Varifocal optical system

The present disclosure relates to the field of imaging technologies and, particularly, relates to a varifocal optical system. The varifocal optical system includes a front fixing group, a triple prism, a zooming group, a compensating group, an objective lens group and an image plane which are successively arranged along an incident direction of light, and a total focal length of the varifocal optical system is a fixed value; the front fixing group includes a first lens; the zooming group includes a second lens; the compensating group includes a third lens and a fourth lens arranged along an optical path direction; the object lens group includes a fifth lens; and the first lens, the second lens, the third lens, the fourth lens and the fifth lens are all glass lenses. The present disclosure can satisfy the developing trend of miniaturization on electronic devices, and has a better imaging performance.

TECHNICAL FIELD

The present disclosure relates to the field of imaging technologies and, particularly, relates to a varifocal optical system.

BACKGROUND

With the development of miniaturization of electronic devices such as cellphone, camera etc., imaging devices thereon are also being miniaturized. In varifocal optical systems of the prior art, a clear imaging is generally achieved by focal length switching through a zooming group and a compensating group. However, the structure of such a varifocal optical system is relatively complex, and the total length is relatively long, which cannot satisfy the developing trend of miniaturization of the electronic devices.

REFERENCE SIGNS

The drawings herein are incorporated into the specification and form a part thereof, which show embodiments of the present disclosure, and are used to explain the principles of the present disclosure together with the specification.

DESCRIPTION OF EMBODIMENTS

The present disclosure will be described in further detail with reference to the following embodiments and the accompany drawings.

As shown fromFIG. 1toFIG. 3, the embodiments of the present disclosure provides a varifocal optical system, including: a front fixing group, a triple prism TP, a zooming group, a compensating group, an object lens group and an image plane W which are successively arranged along an incident direction of light. A total length of the varifocal optical system remains unchanged, and the varifocal optical system adopts a continuous zooming manner. Specifically, the front fixing group includes a first lens L1, the first lens L1is a front-convex-and-back-concave lens, that is, an object-side surface of the first lens L1is a convex surface, and an image-side surface is a concave surface. The triple prism TP is configured for refraction of optical path, which can be a right-angle triple prism, a right-angle edge of the triple prism TP is facing the first lens L1, and the other right-angle edge is facing the second lens L2. The zooming group and the compensating group can move between the triple prism TP and the object lens group along an optical path direction, so as to achieve continuous switching of the focal length. The zooming group includes a second lens L2, the second lens L2is a double-convex lens. The compensating group includes a third lens L3and a fourth lens L4which are arranged along the optical path direction, the third lens L3is a double-concave lens, the fourth lens L4is a double-convex lens. The object lens group can move with respect to the image plane W along the optical path direction to achieve fine adjustment of the object lens group, so as to be used for focusing in scenarios at different distances. The object lens group includes a fifth lens L5, the fifth lens L5is a double-convex lens. It should be noted that, the above ‘front’ refers to a side along the optical path direction close to the incident end, the ‘back’ refers to a side along the optical path direction away from the incident end. The first lens L1, the second lens L2, the third lens L3, the fourth lens L4and the fifth lens L5are all glass lenses.

In the above-mentioned structure, through adding the triple prism TP, the optical path can be refracted to reduce the total length of the optical system, so as to be adapted to the developing trend of miniaturization on electronic device. The cooperation of the zooming group and the compensating group allows the object to be clearly imaged clearly from a limited distance to an infinite distance.

Further, the first lens L1and the third lens L3have negative focal power, the second lens L2, the fourth lens L4and the fifth lens L5have positive focal power.

Among an object-side surface and an image-side surface of the second lens L2, an object-side surface and an image-side surface of the third lens L3, an object-side surface and an image-side surface of the fifth lens L5, at least one of them is an aspheric surface, and the rest are spherical surfaces. Optionally, the object-side surface and the image-side surface of the first lens L1, the object-side surface and the image-side surface of the fourth lens L4are all spherical surfaces; and the object-side surface and the image-side surface of the second lens L2, the object-side surface and the image-side surface of the third lens L3, the object-side surface and the image-side surface of the fifth lens L5are all aspheric surfaces. That is to say, all points on the object-side surface and the image-side surface of the first lens L1, and all points on the object-side surface and the image-side surface of the fourth lens L4satisfy a spherical equation. All points on the object-side surface and the image-side surface of the second lens L2, all points on the object-side surface and the image-side surface of the third lens L3, and all points on the object-side surface and the image-side surface of the fifth lens L5satisfy an aspheric equation. The arrangement of the spherical surfaces and the aspheric surfaces can better correct aberration, so as to improve imaging quality.

In the above-mentioned embodiments, the varifocal optical system satisfies the following conditional expression,
7.5 mm≤D1≤12.5 mm;  (1)

D1is an axial distance from the object-side surface of the first lens L1to an end of the triple prism TP away from the first lens L1, as shown inFIG. 1.

Conditional expression (1) defines the axial distance between the first lens L1and the triple prism TP, for example D1=7.5 mm, 8.0 mm, 9 mm, 9.8 mm, 9.85 mm, 10 mm, 11 mm, 12.5 mm etc., which decreases the size of the varifocal optical system in the incident direction of light as much as possible while guaranteeing the imaging quality, so as to be better adapted to the development trend of miniaturization on the electronic device.

The varifocal optical system in the above-mentioned embodiments satisfies the following conditional expressions,
D1/ΣTi<1.1;  (2)
ΣTi=T1+T5+T8+T10+T12;  (3)

the D1is the axial distance from the object-side surface of the first lens L1to an end of the triple prism TP away from the first lens L1, as shown inFIG. 1;

ΣTi is a sum of central thicknesses of the first lens L1, the second lens L2, the third lens L3, the fourth lens L4and the fifth lens L5;

T1is a central thickness of the first lens L1;

T5is central thickness of the second lens L2;

T8is central thickness of the third lens L3;

T10is central thickness of the fourth lens L4;

T12is central thickness of the fifth lens L5.

The above-mentioned conditional expressions (2), (3) define the ratio of the radial dimension to the length of the optical system, so as to further reduce the volume of the entire optical system, and be adapted to the development trend of miniaturization on the electronic device.

nTP is a refractive index of the triple prism;

n1is a refractive index of the first lens L1;

n2is a refractive index of the second lens L2;

n3is a refractive index of the third lens L3;

n4is a refractive index of the fourth lens L4;

n5is a refractive index of the fifth lens L5.

In the above-mentioned conditional expressions, conditional expression (5) defines the refractive index of the triple prism TP, for example nTP=1.75, 1.8, 1.8042, 1.85 etc., in the range defined by conditional expression (5), it is advantageous to increase the angle of incidence, and increase imaging range of the optical system. It is preferred that the nTP approaches an upper limit. If the nTP is below a lower limit, it is disadvantageous to increase the angle of incidence, result in a reduced imaging range; if the nTP is beyond the upper limit, the material selection will be limited, leading to cost increase.

Conditional expressions (4), (6), (7), (8), (9) respectively define the refractive indexes of the first lens L1, the second lens L2, the third lens L3, the fourth lens L4and the fifth lens L5, for example n1=1.55, 1.58, 1.6, 1.638542, 1.65 etc., n2=1.55, 1.58, 1.6, 1.61 etc., n3=1.7, 1.72, 1.74, 1.75 etc., n4=1.53, 1.56, 1.59, 1.6 etc., n5=1.55, 1.57, 1.6, 1.61. In the ranges defined in the above-mentioned conditional expressions, it is advantage to correct chromatic aberration, and thus improve imaging quality. If refractive indexes are below the lower limit or beyond the upper limit defined in the conditional expressions (4), (6), (7), (8), (9), it is disadvantageous to correct chromatic aberration, and material selection will be limited, leading to cost increase.

f1is a focal length of the first lens;

f2is a focal length of the second lens;

f3is a focal length of the third lens;

f4is a focal length of the fourth lens;

f5is a focal length of the fifth lens;

fs is a focal length of the varifocal optical system in a short focus state.

Conditional expressions (10)-(14) define the ratio of the focal length of each lens to the focal length of the varifocal optical system in the short focus state, for example f1/fs=−3.0, −2.8, −2.634, −2.5, −2.2, −2.1 etc., f2/fs=1.5, 1.6, 1.7317, 1.85, 1.9, 2.0 etc., f3/fs=−1.0, −0.9, −0.6232, −0.6, −0.5, −0.35, −0.3 etc., f4/fs=0.6, 0.65, 0.8, 0.9, 1.171, 1.2, 1.5 etc., f5/fs=1.75, 1.8, 2.0, 2.1, 2.1574, 2.25, 2.45 etc., in the ranges defined in the conditional expressions (10)-(14), the allocation of focal power of each lens is reasonable, which can effectively control the curvature field of the system.

The varifocal optical system in the above-mentioned embodiments satisfies the following conditional expression,
T4>2 mm;  (15)

T4is an axial distance from the image-side surface of the triple prism TP to the object-side surface of the second lens L2, when the system is in long focal length.

Conditional expression (15) defines the axial distance from the image-side surface of the triple prism TP to the object-side surface of the second lens L2, such as T4=2.0 mm, 2.34 mm, 3 mm etc., so as to guarantee all incident light enters into the second lens L2after being refracted by the triple prism TP, thereby guaranteeing imaging quality.

Generally, the varifocal optical system also includes a diaphragm S, along the optical path direction, the diaphragm S is located between the second lens L2and the third lens L3, the varifocal optical system satisfies the following conditional expressions,
T6>1.5 mm;  (16)
T7>1.5 mm;  (17)

T6is an axial distance from the image-side surface of the second lens L2to the diaphragm S;

T7is an axial distance from the diaphragm S to the object-side surface of the third lens L3.

Conditional expressions (16), (17) define the distance between the diaphragm S and the second lens L2, and the distance between the diaphragm S and the third lens L3. For example T6, T7can respectively be selected from the following values: 1.55 mm, 1.6 mm, 1.9 mm, 2.2 mm, 2.34 mm, 2.5 mm etc., in the range defined by conditional expressions (16), (17), it is guaranteed that enough light can pass through, so as to guarantee intensity of the light, and thereby improving imaging quality.

Generally, the varifocal optical system also includes a glass plate G1, along the optical path direction, the glass plate G1is arranged between the fifth lens L5and the image plane W.

The following parameters in accordance with an exemplary embodiment will illustrate the varifocal optical system provided in the present disclosure. Table 1 lists parameters of the first lens L1, the second lens L2, the third lends L3, the fourth lens L4, the fifth lens L5, the triple prism TP and the diaphragm S in the present embodiment. Table 2 lists aspheric coefficients of each aspheric lens in the present embodiment. Table 3 lists various parameters of the present embodiment in a short focus state, a medium focus state, and a long focus state. Table 4 lists specific parameters of each conditional expression in the present embodiment of the present disclosure. The parameters in the tables are defined as follows:

R11: curvature radius of object-side surface of the first lens L1;

R12: curvature radius of image-side surface of the first lens L1;

R21: curvature radius of object-side surface of the second lens L2;

R22: curvature radius of image-side surface of the second lens L2;

R31: curvature radius of object-side surface of the third lens L3;

R32: curvature radius of image-side surface of the third lens L3;

R41: curvature radius of object-side surface of the fourth lens L4;

R42: curvature radius of image-side surface of the fourth lens L4;

R51: curvature radius of object-side surface of the fifth lens L5;

R52: curvature radius of image-side surface of the fifth lens L5;

n1: refractive index of the first lens L1;

nTP: refractive index of the triple prism TP;

n2: refractive index of the second lens L2;

n3: refractive index of the third lens L3:

n4: refractive index of the fourth lens L4:

n5: refractive index of the fifth lens L5:

n6: refractive index of the glass plate G1;

V1: chromatic dispersion coefficient of the first lens L1;

VTP: chromatic dispersion coefficient of the triple prism;

V2: chromatic dispersion coefficient of the second lens L2;

V3: chromatic dispersion coefficient of the third lens L3;

V4: chromatic dispersion coefficient of the fourth lens L4;

V5: chromatic dispersion coefficient of the fifth lens L5;

V6: chromatic dispersion coefficient of the glass plate G1;

T1: central thickness of the first lens L1;

T2: axial distance from the image-side surface of the first lens L1to the object-side surface of the triple prism TP;

T31: horizontal central thickness of the triple prism TP;

T32: vertical central thickness of the triple prism TP;

T4: axial distance from the image-side surface of the triple prism TP to the object-side surface of the second lens L2;

T5: central thickness of the second lens L2;

T6: axial distance from the image-side surface of the second lens L2to the diaphragm S;

T7: axial distance from the diaphragm S to the object-side surface of the third lens L3;

T8: central thickness of the third lens L3;

T9: axial distance from the image-side surface of the third lens L3to the object-side surface of the fourth lens L4;

T10: central thickness of the fourth lens L4;

T11: axial distance from the image-side surface of the fourth lens L4to the object-side surface of the fifth lens L5;

T12: central thickness of the fifth lens L5;

T13: axial distance from the image-side surface of the fifth lens L5to the object-side surface of the glass plate G1;

T14: central thickness of the first glass plate G1;

T15: axial distance from the image surface of the first glass plate G1to the image plane W;

f: focal length of the varifocal optical system;

fs: focal length of the varifocal optical system in a short focus state;

f1: focal length of the first lens L1:

f2: focal length of the second lens L2;

f3: focal length of the third lens L3;

f4: focal length of the fourth lens L4;

f5: focal length of the fifth lens L5;

FOV: field of view;

H: horizontal field of view;

V: vertical field of view;

D1: axial distance from the object-side surface of the first lens L1to an end of the triple prism TP away from the first lens L1;

For ease of convenience, the aspheric surface of each lens surface satisfies the aspheric equation of conditional expression (18), in the equation, R is axial curvature radius, k is cone coefficient, A4, A6, A8, A10, A12, A14, A16 are aspheric surface coefficients.

It should be noted that, in Table 1, the T4, T6, T11, T15are distance values in medium focus state; when in long focus state, T4=2.001, T6=4.738, T11=8.071, T15=0.96; when in a short focus state, T4=11.498, T6=2.915, T11=0.5, T15=0.963.

It should be noted that, the height of the image of the present embodiment by adopting parameters in Tables 1-3 is 2.45 mm.

FIG. 1shows a structural schematic diagram of the embodiment adopting the parameters in Tables 1-3 being in a short focus state,FIG. 4shows vertical axis chromatic aberration in a short focus state,FIG. 7shows the field curvature and distortion in a short focus state;FIG. 2is a structural schematic diagram of the embodiment adopting the parameters in Tables 1-3 in a medium focus state,FIG. 5shows vertical axis chromatic aberration in a medium focus state,FIG. 8shows the field curvature and distortion in a medium focus state;FIG. 3is a structural schematic diagram of the embodiment adopting the parameters in Tables 1-3 in a long focus state,FIG. 6shows vertical axis chromatic aberration in a long focus state,FIG. 9shows the field curvature and distortion in a long focus state. Curves S1, S3, S5are the vertical axial chromatic aberration of the short wave length with respect to the long wave length. Curves S2, S4, S6are the vertical axial chromatic aberration of the short wave length corresponding to the main wave length. It can be seen fromFIG. 1toFIG. 3, during zooming from short focus to medium focus, and to long focus, the zooming group (i.e., second lens L2) is gradually away from the triple prism TP, the compensating group (i.e., the third lens L3) is gradually close to the image plane W; It can be seen fromFIG. 4toFIG. 9, adopting the embodiments provided by the present disclosure can obtain better imaging quality.

The above are just the preferred embodiments of the present disclosure, which will not limit the present disclosure. For those skilled in the art, the present disclosure can have various modifications and variations. Any modifications, equivalent replacements and improvements made within the spirits and principles of the present disclosure shall all fall in the protection scope of the present disclosure.