Patent Description:
Recently, photographing apparatuses such as digital cameras, video cameras, or surveillance cameras are required to have small sizes.

However, imaging devices such as charge coupled devices (CCDs) or complementary metal-oxide semiconductors (CMOSs) included in photographing apparatuses are required to have large areas and many pixels for high pixel count. Thus, optical systems for directing image-forming light to imaging devices have large sizes and thus, it is difficult to reduce an optical aberration over the entire region of the imaging devices. Therefore, there has been an increasing demand for a high-performance zoom lens system for use in small photographing apparatuses and capable of directing image-forming light to imaging devices with low optical aberration.

In addition, along with the increased importance of security, many individuals as well as public institutions or companies now use precise measurement cameras or surveillance cameras for closed-circuit televisions (CCTVs). Surveillance cameras are used at night as well as in the daytime and thus, there has been an increasing need for surveillance camera zoom lens systems capable of effectively correcting aberration in a range from visible light to near infrared light.

<CIT>, <CIT>, and <CIT> disclose a zoom lens system correcting aberration.

"<NPL>, and<NPL> disclose an aberration.

Various aspects of exemplary embodiments of the inventive concept will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments.

According to the invention, there is provided a zoom lens system as defined in claim <NUM>.

The first wavelength may be about <NUM>, and the second wavelength may be about <NUM>.

The distance between the focal point of light having the first wavelength and the focal point of light having the second wavelength may be about <NUM> or less at the wide-angle position.

The zoom lens system may satisfy a following condition: <MAT> where fw is a focal length at the wide-angle position, and ft is a focal length at the telephoto position.

The zoom lens system may satisfy a following condition: <MAT> where TL is a distance between an image plane and an object-side surface of one of lenses of the first lens group that is closest to the object side.

The zoom lens system may further include an aperture stop between the third lens group and the fourth lens group.

The first lens group, the third lens group, and the sixth lens group may be fixed during zooming.

The zoom lens system may satisfy a following condition: <MAT> where f<NUM> is a focal length of the first lens group, and f<NUM> is a focal length of the fourth lens group.

The first lens group may include a doublet lens at a position closest to the object side, the doublet lens including a negative lens and a positive lens sequentially arranged from the object side, and the zoom lens system may satisfy a following condition: <MAT> where Vd<NUM> is to an Abbe number of the negative lens of the doublet lens, and Vd<NUM> is an Abbe number of the positive lens of the doublet lens.

The first lens group may include at least four lenses each having an Abbe number of about <NUM> or greater.

Each of the second lens group, the third lens group, the fourth lens group may include at least one lens having an aspherical surface.

The zoom lens system may include at least five lenses each having a refractive index of about <NUM> or greater.

The sixth lens group may include a triplet lens including three lenses cemented together.

Other aspects will be clearly understood from the following detailed description, claims, and accompanying drawings.

These and/or other aspects will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings, in which:.

Reference will now be made in detail to exemplary embodiments which are illustrated in reference to the accompanying drawings. Effects and features of the exemplary embodiments, and implementation methods thereof will be clarified through the following descriptions in reference to the accompanying drawings. In this regard, these exemplary embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. In the drawings, like reference numerals denote like elements, and overlapping descriptions thereof will be omitted.

In the following descriptions of the exemplary embodiments, although the terms "first" and "second" are used to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another element.

In the following descriptions of the exemplary embodiments, the terms of a singular form may include plural forms unless referred to the contrary.

In the following descriptions of the exemplary embodiments, the meaning of "include," "comprise," "including," or "comprising" specifies a property, a region, a fixed number, a step, a process, an element, a component, and a combination thereof but does not exclude other properties, regions, fixed numbers, steps, processes, elements, components, and combinations thereof.

In the drawings, the sizes of elements may be exaggerated for clarity. For example, in the drawings, the size or thickness of each element may be arbitrarily shown for illustrative purposes and thus, the inventive concept should not be construed as being limited thereto.

<FIG>, <FIG>, and <FIG> illustrate optical arrangements of zoom lens systems according to exemplary embodiments.

Referring to <FIG>, <FIG>, and <FIG>, each of the zoom lens systems may include a first lens group G1 having a positive refractive power, a second lens group G2 having a negative refractive power, a third lens group G3 having a positive refractive power, a fourth lens group G4 having a positive refractive power, a fifth lens group G5 having a positive refractive power, and a sixth lens group G6 having a positive refractive power. The first to sixth lens groups G1 to G6 are sequentially arranged from an object (O) side to an image plane (IP) side. An aperture stop ST may be placed between the third lens group G3 and the fourth lens group G4, and optical blocks B1 and B2 may be placed between the sixth lens group G6 and an image plane IP.

According to an exemplary embodiment, chromatic aberration of each of the zoom lens systems may be corrected in a range from a visible ray to a near infrared (NIR) ray and thus, the zoom lens system may be applied to photographing apparatuses such as surveillance cameras that operate both at night as well as in the daytime.

According to an exemplary embodiment, the optical blocks B1 and B2 may be an NIR-cut filter B1 and cover glass B2, respectively. The NIR-cut filter B1 may be turned on/off by a controller (not shown) which may be implemented by hardware, software, firmware or a combination thereof. For example, in the daytime, the NIR-cut filter B1 may be disposed on a path of light between the sixth lens group G6 and the image plane IP so as to use a visible ray for photographing, and at night, the NIR-cut filter B1 may be outside the path so as to use an NIR ray for photographing. In <FIG> and <FIG>, only the optical block B1 is illustrated. However, each of the zoom lens systems illustrated in <FIG> and <FIG> may include two optical blocks, for example, the optical blocks B1 and B2.

Each of the zoom lens systems may perform zooming by moving the second lens group G2, the fourth lens group G4, and the fifth lens group G5 along an optical axis OA, and the first lens group G1, the third lens group G3, and the sixth lens group G6 may be fixed. The aperture stop ST may be adjacent to the third lens group G3 and may not be moved during zooming.

According to an exemplary embodiment, each of the zoom lens systems may perform zooming from a wide-angle position to a telephoto position, and may have high optical performance in an NIR region as well as in a visible region. When a wavelength corresponding to green light is referred to as a first wavelength λ<NUM> and a wavelength corresponding to NIR light is referred to as a λ<NUM>, a distance between a focal point of light having the first wavelength λ<NUM> and a focal point of light having the second wavelength λ<NUM> may be about <NUM> or less at the wide-angle position and the telephoto position. The first wavelength λ<NUM> may be about <NUM>, and the second wavelength λ<NUM> may be about <NUM>. The above-mentioned distance is a distance measured in a direction parallel to the optical axis OA.

According to an exemplary embodiment, since a difference between a focal point of visible light directed to the image plane IP and a focal point of NIR light directed to the image plane IP is very small over an entire region of the image plane IP, focal point misalignment may be suppressed during transition from a daytime photographing mode to a night photographing mode and thus, high-resolution photographing may be possible at night as well as in the daytime. That is, the sharpness of images captured at night may be improved and thus, double images may not be captured.

According to an exemplary embodiment, at the wide-angle position, the distance between the focal point of light having the first wavelength λ<NUM> and the focal point of light having the second wavelength λ<NUM> may be about <NUM> or less, for example, about <NUM> or less. This will be described later with reference to <FIG>.

Each of the zoom lens systems of the exemplary embodiments may satisfy Condition <NUM> below: <MAT> where fw is a focal length at the wide-angle position, and ft is a focal length at the telephoto position.

Condition <NUM> represents zoom magnification of each of the zoom lens systems. If each of the zoom lens systems satisfies Condition <NUM>, each of the zoom lens systems has zoom magnification of greater than <NUM>. That is, according to an exemplary embodiment, each of the zoom lens systems may have zoom magnification greater than <NUM> and may correct aberration in a range from a visible ray to an NIR ray.

Each of the zoom lens systems of the exemplary embodiments may satisfy Condition <NUM> below: <MAT> where TL is a distance between the image plane IP and an object-side surface of a first lens L11 of the first lens group G1 which is closest to the object side to the image plane IP.

Condition <NUM> refers to a ratio of an entire length of each of the zoom lens systems to the zoom magnification of each of the zoom lens systems. If the ratio is equal to or greater than an upper limit, the entire length of each of the zoom lens systems may be excessively large compared to the zoom magnification of a zoom lens system. That is, each of the zoom lens systems may have a large size, and it may be difficult to miniaturize the zoom lens systems.

The number of lenses and/or the entire length of a zoom lens system may be increased so as to correct aberration of the zoom lens system in a wide range. However, since lenses of the lens groups of the zoom lens systems of the exemplary embodiments are optimized, the zoom lens systems may be miniaturized to have an entire length of less than about <NUM> even though the zoom lens systems have high zoom magnification on a level of greater than about <NUM>.

Each of the zoom lens systems of the exemplary embodiments may satisfy Condition <NUM> below: <MAT> where f<NUM> is a focal length of the first lens group G1, and f<NUM> is a focal length of the fourth lens group G4.

Condition <NUM> represents a range of a ratio of the focal length of the first lens group G1 to the focal length of the fourth lens group G4 for correcting chromatic aberration of a zoom lens system. If the ratio is outside the range, it may be difficult to correct chromatic aberration of each of the zoom lens systems.

The first lens group G1 has a positive refractive power, and five lenses may be included in the first lens group G1. The first lens group G1 may be fixed and may include the first lens L11 having a negative refractive power, a second lens L12 having a positive refractive power, a third lens L13 having a positive refractive power, a fourth lens L14 having a positive refractive power, and a fifth lens L15 having a positive refractive power. Here, the first to fifth lenses L11 to L15 may be sequentially arranged from the object side to the image plane side. The first lens group G1 may include one negative lens and four positive lenses, and a negative lens (the first lens L11) and a positive lens (the second lens L12) which are closest to the object side and sequentially arranged from the object side may form a doublet lens.

That is, the first lens L11 and the second lens L12 may form a doublet lens that is closest to the object side among the lenses of the first lens group G1 and may be used for correcting chromatic aberration at the wide-angle position.

Each of the zoom lens systems of the exemplary embodiments may satisfy Condition <NUM> below: <MAT>.

In Condition <NUM>, Vd<NUM> is an Abbe number of the negative lens (the first lens L11) of the doublet lens of the first lens group G1, and Vd<NUM> is an Abbe number of the positive lens (the second lens L12) of the doublet lens of the first lens group G1. If Vd<NUM>/Vd<NUM> is outside the above range, it may be difficult to focus an NIR ray and a visible ray at the same time and miniaturize a zoom lens system. Vd<NUM> and Vd<NUM> are the Abbe numbers measured at a D-line wavelength of about <NUM>. In the following description, refractive indexes and Abbe numbers are values measured at the D-line wavelength of about <NUM>.

The first lens group G1 includes at least four lenses each having an Abbe number of about <NUM> or greater. According to an exemplary embodiment, each of the second lens L12, the third lens L13, the fourth lens L14, and the fifth lens L15 may have an Abbe number of about <NUM> or greater. That is, since the first lens group G1 includes four low-dispersion lenses, aberration may be corrected in the NIR region.

The second lens group G2 has a negative refractive power, and five lenses may be included in the second lens group G2. The second lens group G2 may be moved along the optical axis OA during zooming and may include a sixth lens L21 having a negative refractive power, a seventh lens L22 having a negative refractive power, an eighth lens L23 having a negative refractive power, a ninth lens L24 having a negative refractive power, and a tenth lens L25 having a positive refractive power. Here, the sixth to tenth lenses L21 to L25 may be sequentially arranged from the object side to the image plane side.

The second lens group G2 may include four negative lenses and one positive lens, and a negative lens (the ninth lens L24) and a positive lens (the tenth lens L25) which are closest to the image plane side may form a doublet lens. The second lens group G2 may include at least one lens having an aspherical surface. According to an exemplary embodiment, both surfaces of each of the seventh lens L22 and the eighth lens L23 may be aspherical.

The third lens group G3 has a positive refractive power, and two lenses are included in the third lens group G3. The third lens group G3 may be fixed and includes an eleventh lens L31 having a positive refractive power and a twelfth lens L32 having a negative refractive power. Here, the eleventh lens L31 and the twelfth lens L32 are sequentially arranged from the object side to the image plane side. The negative lens (the twelfth lens L32) of the third lens group G3 have at least one aspherical surface. According to an exemplary embodiment, both surfaces of the twelfth lens L32 may be aspherical.

An Abbe number of the positive lens (the eleventh lens L31) of the third lens group G3 is less than an Abbe number of the negative lens (the twelfth lens L32) of the third lens group G3, and the negative lens (the twelfth lens L32) satisfies Condition <NUM> below: <MAT> where Vd<NUM> refers to the Abbe number of the negative lens of the third lens group G3.

The third lens group G3 is fixed and configured to compensate for tilting of the first lens group G1. That is, if the first lens group G1 is tilted by an angle of about <NUM> degree to about <NUM> degree due to assembly tolerance, optical performance of a zoom lens system may decrease. In each of the zoom lens systems of the exemplary embodiments, however, tilting of the first lens group G1 is compensated for by the third lens group G3 including two lenses and an aspherical surface formed on at least one of the two lenses. That is, although the first lens group G1 is tilted by a certain angle, the third lens group G3 compensates for the tilting of the first lens group G1 and thus, prevents decrease in the optical performance of the zoom lens system. For example, the third lens group G3 may correct comma aberration caused by tilting of the first lens group G1.

The fourth lens group G4 has a positive refractive power, and three lenses may be included in the fourth lens group G4. The fourth lens group G4 may be moved along the optical axis OA during zooming and may include a thirteenth lens L41 having a positive refractive power, a fourteenth lens L42 having a positive refractive power, and a fifteenth lens L43 having a negative refractive power. Here, the thirteenth to fifteenth lenses L41 to L43 may be sequentially arranged from the object side to the image plane side. The fourth lens group G4 may include at least one lens having an aspherical surface. According to an exemplary embodiment, both surfaces of each of the fourteenth lens L42 and the fifteenth lens L43 may be aspherical.

As described above, each of the second lens group G2, the third lens group G3, and the fourth lens group G4 includes a lens having an aspherical surface and thus, aberration of a zoom lens system may be corrected while minimizing the number of lenses of the zoom lens system. That is, the zoom lens system may have a small size and a high degree of resolution at the same time.

The fifth lens group G5 has a positive refractive power, and two lenses may be included in the fifth lens group G5. The fifth lens group G5 may be moved along the optical axis OA during zooming and may include a sixteenth lens L51 having a positive refractive power and a seventeenth lens L52 having a negative refractive power. Here, the sixteenth lens L51 and the seventeenth lens L52 may be sequentially arranged from the object side to the image plane side. The sixteenth lens L51 and the seventeenth lens L52 may form a doublet lens.

The sixth lens group G6 has a positive refractive power, and six lenses may be included in the sixth lens group G6. The sixth lens group G6 may be fixed, and may include an eighteenth lens L61 having a negative refractive power, a nineteenth lens L62 having a positive refractive power, a twentieth lens L63 having a negative refractive power, a twenty-first lens L64 having a positive refractive power, a twenty-second lens L65 having a negative refractive power, and a twenty-third lens L66 having a positive refractive power. Here, the eighteenth to twenty-third lenses L61 to L66 may be sequentially arranged from the object side to the image plane side. The eighteenth lens L61, the nineteenth lens L62, and the twentieth lens L63 may form a triplet lens, and the twenty-first lens L64 and the twenty-second lens L65 may form a doublet lens.

The sixth lens group G6 is closest to the image plane side, and includes the triplet lens and the doublet lens. Thus, chromatic aberration may be easily corrected in a range from a visible ray to an NIR ray without increasing an entire length of a zoom lens system.

According to an exemplary embodiment, a zoom lens system may include at least five lenses each having a refractive index of about <NUM> or greater. According to an exemplary embodiment, at least one lens having a refractive index of about <NUM> or greater may be included in the second lens group G2, the fifth lens group G5, and the sixth lens group G6 that are required to have a sufficiently high degree of refractive power.

The above-described configuration makes it possible to implement six-group zoom lens systems having high magnification and guaranteeing high-resolution photographing at night as well as in the daytime by correcting chromatic aberration in a range from a visible ray to an NIR ray. In addition, a zoom lens system of which aberration is corrected in a peripheral region as well as in a center region of a screen may be implemented.

Hereinafter, design data of the zoom lens systems of the exemplary embodiments will be described with reference to Tables <NUM> to <NUM>.

In the design data, f refers to a focal length [mm], Fno refers to an F-number, R refers to a radius of curvature of each lens in millimeters [mm] (R of a flat surface is indicated by infinity), and Dn refers to a distance between lens surfaces along an optical axis in millimeters [mm], that is, a thickness of a lens or the distance between lenses. Nd refers to a refractive index of each lens at the D-line, and vd refers to an Abbe number of each lens at the D-line.

In the exemplary embodiments, the term "aspherical surface (ASP)" is defined as follows: <MAT>.

In the exemplary embodiments, aspherical surfaces of the zoom lens systems may be expressed by a condition shown above, where a z-axis denotes an optical axis, an h-axis denotes a direction perpendicular to the optical axis, and a propagation direction of light rays is denoted as being positive. In addition, z denotes a distance measured from a vertex of a lens in the direction of the optical axis of the lens, h denotes a distance measured from the optical axis in a direction perpendicular to the optical axis, K denotes a conic constant, A, B, C, and D denote aspherical surface coefficients, and c denotes a reciprocal (<NUM>/R) of a radius of curvature at the vertex of the lens.

Table <NUM> shows design data of the zoom lens system illustrated in <FIG> according to a first exemplary embodiment. A surface number Si is given to an ith surface as follows: an object-side surface of the first lens L11 is numbered S1, and other surfaces of lenses are sequentially numbered in a direction toward the image plane side.

In Table <NUM> above, the mark "*" refers to an aspherical surface, and Table <NUM> below shows aspherical surface coefficients of aspherical surfaces of the zoom lens system of the first exemplary embodiment illustrated in <FIG>. E-m (m is an integer) in aspherical surface coefficients refers to ×<NUM>-m.

Table <NUM> below shows a focal length, F-number Fno, and variable distances of the zoom lens system of the first exemplary embodiment illustrated in <FIG> at the wide-angle position and the telephoto position.

<FIG> illustrates longitudinal spherical aberration, astigmatism, and distortion of the zoom lens system of the first exemplary embodiment at the wide-angle position.

In <FIG>, the longitudinal spherical aberration is illustrated for light wavelengths of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, and astigmatism and distortion are illustrated for a light wavelength of <NUM>. In the astigmatism, a dotted curve T refers to a tangential surface T, and a solid line refers to a sagittal surface S.

Referring to <FIG>, aberration is corrected over an entire image height from a center region to an edge region of the image plane IP. For example, chromatic aberration virtually does not occur from a center region to an edge region of the image plane IP in a range from a blue wavelength of <NUM> to an NIR wavelength of <NUM>. According to an exemplary embodiment, a maximum distance between focal points of light having wavelengths in a range of <NUM> to <NUM> may be about <NUM> or less.

<FIG> illustrates longitudinal spherical aberration of the zoom lens system of the first exemplary embodiment at the wide-angle position and the telephoto position for light having a first wavelength λ<NUM> and light having a second wavelength λ<NUM>.

Referring to <FIG>, a distance between a focal point of light having a first wavelength λ<NUM> corresponding to the green light and a focal point of light having a second wavelength λ<NUM> corresponding to the NIR light is about <NUM> or less both at the wide-angle position and the telephoto position (refer to d<NUM> and d<NUM> in <FIG>). According to an exemplary embodiment, the first wavelength λ<NUM> may be about <NUM>, and the second wavelength λ<NUM> may be about <NUM>. The above-mentioned distance is a distance measured in a direction parallel with the optical axis OA.

According to an exemplary embodiment, since a difference between the focal point of visible light directed to the image plane IP and the focal point of NIR light directed to the image plane IP is very small over the entire region of the image plane IP, focal point misalignment may be suppressed during transition from a daytime photographing mode to a night photographing mode and thus, high-resolution photographing may be possible at night as well as in the daytime. That is, the sharpness of images captured at night may be improved and thus, double images may not be captured.

Table <NUM> shows design data of the zoom lens system illustrated in <FIG> according to a second exemplary embodiment.

In Table <NUM> above, the mark "*" refers to an aspherical surface, and Table <NUM> below shows aspherical surface coefficients of aspherical surfaces of the zoom lens system of the second exemplary embodiment illustrated in <FIG>. E-m (m is an integer) in aspherical surface coefficients refers to ×<NUM>-m.

Table <NUM> below shows a focal length, F-number Fno, and variable distances of the zoom lens system of the second exemplary embodiment illustrated in <FIG> at the wide-angle position and the telephoto position.

<FIG> illustrates longitudinal spherical aberration, astigmatism, and distortion of the zoom lens system of the second exemplary embodiment at the wide-angle position.

In <FIG>, the longitudinal spherical aberration is illustrated for light wavelengths of <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and <NUM>, and the astigmatism and distortion are illustrated for a light wavelength of <NUM>. In the astigmatism, a dotted curve T refers to a tangential surface T, and a solid line refers to a sagittal surface S.

Referring to <FIG>, aberration is corrected over an entire image height from a center region to an edge region of the image plane IP. For example, chromatic aberration virtually does not occur from a center region to an edge region of the image plane IP in a range from a blue wavelength of <NUM> to an NIR wavelength of <NUM>. According to an exemplary embodiment, a maximum distance between the focal points of light having wavelengths in a range of <NUM> to <NUM> may be about <NUM> or less.

Table <NUM> shows design data of the zoom lens system illustrated in <FIG> according to a third exemplary embodiment.

In Table <NUM> above, the mark "*" refers to an aspherical surface, and Table <NUM> below shows aspherical surface coefficients of aspherical surfaces of the zoom lens system of the third exemplary embodiment illustrated in <FIG>. E-m (m is an integer) in aspherical surface coefficients refers to ×<NUM>-m.

Table <NUM> below shows a focal length, F-number Fno, and variable distances of the zoom lens system of the third exemplary embodiment illustrated in <FIG> at the wide-angle position and the telephoto position.

<FIG> illustrates longitudinal spherical aberration, astigmatism, and distortion of the zoom lens system of the third exemplary embodiment at the wide-angle position.

Referring to <FIG>, aberration is corrected over an entire image height from a center region to an edge region of the image plane IP. For example, the chromatic aberration virtually does not occur from a center region to an edge region of the image plane IP in a range from a blue wavelength of <NUM> to an NIR wavelength of <NUM>. According to an exemplary embodiment, a maximum distance between the focal points of light having wavelengths in a range of <NUM> to <NUM> may be about <NUM> or less.

Table <NUM> below shows that the zoom lens systems of the exemplary embodiments satisfy Conditions <NUM> to <NUM>.

As described above, the exemplary embodiments provide six-group zoom lens systems having high magnification and guaranteeing high-resolution photographing at night as well as in the daytime by correcting chromatic aberration in a range from a visible ray to an NIR ray. In addition, zoom lens systems of which aberration is corrected in a peripheral region as well as in a center region of a screen may be provided.

Claim 1:
A zoom lens system comprising:
a first lens group (G1) having a positive refractive power;
a second lens group (G2) having a negative refractive power;
a third lens group (G3) having a positive refractive power;
a fourth lens group (G4) having a positive refractive power;
a fifth lens group (G5) having a positive refractive power; and
a sixth lens group (G6) having a positive refractive power,
wherein the first to sixth lens groups (G1 to G6) are sequentially arranged along an optical axis (OA) from an object side (<NUM>) to an image plane side (IP), and wherein the zoom lens system is configured to perform zooming between a wide-angle position and a telephoto position by moving at least one of the second lens group (G2), the fourth lens group (G2) and the fifth lens group (G5) along the optical axis (OA),
wherein a distance between a focal point of light having a first wavelength (λ<NUM>) and a focal point of light having a second wavelength (λ<NUM>) is about <NUM> or less at the wide-angle position and the telephoto position, and
wherein the first wavelength (λ<NUM>) corresponds to green light, the second wavelength (λ<NUM>) corresponds to near infrared (NIR) light,
characterized in that
the third lens group (G3) comprises a positive lens and a negative lens sequentially arranged from the object (O) side, and at least one surface of the negative lens is aspherical, and an Abbe number of the positive lens of the third lens group (G3) is less than an Abbe number of the negative lens of the third lens group (G3), and
in that the zoom lens system satisfies a following condition: <MAT> where Vd<NUM> is an Abbe number of the negative lens of the third lens group (G3),
and in that the third lens group (G3) consists of two lenses.