EP3507639A1

EP3507639A1 - Wide-angle optical system with variable focus

Info

Publication number: EP3507639A1
Application number: EP17771488.8A
Authority: EP
Inventors: Emmanuel Hugot; Wilfried Jahn; Marc Ferrari
Original assignee: Aix Marseille Universite; Centre National de la Recherche Scientifique CNRS
Current assignee: Aix Marseille Universite; Centre National de la Recherche Scientifique CNRS
Priority date: 2016-09-01
Filing date: 2017-08-30
Publication date: 2019-07-10
Also published as: WO2018042129A1; FR3055428B1; FR3055428A1

Abstract

A fisheye-type wide-angle optical system (1) with variable focus mounted on an electronic image formation system (2) comprising a non-planar electronic image sensor (3) with fixed radius of curvature. The optical system comprises a first group of lenses (A) with negative refractive power and a second group of lenses (B) with positive refractive power moving with respect to each other between a shorter focal distance configuration and a longer focal distance configuration. The optical system satisfies condition (I) where C is the radius of curvature of the image formed by the optical system and bfw is the distance between the end lens of the imaged space of the second group of lenses and the object image formed by the optical system in the shorter focal distance configuration.

Description

Wide angle optical system with variable focus.

The present invention relates to electronic image forming systems having fisheye wide angle optical systems with variable focal length.

More particularly, the invention relates to a fisheye-type wide angle optical system with variable focal length capable of passing from a diagonal fisheye to a circular fisheye.

Such an optical system is intended to be mounted on an electronic image forming system comprising an electronic image sensor, being associated with said sensor positioned in the image space of the optical system.

"Fisheye wide-angle optical system" means that the optical system has a maximum field of view greater than 120 degrees in at least one general direction of the field of view of the optical system, and preferably a higher maximum field of view. at 160 degrees or even a maximum angle of field close to 180 degrees.

By "Fisheye wide angle optical system with variable focal length" is meant a change in the region of the image formed by the optical system on which the maximum angle of view is reached. In particular, such an optical system may be such that the field of view is fixed during the change of focal length.

Thus, "circular fisheye" means a configuration in which the entire field of view, whose maximum angle is close to 180 degrees, is written in the electronic sensor. The entire field of view is thus imaged in a circle inscribed on the sensor.

And by "diagonal fisheye" is meant a configuration in which the maximum angle of example close to 180 degrees, is obtained only in a few directions, especially on diagonals of the image sensor.

Such an optical system is for example described in the document EP 2 407 809 A1 and usually comprises substantially aligned along an optical axis OI from an object space 0 to an image space I of the optical system: a first group of lenses A negative vergence , and

a second group of positive-verging lenses B, the first group of lenses A and the second group of lenses B moving relative to one another along the optical axis between a configuration of shorter focal length fw and a configuration of longer focal length ft.

The configuration of shorter focal length then corresponds to a circular fisheye while the configuration of longer focal length corresponds to a diagonal fisheye.

The optical system described in the document

However, EP 2 407 809 A1 has several disadvantages. This system comprises a large number of lenses (14 lenses) made of a similarly large number of different materials (10 different materials). Many of these lenses have aspheric surfaces that are expensive to manufacture and that require demanding manufacturing and alignment tolerances. Furthermore, this optical system has a large vignetting which limits the optical performance of the system in terms of nonuniformity of the distribution of light intensity on the sensor causing a significant drop in this intensity as one moves away from the center of the image. There is a need to improve the optical characteristics of an electronic imaging system having such an optical system, as well as to reduce the complexity and cost of manufacture.

The present invention is intended to overcome these disadvantages.

For this purpose, according to the invention, an electronic image forming system comprising an optical system of the kind in question and a non - planar electronic image sensor, the optical system being specially adapted to be mounted on the training system. An electronic image comprising the non - planar electronic image sensor associated with said sensor positioned in the optical system image space is characterized by further satisfying the following condition:

VS

3.5 <- <7.5

bfw

where C is the radius of curvature of the image formed by the optical system and bfw is the distance between the end lens of the image space of the second lens group B and the object image formed by the optical system in the configuration of shorter focal length, and in that the non-planar electronic image sensor has a fixed radius of curvature, in particular fixed during a change in focal length of the optical system.

Thanks to these arrangements, it is possible to use a reduced number of lenses and a reduced number of manufacturing materials. It is also possible to improve the optical performance of the system by significantly removing vignetting.

In preferred embodiments of the invention, one or more of the following provisions may be used:

the optical system satisfies the following condition:

\ F1 \ .F2

0.20 <- ^■ - _T ≤ 0.30

(bf _W y

where Fl is a focal length of the fixed lens group A, F2 is a focal length of the second lens group B and bfw is the distance between the end lens of the image space of the second lens group B and the image object formed by the optical system in the configuration of shorter focal length;

said second lens group B comprises, in order from an image space thereof, a positive lens and a negative lens, in particular a positive lens, a negative lens and a positive lens;

said second group of lenses B comprises less than eight lenses, preferably less than seven lenses, even more preferably less than six lenses;

the optical system satisfies the following condition:

bfw

2.50 <-L- <3.40 where bfw is the distance between the end lens of the image space of the second lens group B and the object image formed by the optical system in the shorter focal length configuration and Fl is a focal length of the first lens group A;

the optical system satisfies the following condition:

0.30 <SF1≤ 0.50

where SF1 is a form factor of a first lens from the object space of the fixed lens group A such that SFL1 = (r1-r2) / (r1 + r2) where r1 is a radius of curvature from the object space of said first lens and r2 is a radius of curvature from the image space of said first lens;

the optical system satisfies the following condition:

M2

0.45 <<0.6

F2

where M2 is an axial displacement of the second lens group B between the shorter focal length configuration and the longer focal length configuration and F2 is a focal length of the second lens group B;

said second group of lenses B comprises an alternating juxtaposition of positive and negative lenses, starting with a positive lens from the object space of said lens group;

the non-planar electronic image sensor is convex.

Other features and advantages of the invention will become apparent from the following description of several of its embodiments, given by way of non-limiting examples, with reference to the accompanying drawings.

On the drawings:

FIG. 1 diagrammatically illustrates a first exemplary embodiment of an optical system according to the invention in which the first lens group comprises five lenses and the second lens group of the optical system also comprises five lenses,

FIG. 2A illustrates four aberration curves of the optical system of FIG. 1 at shorter focal length, respectively from left to right, spherical aberration for three wavelengths 0.4861 μm, 0.5876 μm and 0.6563 μm, the curvature from field to field wavelength 0.5876 μm, side chromaticism at wavelength 0.5876 μm and distortion at wavelength 0.5876 μm,

FIG. 2B illustrates the aberration curves of FIG. 2A for an intermediate focal distance of the example of FIG. 1,

FIG. 2C illustrates the aberration curves of FIG. 2A for the largest focal length of the example of FIG. 1,

FIG. 3 schematically illustrates a second exemplary embodiment of an optical system according to the invention in which the first lens group comprises five lenses and the second lens group of the optical system comprises six lenses,

FIG. 4A illustrates four aberration curves of the optical system of FIG. 3 at shorter focal length, respectively from left to right, spherical aberration for three wavelengths 0.4861 μm, 0.5876 μm and 0.6563 μm, the curvature field strength at wavelength 0.5876 μm, side chromaticism at wavelength 0.5876 μm and distortion at wavelength 0.5876 μm,

FIG. 4B illustrates the aberration curves of FIG. 4A for an intermediate focal length of the example of FIG. 3,

FIG. 4C illustrates the aberration curves of FIG. 4A for the greatest focal length of the example of FIG. 3,

FIG. 5 schematically illustrates a third exemplary embodiment of an optical system according to the invention in which the first lens group comprises five lenses and the second lens group of the optical system comprises seven lenses,

FIG. 6A illustrates four aberration curves of the optical system of FIG. 5 at shorter focal length, respectively from left to right, spherical aberration for three wavelengths 0.4861 μm, 0.5876 μm and 0.6563 μm, the curvature field strength at wavelength 0.5876 μm, side chromaticism at wavelength 0.5876 μm and distortion at wavelength 0.5876 μm,

FIG. 6B illustrates the aberration curves of FIG. 6A for an intermediate focal length of the example of FIG. 5,

FIG. 6C illustrates the aberration curves of FIG. 6A for the largest focal length of the example of FIG. 5,

FIG. 7 schematically illustrates a fourth embodiment of an optical system according to the invention in which the first lens group comprises five lenses and the second lens group of the optical system comprises six lenses,

FIG. 8A illustrates four aberration curves of the optical system of FIG. 7 at shorter focal length, respectively from left to right, spherical aberration for three wavelengths 0.4861 μm, 0.5876 μm and 0.6563 μm, the curvature of FIG. field at wavelength 0.5876 μm, side chromaticism at wavelength 0.5876 μm and distortion at wavelength 0.5876 μm,

FIG. 8B illustrates the aberration curves of FIG. 8A for an intermediate focal length of the example of FIG. 7,

FIG. 8C illustrates the aberration curves of FIG. 8A for the greatest focal length of the example of FIG. 7,

FIG. 9 illustrates an angle between the optical axis and a main ray of an off-axis light beam arriving on the lens closest to the object space, and

Fig. 10 is a schematic illustration of an electronic image forming system according to one embodiment of the invention.

In the different figures, the same references designate identical or similar elements.

FIG. 1 illustrates a first embodiment of an optical system 1 integrated in an electronic image forming system 2 comprising a non-planar electronic image sensor 3 according to the invention. The optical system 1 extends with axial symmetry along an optical axis O from an object space to an image space of the optical system.

The optical system comprises, in order from the object space, a first group of lenses A and a second group of lenses B.

The first group of lenses A has a negative vergence.

The second group of lenses B has a positive vergence.

The first group of lenses A and the second group of lenses B move relative to each other along the optical axis during a change of focal length of the optical system.

More specifically, during a change of focal length of the optical system from a short focal length fw to a long focal length ft, the first lens group A and the second group of lenses B move relative to each other between a configuration of shorter focal length and a configuration of longer focal length.

The configuration of shorter focal length corresponds for example to a circular fisheye while the configuration of longer focal length corresponds to a diagonal fisheye.

The region of the image formed by the optical system on which the maximum field angle is reached is thus varied between the shorter focal length configuration and the longer focal length configuration.

The size of the image formed by the optical system is thus varied between the configuration of shorter focal length and the configuration of longer focal length.

Advantageously, the optical system is such that

r = 2 * / * sinQ (85 ° <0 <90) (1)

where f is a focal length such that fw <f <ft and Y is the image size of an incident ray with an angle Θ, where Θ is the angle, shown in Figure 9, between the optical axis (0) and a main ray (P) of an off-axis light beam arriving on the lens closest to the object space. This projection performed by the wide-angle optical system is called an equisolide angle projection.

In this way, the optical system has a maximum near-field angle of 180 degrees with reduced distortion at the center of the image.

On the other hand, the optical system can also satisfy the following condition: where Yt is a larger image size for the optical system in the longer focal length configuration and Yw is a larger image size for the optical system in the shorter focal length configuration.

Condition (2) provides an optical system capable of passing from a configuration of shorter focal length corresponding to a circular fisheye to a configuration of longer focal length corresponding to a diagonal fisheye.

In particular, the condition (2) guarantees the obtaining of a circular fisheye and a diagonal fisheye for a variety of image sensor formats, in particular APS-H format image sensors (with dimensions of 28.1 mm x 18.7 mm and a diagonal of 33.8 mm), APS-C image sensors (with dimensions of 22.5 mm x 15.0 mm and a diagonal of 27.0 mm) and full-size image sensors -size "(with dimensions of 36 mm x 24 mm and a diagonal of 43.2 mm).

Moreover, the optical system is such that:

3.5 <- ^ <7.5 (3)

bfw

where C is a radius of curvature of the image formed by the optical system and bfw is a draw of the objective, that is to say a distance between, on the one hand, a last lens of the second group of lenses B disposed the image space of said second lens group, and secondly, an image formed by the optical system in the configuration of shorter focal length.

vs

If the ratio is below the lower limit bfw

of condition (3), the radius of curvature required for the image sensor is too large and the aberrations optical becomes prejudicial or the manufacture of the image sensor becomes difficult or impossible. If the c

When the ratio is greater than the upper limit of the condition (3), the required radius of curvature for the image sensor is too large and the system has the drawbacks of prior art systems discussed above, such as greater manufacturing complexity and lower optical performance.

Considering first more particularly the first group of lenses A, the optical system can satisfy the following condition:

2.50 <^ <3.40 (4)

| F1 |

where Fl is a focal length of the first group of lenses A.

bfw

When -y ^ j exceeds the upper limit of condition (4), the first lens group A has a short focal length and it becomes difficult to correct the field curvature and chromatic side aberrations. The system then has the disadvantages of prior systems discussed above, including greater manufacturing complexity and lower optical performance due to increased aberrations.

bfw

When --- is below the lower limit of

| F1 |

the condition (4), the first lens group A has a long focal length, it is difficult to obtain a circular fisheye in the configuration of shorter focal length while maintaining a draw bfw sufficient to fit the electronic boxes current.

The optical system can also satisfy the following condition:

0.30 <SF1≤ 0.50 (5)

where SF1 is a shape factor of a first lens Al of the object space of the fixed lens group A such that SF1 = (r1-r2) / (r1 + r2) where r1 is a radius of curvature from space object of said first lens and r2 is a radius of curvature from the image space of said first lens.

When SF1 exceeds the upper limit of the condition (5), the lens A1 has too strong a vergence and it becomes difficult to correct the aberration of curvature generated as the lateral chromatic aberration.

When SF1 is lower than the lower limit of condition (5), the first lens of the object space of the first lens group A has a weak vergence and it is difficult to obtain a wide angle fisheye lens.

Now considering more particularly the second group of lenses B, the optical system can satisfy the following condition:

0.45≤-≤ 0.6 (6)

F2

where M2 is an axial displacement of the second lens group B between the shorter focal length configuration and the longer focal length configuration and F2 is a focal length of the second lens group B.

M2

When - exceeds the upper limit of the condition

(6), the maximum displacement value M2 of the second lens group B becomes excessively large and the optical aberration variations become too large to be sufficiently corrected. M2

When - is below the lower limit of the

^ F2

condition (6), the maximum displacement value M2 of the second lens group B becomes too small to provide the desired zoom factor.

The optical system can also satisfy the following condition:

0.2 <- <0.5 (7)

F2

When exceeds the upper limit of condition (7), the first lens group A has a very large focal length and it becomes difficult to achieve a circular fisheye.

When - is below the lower limit of the condition (7), the first lens group A has a small focal length and it becomes difficult to correct the field curvature and chromatic side aberrations.

The optical system may further satisfy the following condition:

_{0 20 ≤} \ FllF2 _{≤ 0}

{pfw

When \ ^pl \ - ^F2 exceeds the upper limit of the

{pfwy

condition (8), it becomes difficult to achieve a circular fisheye while maintaining a draw bfw sufficient to adapt to current electronic boxes.

When ^ ^F1 ^ ^F2 is less than the lower limit of

{pfwy

condition (8), it becomes difficult to correct the field curvature and lateral chromatic aberrations.

Four embodiments of the invention will now be described in more detail.

In each of these embodiments, the first A group of lenses has a negative vergence while the second group of lenses B has a positive vergence.

Moreover, the first group of lenses A has five lenses referenced A1 to A5 from the object space to the image space of the optical system.

The first group of lenses A comprises:

a first lens A1 which is a negative meniscus with a convex surface towards the object space,

a second lens A2 which is also a negative meniscus,

a third lens A3 which is also a negative meniscus,

a fourth lens A4 which is a positive lens, and

a fifth lens A5 which is a negative lens.

The precise characteristics of the lenses A1-A5 of the first group of lenses A are variable according to the embodiments of the invention described hereinafter.

FIRST EMBODIMENT

A first embodiment of the invention is illustrated in FIG.

In this embodiment, the second group of lenses B of the optical system comprises five lenses referenced Bl-1 to B5-1 from the object space to the image space of the optical system.

The second group of lenses B thus comprises, from the object space to the image space of the optical system:

a first lens Bl-1 which is a positive lens, an optical triplet (B2-1, B3-1, B4-1) formed of a second lens B2-1 which is a negative lens, a third lens B3-1 which is a positive lens, and a fourth lens B4-1 lens which is a negative lens,

and a fifth lens B5-1 which is a positive lens.

It is thus noted that the second group of lenses B of the optical system comprises, in order from the image space, a positive lens, a negative lens and a positive lens.

The second group of lenses B of the optical system thus does not include a field flattener, in particular formed by a succession of several positive lenses at the end of the image space of the optical system.

More generally, in this first embodiment, the second group of lenses B comprises an alternating juxtaposition of positive and negative lenses, starting with a positive lens from the object space of said lens group.

For the following four embodiments, r is the radius of curvature in millimeters of each surface, d is the distance in millimeters separating two successive surfaces, nd is the refractive index of the material for the wavelength of the line. sodium d (587.56 nm), and vd is the corresponding Abbe number.

effective

Area

r d nd vd semi-n °

diameter

1 54,398 2,000 1.8502 32.17 33.50

2 24,959 13,870 22.92

3,218,238 1,500 1.5952 67.74 22.92 4 18,550 10,682 16.02

5 -87.368 1.522 1.5891 61.27 16.02

6 18.131 9.022 1.7847 26.08 14.60

7 -90.911 2.951 14.60

8 -32.499 1.396 1.8340 37.30 14.20

9 -116.119 (VARIABLE) 14.20

10 26,682 6,958 1.7725 49.62 7.80

11 -56.429 0.035 7.80

12 STOP 1.122 5.34

13 -25,931 5.741 1.8340 37.30 5.60

14 22,342 5.655 1.4875 70.41 8.20

15 -10.442 3.724 1.8061 40.61 8.20

16 -16.760 0.000 10.70

17 64,436 3.843 1.4970 81.61 12.90

18 -40.124 (VARIABLE) 12.90

Plan

221.16

picture

SECOND EMBODIMENT A second embodiment of the invention is illustrated in FIG.

In this embodiment, the second group of lenses B of the optical system comprises six lenses referenced Bl-2, B2-2, B3-2, B4-2, B5-2 and B6-2 from the object space to the image space of the optical system.

a first lens Bl-2 which is a positive lens,

an optical doublet (B2-2, B3-2) formed of a lens B2-2 which is a negative lens and a third lens B3-2 which is a positive lens,

an optical doublet (B4-2, B5-2) formed of a fourth lens B4-2 which is a positive lens, and a fifth lens B5-2 which is a negative lens,

and a sixth lens B6-2 which is a positive lens.

It is also noted here that the second group of lenses B of the optical system comprises, in order from the image space, a positive lens, a negative lens and a positive lens and thus does not include a field flattener.

effective

Area

r d nd vd semi-n °

diameter

1 48,607 2,000 1.8502 32.17 32.40

2 25,344 12,399 23.04

3 111.048 1.502 1.5952 67.74 23.00

4 16.696 10.980 15.16 5 -101.843 1.998 1.5952 67.74 15.10

6 15,843 8.120 1.7552 27.58 12.98

7,535,082 4,414 12.60

8 -25.266 7.550 1.8830 40.76 11.80

9 -49.708 (VARIABLE) 12.00

10 28.745 2.106 1.7858 44.05 8.90

11 -170,673 5,372 8.70

12 STOP 1.379 5.23

13 -22.641 2.117 1.8830 40.76 5.60

14 24.206 3.841 1.4970 81.61 7.00

15 -23.143 0.000 7.90

16 171,813 5,195 1.48.75 70.41 9.24

17 -13.077 3.360 1.8830 40.76 9.60

18 -21.915 0.000 11.90

19 210,431 4,782 1.4970 81.61 13.80

20 -26.933 (VARIABLE) 13.90

Plan

162.04

picture

THIRD EMBODIMENT

A third embodiment of the invention is illustrated in FIG.

In this embodiment, the second group of lenses B of the optical system comprises seven lenses referenced Bl-3, B2-3, B3-3, B4-3, B5-3, B6-3 and B7-3 from the object space to the image space of the optical system.

a first lens Bl-3 which is a positive lens,

a first optical doublet (B2-3, B3-3) formed of a second lens B2 which is a negative lens and a third lens B3-3 which is a positive lens,

a second optical doublet (B4-3, B5-3) formed of a fourth lens B4 which is a positive lens, and a fifth lens B5 which is a negative lens, and a third optical doublet formed of a sixth lens additional B6-3 which is a negative lens and a seventh lens B7-3 which is a positive lens.

Note that the second group of lenses B of the optical system comprises, in order from the image space, a positive lens, a negative lens and a negative lens and thus does not include a field flattener.

Effective Surface r d nd vd

no. semi-diameter

1 47,855 2,500 1.8502 32.17 30.52

2 17.158 15.548 17.06

3 -1093.713 1.600 1.5952 67.74 16.90

4 28.795 6.007 14.30

5 -61.102 1.500 1.5952 67.74 14.03

6 16,926 7.238 1.7552 27.58 12.71

7 -79.977 2.758 12.60

8 -24.685 1.200 1.8830 40.76 12.31 9 -48.508 (VARIABLE) 12.42

10 26,873 1,833 1.8830 40.76 8.29

11 -1433.545 4.133 8.16

12 Infinity 1.953 5.78

13 -29.287 1.797 1.8830 40.76 6.28

14 21.022 3.976 1.4875 70.41 7.45

15 -25.623 0.000 8.18

16,298,549 4,602 1.4875 70.41 9.23

17 -13.218 2.499 1.8830 40.76 9.44

18 -24.073 0.000 11.32

19 112.279 1.000 1.8502 32.17 13.12

20 65,848 5,939 1.4970 81.61 13.43

21 -24.712 (VARIABLE) 13.57

Plan

261.75

picture

FOURTH EMBODIMENT

A fourth embodiment of the invention is illustrated in Figure 7.

In this embodiment, the second group of lenses B of the optical system comprises six lenses referenced Bl-4, B2-4, B3-4, B4-4, B5-4 and B6-4 from the object space to the image space of the optical system.

a first lens Bl-4 which is a positive lens,

an optical doublet (B2-4, B3-4) formed of a lens B2-4 which is a negative lens and a third lens B3-4 which is a positive lens,

an optical doublet (B4-4, B5-4) formed of a fourth lens B4-4 which is a positive lens, and a fifth lens B5-4 which is a negative lens,

and a sixth lens B6-4 which is a positive lens.

effective

Area

r d nd vd semi-n °

diameter

1 70,456 3,000 1.9037 31.42 37.00

2 27.196 13.474 25.02

3 102.014 2.000 1.5935 67.33 23.50

4 20,378 11,656 17.22

5 -70.549 2.000 1.5935 67.33 16.5

6 20,676 9,925 1.7847 25.72 15.00

7 -90.768 5.843 14.50 8 -28.19 6.179 1.883 40.81 11.50

9 -110.466 (VARIABLE) 11.50

10 24,447 2,451 1,788 47.52 8.00

11 -174.400 1.396 8.00

12 STOP 2.298

13 -23,968 3,617 1,883 40.81 7.00

14 23,525 3,911 1.49 70.42 8.70

15 -23.127 0.1 8.70

16 482.532 3.745 1.497 81.60 10.00

17 -19,170 1,189 1,883 40.81 10.50

18 -27.411 0.1 11.00

19 144,600 3,708 1,497 81.60 12.50

20 -32.942 (VARIABLE) 12.50

Plan

280

picture

Of course, the present invention is not limited to the embodiments described above as examples; it extends to other variants.

Other achievements are possible.

The following table summarizes the values of each conditional expression for the different examples detailed above.

where SOI is the first embodiment, SO2 is the second embodiment, SO3 is the third embodiment and SO4 is the fourth embodiment.

FIG. 10 illustrates the integration of an optical system 1 in an electronic image forming system 2 comprising a non-planar electronic image sensor 3 according to the invention.

The non-planar electronic image sensor 3 is placed in the image space of the optical system 1 at the location of the image formed by said optical system.

The image sensor 3 comprises a photo element transducer array and forms a CCD sensor or a CMOS sensor for example.

The image sensor 3 is not flat and therefore has a non-infinite radius of curvature. The image sensor may be a sensor of variable radius of curvature or, advantageously, perhaps an image sensor with a fixed radius of curvature, so as to simplify the operation of the electronic image forming system.

Advantageously, the image sensor 3 has a fixed radius of curvature, in particular fixed during a change in the focal length of the optical system 1. A fixed curvature has certain advantages, especially with respect to a variable or adjustable curvature:

a variable curvature necessitates a deformation mechanism of the additional image sensor, which is heavy, bulky and subject to the fatigue of the materials;

- a variable curvature can lead to additional fragilities of the image sensor which can degrade the quality of the image in the long run; and

- A flat image sensor can easily be replaced by a non-planar image sensor with a fixed radius of curvature in an electronic box. Conversely, the replacement of a planar image sensor by an image sensor with a variable radius of curvature necessitates major modifications of the case, which implies a different design or production line in order to realize the system of production. image formation.

In addition, such an image sensor 3 with a fixed radius of curvature makes it possible to correct the optical aberrations even when the field of view has a high maximum angle, for example close to 180 degrees. It is therefore not necessary to use an image sensor 3 having a variable radius of curvature.

In addition, the image sensor 3 is advantageously convex. This is notably different from the majority of electronic image forming systems which have a sensor having a concave radius of curvature.

The radius of curvature of the image sensor may for example be between 161 and 280 mm, or even between 161 and 262 mm.

The electronic image forming system 2 may be a digital SLR camera. In this mode of embodiment, the optical system 1 can be integrated and mounted on a camera body 4 comprising the image sensor 3. The lens can in particular be removably mounted on the camera body 4.

The apparatus body 4 may furthermore comprise a removable mirror 5 adapted, in a first position, to be placed on the optical axis to reflect the light coming from the optical system 1 in the direction of a viewing optical system 6 comprising a ocular 7 and, in a second position, deviate from the optical axis of the optical system 1 to let light from the optical system 1 reach the image sensor 3 disposed in the camera body 4.

Claims

1. Electronic image formation system (2) comprising:

- a wide-angle fisheye type optical system (1) with variable focal length,

a non-planar electronic image sensor (3), the optical system (1) being specially intended to be mounted on the electronic image forming system (2) comprising the non-planar electronic image sensor (3) by being associated with said sensor positioned in the image space of the optical system,

the optical system (1) comprising, substantially aligned along an optical axis from the object space towards the image space of the optical system:

a first group of lenses A with negative vergence, and

a second group of lenses B with positive vergence, the first group of lenses A and the second group of lenses B being able to move relative to each other along the optical axis between a configuration of shorter focal length fw and a configuration of longer focal length ft when changing the focal length f of the optical system such as:

the optical system satisfying the following conditions:

Y = 2 * / * sin (85° < Θ≤ 90°)

where Yt is the largest image size for the optical system in the longest configuration focal length ft, Yw is the largest image size for the optical system in the shortest focal length configuration, f is a focal length such that fw < f < ft and Y is an image size of one ray incident with an angle Θ,

characterized in that the optical system further satisfies the following condition:

VS

3.5 <—— < 7.5

bfw

where C is the radius of curvature of the image formed by the optical system and bfw is the distance between the end lens of the image space of the second group of lenses B and the object image formed by the optical system in the shorter focal length configuration,

and in that the non-planar electronic image sensor (3) has a fixed radius of curvature, in particular fixed during a change in focal length of the optical system (1).

2. Electronic image forming system (2) according to claim 1, wherein the optical system satisfies the following condition:

\F1\.F2

0.20 <- ^■ — _T ≤ 0.30

(bfw) ²

where Fl is a focal length of the fixed lens group A, F2 is a focal length of the second lens group B and bfw is the distance between the end lens of the image space of the second lens group B and the image object formed by the optical system in the shortest focal length configuration.

3. Electronic imaging system (2) according to one of claims 1 and 2, in which said second group of lenses B comprises, in order from an image space thereof, a positive lens and a negative lens, in particular a positive lens, a negative lens and a positive lens.

4. Electronic image forming system (2) according to one of claims 1 to 3, wherein said second group of lenses B comprises less than eight lenses, preferably less than seven lenses, even more preferably less than six lenses .

5. Electronic image forming system (2) according to one of claims 1 to 4, in which the optical system satisfies the following condition:

bfw

2.50 < -L- < 3.40 where bfw is the distance between the end lens of the image space of the second group of lenses B and the object image formed by the optical system in the shortest focal length configuration and Fl is a focal length of the first group of lenses A.

6. Electronic image forming system (2) according to one of claims 1 to 5, in which the optical system satisfies the following condition:

0.30 < SF1≤ 0.50

where SF1 is a form factor of a first lens from the object space of the fixed lens group A such that SFL1 = (rl— r2)/(rl + r2) where rl is a radius of curvature from the object space of said first lens and r2 is a radius of curvature from the image space of said first lens.

7. Electronic image forming system (2) according to one of claims 1 to 6, in which the optical system satisfies the following condition:

M2

0.45 < < 0.6

F2

8. Electronic image forming system (2) according to one of claims 1 to 7, wherein said second group of lenses B comprises an alternating juxtaposition of positive and negative lenses, starting with a positive lens from the object space of said group of lenses.

9. Electronic image forming system (2) according to one of claims 1 to 8, wherein the non-planar electronic image sensor (3) is convex.