
focal length.)
The ratio of the focal power of the crown-glass lens to that of the flint-glass lons of
an achromatic combination is as the ratio of the dispersive power of the flint glass
to that of the crown glass employed for the construction of the lenses, and is inversely
as the ratio of the focal power of the crown-glass lens to that of the combination.
From this follows that, the greater the dispersive power of the flint glass employed,
the lower a focal power of the crown-glass lens will be required for a given focal
power of the combination or achromatic lens and, consequently, the smaller the
amount of colour-dispersion, to be corrected by the flint-glass lens, will be.
Furthermore, regarding the established fact that in an achromatic lens neither the
spherical nor the chromatic aberration can be completely neutralized, but that a more
or less small proportion of said errors-the aberrations of second order-(consisting
in an incomplete or under-correction of the central part, or toward the center, and a
corresponding over-correction toward the edge of the lens) will always remain, it
follows from the foregoing conclusions, that the amount of chromatic aberration of
second order in an achromatic combination will be the smaller; the greater the dis-
persive power of the flint glass employed.
While, thus, the amount of chromatic aberration of a single or uncorrected lens
depends, with a given kind of glass, on its focal power or focal length alone, and is to
be corrected accordingly, as was shown above, its spherical aberration depends also,
and in a higher degree, on the form of the lens, that is, on the difference of the cur-
vatures of its surfaces, and is the greater in amount, the more the curvatures differ
רשה.'.D

page_text: from each other. (An equal-sided lens is, under ordinary conditions, of minimum, a
meniscus of maximum spherical aberration.)
The spherical aberration of a positive lens may, therefor, be corrected by a negative
lens of almost any focal power, if the curvatures of the same can be made to differ
sufficiently, to produce an amount of (negative) spherical aberration equal to that
(positive) of the positive lens. If, therefor, the positive lens is of minimum spherical
aberration, that is, equal-sided, or nearly so, a negative meniscus of much lower foca!
power than that of the positive lens may neutralize or correct said aberration and,
since the differential part of the focal powers of the components forms the focal power
of the combination, the latter may, in this case, be made to be nearly as great as that
of the positive component of the combination.
But if the positive lens of the combination is itself a meniscus, as is the case with
the positive components of the modern photographic objective, then the focal power
of its negative or correcting lens cannot be made to be much lower than that of the
positive lens, because it would then be impossible to produce, even with the greatest
admissable difference of its curvatures, the amount of negative spherical aberration
required for the correction of the positive lens, and consequently, the remaining focal
power that of the combination-will be very small in comparison with that of its
positive component.
Hence, the focal power of the crown-glass lens in an achromatic meniscus of a given
focal power is to be comparatively great and, being besides this a meniscus itself, its
spherical aberration must be accordingly great, thus leaving a correspondingly great
amount of aberration of second order, after being combined with the flint-glass lens.
Having thus pointed out the distinguishing characteristics of the two optical aber-
rations of a lens, and explained their different conditions of correction, it remains to
illustrate the principle, after which both corrections are conjointly accomplished in the
meniscus of the modern photographic objective.
Fig. 1 of the accompanying drawing represents, in axial section, an aplanatic or
photographic meniscus of modern type. It is composed of a positive crown-glass
lens, d, and a negative flint-glass lens, c, the latter being of longer (negative) focus or
lower focal power than the crown-glass lens, thus leaving the preponderant portion of
the latter as the actual focal power or focal length of the combination. The sides, e,
of the lenses are of equal radius of curvature, and are cemented together-this
arrangement being a necessity in photographic lenses.
Supposed, now, the flint-glass lens, c, to be made of ordinary flint glass, such as is
commonly used in telescops objectives and similar achromatic lenses, then the internal
sides, e, of the lenses, being, as may be assumed, of the proper, though unusually short
curvature required for the correction of the spherical aberration of the meniscus, will
be too short curved for the proper correction of the chromatic aberration, thus pro-
ducing a corresponding over-correction of the latter. If, then, the curves, e, are
flattened, the dispersion of both lenses will be reduced, of course, but that of the Hint-
glass lens will deminish in a higher ratio than that of the crown-glass lens, owing to
the higher dispersive power of the former, and if the flattening of the curves is
continued, a point will soon be reached, at which the dispersive powers are equal in
amount, which case, as will be remembered, is the condition for the correction of the
aberration.
Fig. 2 is to illustrate this case, showing the inner curves, e, as being considerably
flatter than the corresponding curves of Fig. 1, thus just correcting the chromatic
aberration of the combination, of which the flint-glass lens, c, is made of high
dispersive glass as assumed. The flattening of the inner curves, however, interferes
with the correction of the spherical aberration, which is thereby reduced to a corre-
sponding under-correction, as will be obvious.
It will now be readily 'understood that, owing to the peculiarly conflicting relations
between the two aberrations as shown, it is impossible to combine the corrections of
the two aberrations in a meniscus by the single means of a negative lens made of
ordinary or any high dispersive fint glass, in the manner described and illustrated,
for, with such glasses, the chromatic aberration of the meniscus will always be

page_text: considerably over-corrected, when the spherical aberration is corrected, or the latter
will be under-corrected when the former is right.
Considering, however, the causes of this interference, it suggests itself, that, with a
flint glass of a certain low dispersion, or with two glasses of a correspondingly low
difference of dispersion, the desired result may be fully accomplished, and that such
special properties of the glasses, as required, may be accurately predetermined by
calculation. In fact, all that is required for the purpose is, that the two lenses, d
and c, Fig. 1, have just such relative dispersions as will neutralize each other, when
the inner sides are of such short curvatures (Fig. 1) as to neutralize the spherical
aberrations of the lenses.
This principle was discovered many years ago, by the late mathematician Steinheil,
of Germany, and has been ever since, and up to the present time, generally adopted
by all the leading opticians of the world, as the basis par excellence for the manu-
facture of photographic objectives.
It will be remembered that the amount of chromatic aberration of second order, in
an achromatic lens, will be the smaller, the higher the colour-dispersive power of the
flint glass employed for the negative lens, and further that in an aplanatic meniscus
the spherical aberration of second order is comparatively great, much greater than in
ordinary aplanatic lenses. But notwithstanding this fact, the latter may also be
considerably reduced by the employment of high dispersive flint glass, same as the
chromatic aberration, for the reason that, as a matter of fact, a high dispersive flint
glass has also a correspondingly high (medium) refractive power. Let us now apply
these rules inversely to the meniscus of the modern photographic objective, as repre-
sented in Fig. 1, and it appears that, notwithstanding its valuable and superior
properties as described, a serious defect is nevertheless inherent to the same, consisting
in an excessive amount of both aberrations of second order, as an inevitable con-
sequence of the employment of a flint glass of extremely low dispersive power.
It is, therefor, the object of my invention to reduce these defects to a minimum,
which end I accomplish as follows:
employ for the negative lens a high dispersive flint glass, and give the lens, with
the proper proportional focal power for the correction of the chromatic aberration,
such strong meniscal curvature, irrespective of the form of the crown-glass lens-as
is required for the correction of the spherical aberration, and then adapt the form of
the crown-glass lens to it in such a manner, as to preserve the normal meniscal form
of the combination.
In this way I reduce both aberrations of second order considerably, while, at the
same time, fully preserving the valuable qualities consistent with the present photo-
graphic meniscus, as will be understood from the following.
Let us refer again to the meniscus Fig. 2, in which the negative lens, c, is made of
a high dispersive flint glass and the inner curves, e, are so much flattened that the
chromatic aberration of the combination is corrected.
We remember, that at this point the spherical aberration is only partly, or under-
corrected, and the problem is, therefor, to find a suitable way of altering the form,
respectively shortening the meniscal curvature, of the lens, c, without changing its
focal power, and the external surfaces, a and b.
Let us suppose the lens, c,to be flexible, the lens, d, however, to be in a fluid state;
let us further presume the whole to be enclosed in a vessel, for the purpose of pre-
serving the outer form of the meniscus; the vessel, however, to be a little wider from a
to b, so as to give the lens, d, more body, as shown in Fig. 3.
Now let the lens, c, be bent inward at its edge all around, while at its center it is
held against the side, b, of the vessel (as is indicated by the arrows) in order to shorten
the meniscal curvature of the lens. This operation will produce a space between the
lens, c, and the side, b, of the vessel, which will be filled up gradually, by a corre-
sponding portion of the fluid, d, as the space increases.
Let the operation be continued until the curvature, and the correspondingly
increasing negative spherical aberration of the lens, have reached the point where the
latter (the aberration) is equal in amount to that of the crown-glass lens, (or rather

page_text: the fluid part of the combination) when the desired purpose will be accomplished.
This stage of transformation is indicated in Fig. 3 by dotted lines, and Fig. 4 repre-
sents the new (thus created) type of aplanatic meniscus, as it is to be constructed for
practical use, of the proper materials.
The form of the crown-glass lens, d, Fig. 4, has certainly undergone a material
change at the same time with that of the flint-glass lens, as will be seen, apparently,
and in fact, being now of but little less spherical aberration than the crown-glass
lens, d, Fig. 1, of the ordinary doublet meniscus. But the superfluous portion of it
has passed over to the other side of the flint-glass lens, forming there a new crown-
glass lens, f, of nearly such a form, as if it had been directly separated from d
by a cut.
Thus the two new lenses, d and f, together, or their single equivalent, have a focal
power and a meniscal curvature, which are nearly equal to those of d, Fig. 2, the
aberrations of which latter are considerably less in amount than those of d, Fig. 1, as
was shown before. Hence the correction of the aberrations of the combined lenses, d
and Fig. 4, or their equivalent, will leave accordingly small amounts of the
respective aberrations of second order.
It is well established in the optical science, that the aberrations of second or higher
order are the most essential defects in any known form of aplanatic objectives or
optical apparatus, and that, although their complete removal being an impossibility,
the reduction of the same in any measure, however small, signifies a valuable
enhancement of the optical excellence of said apparatus. Therefor 1 regard my new
photographic meniscus described above and illustrated in Fig. 4, as being an essential
improvement over that invented by Steinheil and represented in Fig. 1. As will be
seen, the lens is a triplet, novel in the form of its components and in the order of
their combination. A particular novelty in this compound is the negative crown-
glass lens, such kind of lenses having not been employed in positive aplanatic
meniscuses heretofore.
DEFINITIONS:
"Positive lens " I term any of the following:
1, a plano-convex lens,
2, a double-convex lens,
3 a concavo-convex lens, the convex side of which has a shorter radius of curvature
than the concave side.
Negative lens" I term any of the following:
1, a plano-concave lens,
2, a double-concave lens,
3 a concavo-convex lens, the convex side of which has a longer radius of curvature
than the concave side.
A meniscus is a concavo-convex lens.
Having now particularly described and ascertained the nature of my said inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :
In a photographic objective, a compound meniscus, consisting of a negative flint-
glass meniscus, a negative crown-glass meniscus, and a positive crown-glass meniscus,
so combined, that the flint-glass meniscus is enclosed by the two crown-glass
meniscuses, facing with its convex side toward the concave side of the negative crown-
glass meniscus, and with its concave side toward the convex side of the positive crown-
glass meniscus.
Dated this First day of August 1890.