GB2528738B - Anamorphic objective zoom lens - Google Patents

Description

ANAMORPHIC OBJECTIVE ZOOM LENS FIELD OF THE INVENTION

The present invention relates to anamorphic objective zoom lenses, and more particularly to anamorphic objective zoom lenses for cinema cameras, with zoom ratios that provide imaging over wide to narrow fields of view.

BACKGROUND OF THE INVENTION

Contemporary anamorphic objective zoom lenses (which are compound lenses) normally have an optical axis and are commonly based on a rear anamorphic lens group or a front anamorphic lens group. Anamorphic objective zoom lenses having a rear anamorphic lens group are typically more commonplace than anamorphic objective zoom lenses having a front anamorphic lens group.

Known anamorphic objective zoom lenses having a rear anamorphic lens group typically have a rear lens group with aligned Y cylinder refractive optical surfaces and a front spherical lens group, with an optical stop in the front spherical lens group in the form of a variable aperture diameter iris or diaphragm (wherein the anamorphic objective zoom lens has a paraxial focal length in the Y direction that is greater than the paraxial focal length in the X direction, being greater by a factor known as the “squeeze ratio”). This anamorphic objective zoom lens arrangement produces images having a circular bokeh (“bokeh” being the aesthetic quality of blur produced in the out-of-focus parts of the image), as compared to the oval or elliptically shaped bokeh produced by fixed focal length anamorphic objective lenses (commonly referred to as “prime” anamorphic objective lenses). The oval or elliptically shaped bokeh is desired by cinematographers to produce a distinctive artistic look that is different from non-anamorphic objective lenses having only spherical lens surfaces. Another common drawback with this anamorphic objective zoom lens arrangement is that the full aperture may be relatively slow (i.e. small aperture, e.g. maximum aperture of f/4.0) as compared to that of anamorphic prime lenses.

Anamorphic objective zoom lenses having a front anamorphic lens group have a front lens group with aligned X cylinder refractive optical surfaces and a rear spherical lens group, with an optical stop in the rear spherical lens group in the form of a variable aperture diameter iris or diaphragm. This anamorphic objective zoom lens arrangement produces images in which out-of-focus objects have an oval or elliptical bokeh, which is desired by cinematographers for the distinctive artistic look. However, these lenses normally provide only small zoom ratios of 2x or 3x, and they tend be large in diameter, typically with corresponding higher weight and cost. They may also exhibit some breathing when focusing, where the breathing is characterized by the field of view or focal length of the lens changing as the lens is focused from distant to close objects, or vice versa. Nevertheless, the front anamorphic objective zoom lens arrangements produce images having numerous residual optical aberrations and characteristics some of which are desired by cinematographers because they produce an artistic look that is different from non-anamorphic objective lenses having only lens surfaces with continuous rotational symmetry (e.g. spherical lens surfaces, including piano lens surfaces).

Many of the less desired residual optical aberrations and characteristics of the known front and rear anamorphic objective zoom lens arrangements were accepted by cinematographers with film based cameras, but with the advent and adoption of electronic sensor based digital cameras some of them have become less acceptable. In particular the amount of residual chromatic aberration has become less tolerable, whereas some field curvature combined with some residual astigmatism is still acceptable.

As well as the oval or elliptical bokeh, another characteristic that is desired, because of the distinctive artistic look produced, is the depth of field being different in the vertical azimuth direction of the field versus the horizontal azimuth direction of the field. In the case of an anamorphic objective zoom lens that squeezes the horizontal field of view by substantially two times as compared to the vertical field of view, the depth of field in the horizontal azimuth direction of the field is substantially two times greater than the depth of field in the vertical azimuth direction of the field.

Improving the optical aberrations and characteristics of anamorphic objective zoom lenses of this arrangement may involve increasing optical surface shape complexity and hence manufacturing cost including adding aspherical and free-form shaped optical surfaces.

Thus, to address the artistic need of cinematographers and maximize the imaging potential of both film and digital cameras, there remains a need for a compact diameter anamorphic objective zoom lens arrangement that provides a useful zoom range (e.g. from wide to narrow fields of view during zooming) with a suitable blend of residual optical aberration correction and characteristics.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an anamorphic objective zoom lens comprising along an optical axis and in order from an object space to an image space: a first lens group having a negative (-) power; an anamorphic second lens group; a third lens group; a fourth lens group having a variable power; a fifth lens group having a positive (+) power, and wherein the first, third and fifth lens groups each consist of lens elements with surface shapes that are continuously rotationally symmetrical about the optical axis, wherein the second lens group comprises at least one lens element with a surface shape that is not continuously rotationally symmetrical about the optical axis (i.e. is not circularly symmetric), wherein the objective zoom lens is configured to transmit radiation from the fifth lens group to form a real image at an image surface, and wherein the objective zoom lens further comprises an optical stop located along the optical axis on the opposite side of the variable power fourth lens group from the third lens group. The optical stop is located between the variable power fourth lens group and the image space.

The second lens group may have at least one cylindrical surface orientated in a first direction and at least one cylindrical surface orientated in a direction substantially perpendicular to the first direction. Having at least one cylindrical surface in a first direction and at least one other cylindrical surface in a substantially perpendicular direction to the first direction enables a high degree of aberration correction over the whole image, whereby the residual longitudinal chromatic aberration and the residual lateral chromatic aberration may be substantially reduced.

The second lens group may comprise at least a first cylindrical surface that is orientated in a first direction and is located between second and third cylindrical surfaces orientated in a direction substantially perpendicular to the first direction. The cylindrical surfaces may be consecutive surfaces, or may be separated by other non-cylindrical surfaces.

The cylindrical surfaces may be on different lens elements of the second lens group, or two of the cylindrical surfaces may be provided on the same lens element.

The third lens group may have a positive (+) power.

The optical stop may be located within the fifth lens group.

The first lens group may be configured to provide focusing. The first lens group may provide focusing by movement of at least one of the lens elements contained therein, and may enable the anamorphic objective zoom lens to exhibit low focus breathing in the focus range. The provision of the negatively powered first lens group on the object side of the second lens group may enable the second lens group and typically the first lens group to be manufactured with lens elements of a smaller diameter.

The first lens group may comprise a sub group of lens elements that is axially moveable relative to other lens elements of the first lens group.

The second lens group may consist of lens elements only having surfaces selected from the group consisting of: cylindrical and spherical surfaces (and where a piano surface is understood to be a specific case of a spherical surface). The provision of the second lens group on the object side of the variable power fourth lens group enables the second lens group and the first lens group to be manufactured with lens elements of a smaller diameter. The anamorphic objective zoom lens may be configured to provide a focal length of 40mm with a lens element that is furthest from the image space (i.e. closest to object space) that has a diameter of less than 120mm.

The anamorphic objective zoom lens may have an image surface, and be configured to transmit radiation from the fifth lens group to a sensor at the image surface, in which the radiation is angled to the optical axis by less than 10 degrees. The positive power fifth lens group, adjacent the image space, delivers the radiation passing through the optical system on to an image sensor with nearly telecentric light paths and suitably high relative illumination, thereby increasing the efficiency of many electronic sensors.

The anamorphic objective zoom lens may have a focal length within the range of from at least 35mm to 140mm in the Y direction, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group.

The anamorphic objective zoom lens may have a focal length within the range of 40mm to 125mm in the Y direction, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group.

The anamorphic objective zoom lens has a paraxial focal length in the Y direction that is greater than a paraxial focal length in the X direction by a squeeze ratio. The anamorphic objective zoom lens may have a paraxial focal length in the Y direction that is greater than a paraxial focal length in the X direction by a squeeze ratio of 1.9 to 2.1.

The anamorphic objective zoom lens may be configured to provide an image of out of focus objects close to the optical axis with an elliptical bokeh shape.

The anamorphic objective zoom lens may provide different depths of field in the vertical and horizontal azimuth directions of the field of view.

The optical stop may have a full aperture (i.e. maximum aperture) of up to f/3.1.

The lens groups may be fabricated of lens elements made of glass.

The first, third and fifth lens groups may consist of lens elements with surface shapes selected from the group consisting of: spherical surfaces and piano surfaces (e.g. planar surfaces).

The anamorphic objective zoom lens may be configured to operate over a waveband of 455-656nm.

The second lens group may comprise seven cylindrically surfaced lens elements with the following lens surface shapes: eight Y cylindrical surfaces, five X cylindrical surfaces, and one piano surface, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group.

The first lens group may comprise five lens elements, three of which are axially moveable relative to the other lens elements.

The fifth lens group may comprise nine lens elements. The provision of the positively powered fifth lens group on the image side of the second lens group enables the second lens group to be designed to provide enhanced optical performance.

The anamorphic objective zoom lens may be configured for the third lens group to receive rays from the second lens group, in which the rays of each received field beam diverge in the direction of the image space (i.e. across the focus distance range and zoom range of the anamorphic objective zoom lens). The anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group. The anamorphic objective zoom lens may be configured for the third lens group to receive rays from the second lens group, in which each peripheral ray of the received zero field beam (i.e. the field beam centred on the optical axis 0) diverges by at least 2° relative to the optical axis 0 in the Y direction (i.e. in the YZ plane). The anamorphic objective zoom lens may be configured for the third lens group to receive rays from the second lens group, in which each peripheral ray of the received zero field beam diverges by at least 3° relative to the optical axis 0, in the Y direction (i.e. in the YZ plane). The anamorphic objective zoom lens may be configured for the third lens group to receive rays from the second lens group, in which each peripheral ray of the received zero field beam diverges by at least 4° relative to the optical axis 0 in the Y direction (i.e. in the YZ plane). Configuring the third lens group to receive diverging rays from the second lens group may enable the second lens group (and typically also the first lens group) to be manufactured with lens elements of a smaller diameter.

The third lens group may comprise four lens elements.

The variable power fourth lens group may comprise five lens elements.

The variable power fourth lens group may comprise two lens sub groups having three lens elements and two lens elements, and both sub groups may be axially moveable relative to the image surface. The variable power fourth lens group provides zooming, and uses at least two axially movable lens sub groups. The variable power fourth lens group is located between object space and the optical stop, thus enabling the provision of an approximately constant full aperture through zoom.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the anamorphic objective zoom lens of the invention is further described with reference to the accompanying drawings, in which:

Figure 1 shows lens plots in the YZ elevation (side view) and XZ elevation (plan view) on an optical axis 0 where the Y direction focal length is 51.00mm and the X direction focal length is 26.21mm. In the YZ elevation, three field beams are shown at zero, top and bottom of the field of view. In the XZ elevation, three field beams are shown at zero and both sides of the field of view. In the YZ elevation and in the XZ elevation diagrams an intermediate focus distance arrangement is shown;

Figure 2 shows three lens plots in the YZ elevation (side view) on an optical axis 0 where the Y direction focal lengths are 40.01mm, 67.98mm and 125.01mm with three field beams shown at zero, top and bottom of the field of view, and with the top to bottom diagrams showing far, intermediate and close focus distance arrangements;

Figure 3 shows three lens plots in the XZ elevation (plan view) on an optical axis 0, corresponding with the respective lens plots of Figure 2, where the X direction focal lengths are 20.57mm, 34.94mm and 64.27mm with three field beams shown at zero and both sides of the field of view, and with the top to bottom diagrams showing far, intermediate and close focus distance arrangements;

Table 1 shows the optical prescription data for the anamorphic zoom lens system of Figures 1 to 3;

Table 2 shows the focal length, anamorphic squeeze, illumination and breathing data for the anamorphic zoom lens system of Figures 1 to 3; and

Table 3 shows diffraction based modulation transfer function (MTF) performance data for the anamorphic zoom lens system of Figures 1 to 3.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to anamorphic objective zoom lenses, and in particular to a range of different focal length anamorphic objective lenses covering at least a focal length range from 35mm to 140mm, and preferably 40mm to 125mm, in the Y direction and providing low residual chromatic aberration, a traditional elliptical (oval) bokeh shape and different depths of field in the vertical and horizontal azimuth directions of the field. The anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group.

The term “lens group” as used in connection with the anamorphic objective zoom lens disclosed herein means one or more individual lens elements.

An “optical stop” S50 (also known as an “aperture stop”) is the opening that determines the cone angle of a bundle of rays that come to a focus, on the optical axis 0, in the image surface S59.

The terms “negative power” and “positive power”, in describing a lens group, respectively refer to lens groups (each of one or more lens elements) that have an overall divergent or convergent focusing effect, i.e. they would respectively spread a collimated axial beam away from the optical axis 0 or focus a collimated axial field beam towards the optical axis. However, it will be appreciated that in the case of a lens group comprising more than one lens element (i.e. a compound lens), some of the lens elements may have an optical power that is opposite to the collective power of the respective lens group.

The term “spherical”, in describing a lens group, refers to a group of lens elements in which the optical surface shapes of the lens elements all have circular symmetry (i.e. are continuously rotationally symmetrical) about the optical axis 0, e.g. a spherical or piano surface. In contrast, in the “anamorphic lens group”, at least one lens element surface has a shape that does not have circular symmetry (i.e. is not continuously rotationally symmetrical) about the optical axis 0, for example having at least one cylindrical surface.

The anamorphic objective zoom lens 50 shown in Figure 1 is a preferred embodiment of the invention in which the first (front) lens group G1 is negatively powered, the third lens group G3 is positively powered, the fourth lens group G4 is variable powered, and the fifth (last, rear) lens group G5 is positively powered. Those lens groups are paired with an anamorphic second lens group G2 to work in unison and match the preferred optical interface characteristics of sensors, where near telecentric radiation beams approach the sensor at the image surface S59.

The embodiment discussed below is a medium fast, full aperture anamorphic objective zoom lens (e.g. the optical stop has a full aperture of f/3.1), having a field of view range from moderately wide angle to moderately narrow angle.

In the illustrated embodiment, all of the lens elements are made from glasses, and further details are provided in Table 1. The lens element optical surface shapes in the first, third, fourth and fifth lens groups G1, G3, G4 and G5 are all continuously rotationally symmetrical about the optical axis 0, having only spherical surfaces (which includes piano surfaces), and are each referred to as a spherical lens group. In the anamorphic second lens group G2 at least one lens element surface shape is not continuously rotationally symmetrical about the optical axis, a cylindrical lens surface.

In addition to these features, and providing low breathing and near telecentric radiation output at the sensor, the illustrated embodiment is capable of achieving enhanced levels of performance, including image quality resolution and contrast (usually measured as diffraction based modulation transfer function, MTF), high relative illumination for low shading and efficient optical throughput at the sensor. In particular, the anamorphic objective zoom lens is configured to transmit near telecentric radiation from the positively powered fifth lens group G5 to the image sensor at the image surface S59, in which the telecentric radiation has an angle to the optical axis 0 of less than 10 degrees. The near telecentric radiation provides enhanced efficiency of light collection in the case of a digital image sensor having a surface provided with micro-lenses, which have an efficiency that is beam angle dependent and conventionally optimized for radiation received perpendicular to the sensor.

The illustrated embodiment of the present invention will now be described further, by way of a design example, with the accompanying figures and tables. Referring first to Figure 1, each lens element is identified by a numeral from 1 through 30 and the general configuration of each lens element is depicted, but the actual radius of each lens surface is set forth in Table 1. The lens surfaces are identified by the letter "S" followed by a numeral from S2 through S58 (excepting S50, which indicates an optical stop).

Each lens element has its opposite surfaces identified by a separate but consecutive surface number. For example, lens element 1 has lens surfaces S2 and S3, lens element 11 has lens surfaces S21 and S22, and so forth, as shown in Figure 1, except that for doublet lens element pairs 3, 4 and 14, 15 and 18, 19 and 23, 24 the coincident facing lens surfaces are given a single surface number. For example, doublet lens element pair 3, 4 is comprised of lens element 3 having a front lens surface S6 and a rear lens surface S7 and lens element 4 having a front lens surface S7 (coincidental) and a rear lens surface S8. The location of the object to be photographed, particularly as it relates to focus distance, is identified by a vertical line and numeral "S1" on the optical axis 0, and the real image surface is identified by the numeral S59. All of the spherical lens surfaces (lens groups G1, G3, G4 and G5) have a finite radius of curvature, except S10 which is piano. All of the lens surfaces of the cylindrical lens elements 6 to 12 in the anamorphic second lens group G2 have surfaces S11 to S24 with a finite radius of curvature in either the X or the Y direction, except for lens surface S14 which is piano (and where a surface is described as being an X cylindrical surface, where it is curved in the XZ plane/elevation, when viewed along the Y axis).

Before describing the detailed characteristics of the lens elements, a broad description of the lens groups and their axial positions and movement will be given for the illustrated anamorphic objective zoom lens system 50. Beginning from the end facing the object S1 to be photographed, i.e. the left end of the anamorphic objective zoom lens 50 in Figure 1, the spherical first lens group G1 comprises lens elements 1 and 2 and a lens sub group SG11, comprised of lens elements 3, 4 and 5. The anamorphic second lens group G2 comprises lens elements 6, 7, 8, 9, 10, 11 and 12. The spherical third lens group G3 includes lens elements 13, 14, 15 and 16. The variable power zoom fourth lens group G4 includes, from left to right in Figure 1, lens sub group SG41 including lens elements 17, 18, and 19, and lens sub group SG42 including lens elements 20 and 21. The spherical fifth lens group G5, which is closest to the image space, includes lens elements 22, 23, 24, 25, 26, 27, 28, 29 and 30.

The images of Figure 2 illustrate, in the Y direction, the movement of lens sub group G11 along the optical axis 0, for focusing at three focus positions, and the movement of lens sub groups SG41 and SG42 in along the optical axis for zooming to three different focal lengths.

The images of Figure 3 illustrate in the X direction the movement of lens sub group G11 along the optical axis 0 for focusing at three focus positions, and the movement of lens sub groups SG41 and SG42 along the optical axis for zooming to three different focal lengths.

In Figures 2 and 3 the auxiliary lens group, the spherical fifth lens group G5, remains stationary and at a fixed distance from the real image surface S59.

While only the lens elements are physically shown in Figure 1, it is to be understood that conventional mechanical devices and mechanisms are provided for supporting the lens elements and for causing axial movement of the movable lens groups in a conventional lens housing or barrel.

The Optical Prescription data for the above described anamorphic zoom lens system 50 is set forth in Table 1, which is extracted from data produced by CODE V® optical design software that is commercially available from Synopsis Optical Research Associates, Inc., Pasadena, Calif., U.S.A., which was also used for producing the optical diagrams in Figures 1 to 3. All of the data in Table 1 is given at a temperature of 25° C (77° F) and standard atmospheric pressure (760 mm Hg).

Throughout this specification, including the Tables, all measurements are in millimetres (mm), or as otherwise shown. In Table 1, the first column "Item" identifies each optical element and each location, i.e. object plane, etc., with the same numeral or label as used in Figure 1. The second and third columns identify the "Group" and "Subgroup", respectively, to which that optical element (lens) belongs with the same numerals used in Figure 1. The fourth column "Surface" is a list of the surface numbers and the fifth column “Shape” is the surface shape.

The sixth and seventh columns "Focus Position" and "Zoom Position", respectively, identify the typical focus positions of the spherical lens group and the typical positions of the lens elements in the variable power (zoom) lens group wherein there are changes in the distance (separation) between some of the surfaces listed in the “Separation” column which is the axial distance between that surface (fourth column) and the next surface, for example, the distance between surface S2 to surface S3 is 5.579 mm.

The columns headed by the legends "Y Radius of Curvature" and “X Radius of Curvature” list the optical surface radius of curvature for each surface in the Y and X plane, respectively, with a minus sign (-) meaning the centre of the radius of curvature is to the left of the surface, as viewed in Figure 1, and "Flat" meaning an optically flat surface, described as “Plano” in column 5.

The next four columns (columns 11 to 14), of Table 1 relate to the "Material" between that surface and the next surface to the right in Figure 1, with the eleventh column "Type" indicating whether there is a lens (Glass) or empty space (Air) between those two surfaces. All of the lenses are glass and the column titled "Code" identifies the optical glass. The column marked “Supplier” identifies the source of the lens and the column marked "Name" lists the Supplier’s identification for each glass type, but it is to be understood that any equivalent or adequate glass may be used.

The last column of Table 1, headed "Aperture Half Diameter", provides the maximum aperture half diameter for each surface through which the light rays pass.

The novel configuration of having a negatively powered spherical first lens group G1, an anamorphic second lens group G2 followed by a spherical third lens group G3 (preferably with a positive power), a variable power spherical fourth lens group G4 and a positively powered spherical fifth lens group G5 containing an optical stop S50 within or directly to one side of the positively powered spherical fifth lens group G5 (i.e. between the variable power spherical fourth lens group G4 and the positively powered spherical fifth lens group G5, within the fifth lens group G5, or on the opposite side of the fifth lens group G5 from the variable power spherical fourth lens group G4) provides good overall optical performance. The anamorphic objective zoom lens 50 may produce some residual distortion, astigmatism and field curvature aberrations, but those aberrations, to a tolerable extent, contribute to the anamorphic look as desired by many cinematographers. In particular, the anamorphic objective lens of the invention provides the optical characteristic of an elliptical bokeh (out of focus objects are elliptically shaped, close to the optical axis 0, and approximately maintained across the field of view) in the image formed at the image surface. Further, the anamorphic objective lens provides reduced residual chromatic aberration. The design enables the use of lens elements in the anamorphic second lens group G2 that are small in diameter, relative to the lens elements of the negatively powered spherical first lens group G1, which particularly reduces the cost of the components and assembly during manufacturing. The provision of focusing by the spherical first lens group G1, on the object side of the anamorphic second lens group G2, enables the anamorphic lens group to be designed to provide enhanced optical performance. The provision of the positively powered spherical fifth lens group G5 on the image side of the anamorphic second lens group G2 enables the anamorphic lens group to be designed to provide enhanced optical performance. In addition, a balanced blend of the afore-described lens characteristics may aid in cost reduction of manufacture. With the advent and adoption of digital cinema cameras employing electronic sensors, a large back focal length which was once required for film cinema cameras having a reflex mirror may be less necessary but may still be provided for in the novel anamorphic objective zoom lens.

The preferred embodiment operates with an optical stop up to an aperture of f/3.1 and over a waveband of 455-656nm, and this waveband is what was used in the diffraction based modulation transfer function (MTF) performance data in Table 3. A faster or slower aperture may be required and an extended waveband may be required. The aperture may be increased or reduced and the waveband expanded or contracted and the optical designs re-optimized to maximize image quality over such apertures and wavebands without departing from the invention. Also, during such re-optimization alternate glass types (i.e. alternate to those listed in Table 1) may be used without departing from the scope of the disclosure. Furthermore, more complex optical surface shapes such as aspherical and free-form surfaces may be provided on the surface of one or more lens elements of the anamorphic objective zoom lens, for expanded performance, but at the likely effect of increased manufacturing cost.

Figures 1 to 3 relate to the preferred embodiment in which the focal length in the Y directions are 40.01mm, 67.98mm and 125.01mm and in the X directions are 20.57mm, 34.94mm and 64.27mm. The overall length is 477mm from the first refractive surface vertex of the anamorphic objective zoom lens (i.e. the intersection of the optical axis 0 with surface S2) to the image surface vertex (i.e. the intersection of the optical axis 0 with the image surface S59), the front diameter clear aperture (i.e. the beam width at surface S2) is 114.00mm, the back focal length from the rear refractive surface vertex (i.e. the intersection of the optical axis 0 with surface S58) to the image vertex is 37.17mm, and the close focus distance from the object to the image is 1165mm.

The focal lengths of the spherical first lens group G1 are -116.86mm, -118.72mm and -120.67mm for the far, intermediate and close focus distances. The focal lengths of the anamorphic second lens group G2 are +306.23mm in the Y direction and -284.96mm in the X direction. The focal length of the spherical third lens group G3 is 107.33mm. The focal lengths of the spherical fourth lens group G4 with zooming are -90.88mm, -100.10mm and -70.74mm for the short, medium and long focal length positions. The focal length of the spherical fifth lens group G5 is 86.52mm. The focal lengths of the focusing sub group SG11 and the zooming sub groups SG41 and SG42 are respectively -265.40mm, -46.15mm and 190.83mm.

The spherical third lens group G3 is configured to receive divergent rays from the anamorphic second lens group G3, in which the rays of each received field beam diverge in the direction of the image space S59, throughout the focus distance range and zoom range of the anamorphic objective zoom lens. In the illustrated anamorphic objective zoom lens, each of the peripheral rays of the zero field beam (i.e. the central beam, centred on the optical axis 0) incident onto lens surface S25 of the lens element 13, being the lens element of the third lens group G3 that is closest to the object space, diverges from the optical axis 0 by more than 5°, in the Y direction (i.e. in the YZ plane). The diverging rays in each field beam at lens surface S25 enables the lens system to be manufactured with an anamorphic second lens group (and a spherical first lens group) having lens elements 6 to 12 of a smaller diameter.

Figures 2 and 3 show the locus of axial movement of the focusing sub group SG1 with a long dashed curve and the locus of axial movements of the zooming sub groups SG41 and SG42 with short dashed curves, where the closest approach of the zooming sub group nearest image space to the adjacent stationary lens element 22 lies between zoom positions three (Z3) and five (Z5), with the axial airspace distance of closest approach being given approximately by the data for zoom position Z4 in Table 1.

The lens elements 6 to 12 of the anamorphic second lens group G2 each have at least one cylindrical surface (S11 to S24, excepting piano surface S14). As least one of the lens elements 6 to 12 has a cylindrical surface orientated in a direction that is substantially perpendicular (i.e. “crossed”) to the orientation of the cylindrical surface of another lens element (e.g. a lens element has a cylindrical surface orientated in the X direction and another has a cylindrical surface orientated in the Y direction). Further, the lens elements 6 to 12 of the anamorphic second lens group G2 are preferably arranged in an “intermixed” arrangement with at least one lens element having a cylindrical surface orientated in a first direction that is located between lens elements having a cylindrical surface orientated in a second, substantially perpendicular direction (e.g. X-cylindrical S17 and S18 are located between Y-cylindrical S16 and S19).

The focal lengths of the seven anamorphic lens elements 6 to 12, each containing at least one cylindrical surface, are respectively (in order from object space to image space): -75.75mm (in X direction), -150.63mm (in X direction), -561.89mm (in Y direction), 87.29mm (in X direction), 141.59mm (in Y direction), -3906.70mm (in Y direction) and -230.05mm (in Y direction). It is to be understood that the focal lengths of the seven anamorphic lens elements in the respective other X and Y directions are substantially large and hence have little optical power.

In the illustrated embodiment the lens elements 6 to 12 of the anamorphic second lens group G2 each have either one cylindrical and one circularly symmetric surface (e.g. lens element 7, having X-cylindrical and piano surfaces) or two cylindrical surfaces that are aligned in the same direction (e.g. lens element 8, having two Y-cylindrical surfaces). However, alternatively (not illustrated), one or more lens elements of the anamorphic second lens group G2 may each have two cylindrical refractive surfaces orientated in substantially perpendicular directions, for example being formed by X and Y cylindrical surfaces, or Y and X cylindrical surfaces. In this case, the intermixed arrangement may be formed by providing an arrangement of lens elements in which at least a first cylindrical surface orientated in a first direction is located between second and third cylindrical surfaces orientated in a direction substantially perpendicular to the first direction (i.e. crossed cylindrical surfaces may be provided on a single lens element). An arrangement with crossed cylindrical surfaces on a single lens element may improve the imaging characteristics but likely at an additional manufacturing cost.

The use of cylindrical surfaces arranged in substantially perpendicular directions improves imaging performance (in particular enabling the provision of a better balance of aberrations) than for an anamorphic lens group with only cylindrical surfaces aligned in a single direction. Further, the intermixed arrangement of differently orientated cylindrical surfaces (whether on different lens elements, as in the illustrated embodiment, or having crossed cylindrical surfaces on one or more single elements) provides more degrees of freedom in designing the anamorphic lens group, enabling an enhanced balance of lower residual aberrations over the whole of the field of view of the anamorphic objective zoom lens.

In the preferred embodiment the lens system of the anamorphic objective lens 50 includes a total of thirty lens elements with twenty two singlets and four doublets. The spherical first lens group G1 contains five lens elements 1 to 5 with three elements 3 to 5 that are axially movable for focusing at different distances. The anamorphic second lens group G2 contains seven cylindrical^ surfaced lens elements 6 to 12, with eight Y cylinders, five X cylinders and one piano surface shape. The spherical third lens group G3 contains four lens elements 13 to 16. The variable power fourth lens group G4 contains five lens elements which form two axially movable sub groups SG41 and SG42 for zooming with three lens elements 17 to 19 and two lens elements 20 to 21, respectively. In the illustrated embodiment 50, the optical stop S50 lies within the spherical fifth lens group G5, which contains nine lens elements 22 to 30. The nominal image size is 8.91mm vertical half height and 10.65mm horizontal half width in image space. In this example embodiment the near telecentric radiation output has an angle to the optical axis 0 of about 9.1 degrees at all three focus positions F1 to F3 and over the zoom range Z1 to Z5.

The accompanying Optical Prescription in Table 1 describes a select example of the preferred embodiment of the anamorphic objective zoom lens 50 disclosed herein.

Table 2 which accompanies this specification contains focal length, anamorphic squeeze, illumination and breathing data of the preferred embodiment. In Table 2, it is shown that the anamorphic squeeze ratio is within a small range of about 1.95 to 2.03. In Table 2 it is also shown that the relative illumination is above 30%, which is sufficiently high for low shading at the corner of the field of view when an anamorphic objective zoom lens is used in combination with an electronic sensor at the image surface S59, e.g. when the anamorphic objective zoom lens 50 constitutes part of a digital camera. In Table 2 it is further shown that the focus breathing is consistently very small throughout focusing and zooming.

In Table 3 which accompanies the specification, the polychromatic diffraction based modulation transfer function (MTF) performance at a spatial frequency of 20 cycles/mm is shown for the example embodiment to be greater than 50% at all field positions at the given combination of far, intermediate and close focus distances and short, medium and long focal lengths.

Although the present invention has been fully described in connection with a preferred embodiment thereof with reference to the accompanying drawings and data tables, various changes and modifications could be made thereto, including smaller and larger zoom ranges, smaller and larger focal lengths, smaller and larger anamorphic squeeze ratios, smaller and larger full aperture f/numbers, smaller and larger image sizes, smaller and larger wavebands (e.g., 435 nm to 656 nm), etc. may be made, as will be apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined by the appended

Claims (25)

Claims

1. An anamorphic objective zoom lens comprising along an optical axis and in order from an object space to an image space: a first lens group having a negative (-) power; an anamorphic second lens group; a third lens group; a fourth lens group having a variable power; a fifth lens group having a positive (+) power, and wherein the first, third and fifth lens groups each consist of lens elements with surface shapes that are continuously rotationally symmetrical about the optical axis, wherein the second lens group comprises at least one lens element with a surface shape that is not continuously rotationally symmetrical about the optical axis, wherein the objective zoom lens is configured to transmit radiation from the fifth lens group to form a real image at an image surface, and wherein the objective zoom lens further comprises an optical stop located along the optical axis on the opposite side of the variable power fourth lens group from the third lens group.

2. The anamorphic objective zoom lens of claim 1, wherein the second lens group has at least one cylindrical surface orientated in a first direction and at least one cylindrical surface orientated in a direction substantially perpendicular to the first direction.

3. The anamorphic objective zoom lens of claim 2 wherein the second lens group comprises at least a first cylindrical surface that is orientated in a first direction and is located between second and third cylindrical surfaces orientated in a direction substantially perpendicular to the first direction.

4. The anamorphic objective zoom lens of any one of claims 1, 2 or 3, wherein the third lens group has a positive (+) power.

5. The anamorphic objective zoom lens of any preceding claim, wherein the optical stop is located within the fifth lens group.

6. The anamorphic objective zoom lens of any preceding claim, wherein the first lens group is configured to provide focusing.

7. The anamorphic objective zoom lens of any preceding claim wherein the first lens group comprises a sub group of lens elements that is axially moveable relative to other lens elements of the first lens group.

8. The anamorphic objective zoom lens of any preceding claim, wherein the second lens group consists of lens elements only having surfaces selected from the group consisting of: spherical, cylindrical and piano surfaces.

9. The anamorphic objective zoom lens of any preceding claim having an image surface, and be configured to transmit radiation from the fifth lens group to a sensor at the image surface, in which the radiation is angled to the optical axis by less than 10 degrees.

10. The anamorphic objective zoom lens of any preceding claim having a focal length within the range of from at least 35mm to 140mm in the Y direction, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given zoom position of the variable power fourth lens group.

11. The anamorphic objective zoom lens of claim 10 having a focal length within the range of 40mm to 125mm in the Y direction, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given zoom position of the variable power fourth lens group.

12. The anamorphic objective zoom lens of any preceding claim, wherein having a paraxial focal length in the Y direction that is greater than a paraxial focal length in the X direction by a squeeze ratio of 1.9 to 2.1.

13. The anamorphic objective zoom lens of any preceding claim configured to provide an image of out of focus objects close to the optical axis with an elliptical bokeh shape.

14. The anamorphic objective zoom lens of any preceding claim providing different depths of field in the vertical and horizontal azimuth directions of the field of view.

15. The anamorphic objective zoom lens of any preceding claim, wherein the optical stop has a full aperture of up to f/3.1.

16. The anamorphic objective zoom lens of any preceding claim, wherein the lens groups are fabricated of lens elements made of glass.

17. The anamorphic objective lens of claim 1, wherein the first, third and fifth lens groups consist of lens elements with surface shapes selected from the group consisting of: spherical and piano surfaces.

18. The anamorphic objective zoom lens of any preceding claim, which is configured to operate over a waveband of 455-656nm.

19. The anamorphic objective zoom lens of any preceding claim, wherein the second lens group comprises seven cylindrically surfaced lens elements with the following lens surface shapes: eight Y cylindrical surfaces, five X cylindrical surfaces, and one piano surface, wherein the anamorphic objective zoom lens has an X direction and a Y direction and respective paraxial focal lengths, and the paraxial focal length in the Y direction is greater than the paraxial focal length in the X direction for any given optical zoom position of the variable power fourth lens group.

20. The anamorphic objective zoom lens of any preceding claim, wherein the first lens group comprises five lens elements, three of which are axially moveable relative to the other lens elements.

21. The anamorphic objective zoom lens of any preceding claim, wherein the fifth lens group comprises nine lens elements.

22. The anamorphic objective zoom lens of any preceding claim, wherein the anamorphic objective zoom lens is configured for the third lens group to receive rays from the second lens group, in which the rays of each received field beam diverge in the direction of the image space.

23. The anamorphic objective zoom lens of any preceding claim, wherein the third lens group comprises four lens elements.

24. The anamorphic objective zoom lens of any preceding claim, wherein the variable power fourth lens group comprises five lens elements.

25. The anamorphic objective zoom lens of any preceding claim, wherein the variable power fourth lens group comprises two lens sub groups having three lens elements and two lens elements, and both sub groups are axially moveable relative to the image surface.