Patent Application
20190310449
The field of the present invention is a fixed focal length objective lens (also called prime lens) used for photography or cinematography. In detail, the present invention relates to a fixed focal length objective lens with a very high level of aberration correction and which can be used for a large field of view and a high numerical aperture, in particular for full frame image sensors.
There are five major types of monochromatic aberrations affecting the image: field curvature, spherical aberration, coma, astigmatism and distortion.
The correction of field curvature is the most important correction since in the most general case the image should be on a flat sensor surface. In an optical system including only refractive lens elements, the correction of field curvature could be done by two methods: firstly, spatially separating negative powered groups of lenses from positive powered groups of lenses and secondly using different indices of refraction for different lenses.
Doublets and aspherical lens elements are used to correct all other aberrations.
Objective lenses having a fixed focal length are widely used in photography and cinematography for capturing an image of an object.
U.S. Pat. No. 7,446,944 B2 discloses objective lenses having a plurality of optical elements including two moving lens groups for focusing and an aspherical lens element.
U.S. Pat. No. 8,508,864 B2 also discloses objective lenses for cinematography having a plurality of optical elements arranged into two positive groups and also aspherical lens elements and moving groups for focusing.
Both mentioned documents are disclosing objective lenses having correction means adapted to a small field of view and high aperture.
The document I. Neil: “High performance wide angle objective lens systems with internal focusing optics and multiple aspheric surface for the visible waveband”, SPIE VOL 2774, p. 216-242, describes lenses used for wide angle applications having a plurality of aspherical surfaces.
All disclosed prior art lenses do not present a complete set of means for correcting the image aberrations related to a large image field and a high numerical aperture.
It is, therefore, an object of the present invention to provide a fixed focal length objective lens for full frame image sensors with very high aberration correction. The aberration correction should be such that the modulation transfer function (MTF) is app. 50%, preferably 60% or higher, at 40 lpm and maximum image field height. This will make the objective lens suitable for 8 k image chips. Additionally, the objective lens should cover a large field of view and offer a high numerical aperture.
This aim is achieved by the invention as claimed in the independent claim. Advantageous embodiments are described in the dependent claims.
We suggest a fixed focal length objective lens forming an image of an object comprising a plurality of lens elements and an aperture stop. The aperture stop defines an aperture stop proximity space and at least one field proximity space. Typically, there are two field proximity spaces containing lenses, one on the object side and the other at the image side. The objective lens comprises at least three aspherical surfaces each on any of the lenses. The aspherical surfaces are distributed in the following way: Either two aspherical surfaces are in the aperture stop proximity space and at least one aspherical surface is in a field proximity space. Or at least one aspherical surface is in the aperture stop proximity space and two aspherical surfaces are in a field proximity space.
The two aspherical surfaces in a field proximity space are either both in the same field proximity space, or they are each in one field proximity space, one on the object side of the objective lens and one on the image side.
Two aspherical surfaces can be either on different lenses or on the same lens.
The use of three aspheres allows the correction of aberrations without using supplementary glass material so that glass weight could be controlled.
The aspheres are positioned where they have the highest effect on aberrations, i.e. either in the field proximity space or in the aperture stop proximity space. In this way four main aberrations are corrected: spherical aberration, coma, astigmatism and distortion.
The use of two aspheres in the same space allows a selective correction effect on higher order aberrations. For example since the first aspherical term is influencing third order aberrations, an opposite contribution of the two aspheres could cause a reduced effect on this order of aberration, maintaining the large effect on the 5th and higher order aberrations.
All lens elements/surfaces situated in the aperture stop proximity space have an decisive influence on spherical aberration since the third order spherical aberration coefficient of a particular surface varies with the fourth power of the axial marginal ray height HM at the surface.
Furthermore, all lens elements/surfaces situated in the aperture stop proximity space have an increased influence on coma aberration, since the third order coefficient of coma aberration of a particular surface varies with the third power of the marginal ray height HM at the surface and the first power of the chief ray height HC at the surface.
Also, optical elements/surfaces positioned in field proximity space have an increased influence on distortion, since distortion varies with the third power of the chief ray height HC and the first power of the axial marginal ray height on the surface. They also have an increased influence on astigmatism, since the third order coefficient of astigmatism varies with the second power of the marginal ray height and the second power of the chief ray height on the surface.
The highest influence on some of the monochromatic aberrations can be achieved if the aspherical surfaces in the field proximity space are placed where the ratio HC/HM is larger than 2.5.
If the ratio HC/HM is larger than 4 for at least one aspherical surface in the field proximity space, the influence on distortion is decisive.
Basically the same applies for the aspherical surfaces in the aperture stop proximity space. If the ratio HC/HM is smaller than 0.4 at the position of the at least one aspherical surface in the aperture stop proximity space, an excellent correction of coma and spherical aberration can be achieved.
The objective lens has a fixed first lens group of negative refracting power at the object side. This provides for sufficient compactness and will not alter the length of the objective lens when focusing, which can be an issue if space is restricted.
The objective lens further has a second lens group of positive refracting power following the first lens group in this order coming from the object side. It is, thus, a retrofocus objective lens.
The aperture stop is located in the positive lens group.
For optimal focusing the positive lens group comprises at least two sub lens groups.
Thus, the present invention relates to objective lenses having a first lens group of negative refracting power and a second lens group of positive refracting power and an iris stop located in the positive lens group, each of the lens groups comprising at least one aspherical lens element and the positive lens group comprising at least two moving optical elements for focusing at different object positions.
The present invention describes optimal arrangements of optical group structure and correction means within the optical system used for wide angle applications. This art of configuration is assuring an optimal correction of aberration also keeping a compact sized objective lens.
This structure common to all lenses disclosed in this specification is leading to a high performance. A lens system is considered to have high performance if the MTF (Modulation Transfer Function) has a value of at least 70% on axial field and at least 50% at all other field points calculated at a spatial frequency of 20 line pairs/mm. These values are frequently exceeded by the objective lens according to the present invention.
It has proven particularly positive for focusing, if the first lens element of the second lens group is moving for focusing.
Thereby, the change of those aberrations depending on the chief ray height, like astigmatism and distortion, with the change of object position, can be corrected efficiently.
A good potential for correcting chromatic aberrations is ensured if a Glass anomalous ratio (GAR) between 125<GAR<175 is met.
The best correction of chromatic aberration can be achieved when at least one abnormal glass of the type fluorite crown is used in positive powered lenses and special short flints (see KzFS in
If at least one of the two lens groups comprises two aspherical surfaces, it enables a selective correction effect on higher order aberrations. For example since the first aspherical term is influencing third order aberrations, an opposite contribution of the two aspheres could cause a reduced effect on this order of aberration, maintaining the large effect on the 5th and higher order aberrations.
Preferably, the two aspherical surfaces within a lens group are located on two different lens elements, and the two different lens elements are positioned adjacent to one another. This increases the correction effect on aberrations and enables a selective correction effect on higher order aberrations.
If the first lens group having negative refractive power comprises at least two negative lens elements, then the necessary negative power of the lens group is distributed on at least two elements and the aberration contribution of this two elements is reduced accordingly, since the aberration contribution depends directly on the lens power. A large power lens will have a larger aberration contribution than a low power lens.
If the first lens elements will have a meniscus shape oriented with the convex side toward the object, the aberration contribution of the surfaces will also be reduced since the incidence angle will have smaller values. At normal incidence, the ray is not deviated and so the surface will have no contribution on aberrations at all.
With two aspherical lens elements in the front lens group, a separation of specific aberration correction is achieved on at least two aberration types since one is mainly influencing one aberration for example distortion and the other is mainly affecting a second aberration for example astigmatism.
This can further be improved if the first and the second lens elements of the first group have each a first surface on the object side of aspherical shape.
If both front lens elements are of meniscus type with the convex surface toward the object both will have an optimal shape for aberration contribution since the incidence angle of the ray bundles starting from the field extremity will be reduced.
It is optimal for correction and manufacturability, if the first lens has an aspherical surface on the object side. An aspherical surface on a negative lens in the front group can reduce the power of the lens from the optical axis toward the lens margin, thus allowing a reduced angle of incidence of the rays of the beam coming from the outmost object field and impinging on the surface, particularly so, if the first lens is of meniscus type with the convex side towards the object.
More aspheres will increase the correction means described above. The position of aspherical surfaces is critical for affecting specific aberrations.
Other objects and advantages of the present invention may be ascertained from a reading of the specification and appended claims in conjunction with the drawings therein. For a more complete understanding of the present invention, reference is established to the following description of embodiments made in connection with accompanying drawings. The possibilities to solve the problem are not limited to the embodiments. The exemplary embodiments are shown schematically in the figures. The same reference numerals in the individual figures designate the same or functionally identical or with respect to their functions corresponding elements. In detail:
For a centered optical system the plane formed by the optical axis 108 and the marginal ray 110 is called by convention the meridional plane. The chief ray 112 is also positioned in this meridional plane. The graphic representation of the lenses are always done in the meridional plane, sectioning all the lens elements.
Within the description of this document and according to
Considering the axial symmetry of the lens and imagery, there are two positions where the ratio equals 1 where a chief ray 112 is intersecting a marginal ray 110. At the aperture stop 114 the chief ray height is zero. At the image position the marginal ray height is zero. The space around the aperture stop satisfying the relation HC/HM<0.5 is defined to be the aperture stop proximity space 118.
All lens elements/surfaces situated in this aperture stop proximity space 118 have an increased influence on spherical aberration since the third order spherical aberration coefficient of a particular surface (see e.g. Tab. 7) varies with the fourth power of the axial marginal ray height HM at the surface.
Furthermore, all lens elements/surfaces situated in the aperture stop proximity space 118 have an increased influence on coma aberration, since the third order coefficient of coma aberration of a particular surface (see e.g. Tab. 7) varies with the third power of the marginal ray height HM at the surface and the first power of the chief ray height He at the surface.
The space in front of the aperture stop proximity space 118 is called the object side field proximity space 120. The space beyond the aperture stop proximity space 118 is called the image side field proximity space 122.
Optical elements positioned in this space have an increased influence on distortion, since the third order surface contribution on distortion varies with the third power of the chief ray height at the relevant optical surface. They also have an increased influence on astigmatism, since the third order surface contribution on astigmatism varies with the second power of the marginal ray height and the second power of the chief ray height both on the relevant surface of the.
The first embodiment can be summarized as follows:
The * denotes the aspheres. There are no other lens elements in this objective lens.
With this general setup, the major advantages of the invention can be achieved.
The aspherical surfaces are preferably positioned as follows:
As explained above, at these positions, they have a very strong influence on correcting different aberrations.
An advantageous realization of the first embodiment can be achieved with the following forms of the lenses:
Further advantages can be achieved by using the following orientations of the lenses:
Since the role of the lenses is to bundle the rays emerging from the object, thus forming the image, the shape of the lenses is optimally designed, since each lens has either a reduced incidence angle of the chief ray or a reduced incidence angle of the marginal ray. This enables a reduced contribution of each lens on image aberrations and also a reduced number of correction means.
Further advantages can be achieved by using the following glass types for the lenses:
The definitions of the glass types are given in the glossary.
Lenses also have a contribution on chromatic aberrations since the glass index of refraction varies with wavelength. The selection of glass types is crucial for correcting chromatic aberrations and chromatic variation of all monochromatic aberrations.
Since dispersion is the main property of a glass type connected with aberration correction, there are two different ways of glass type classification.
First of all, glass materials can be classified according to the magnitude of their dispersion characterized with the principal dispersion or the Abbe number. So, a high dispersion glass has an Abbe number lower than 62 and a low dispersion glass has an Abbe number larger than 62.
Secondly, glass materials can be classified according to the behavior of their dispersion in the short wavelength region. So, there are glasses with normal dispersion (most of them situated on a line in the diagram relative partial dispersion vs. Abbe number going through the glasses K7 and F2 from SHOTT AG) and glasses with abnormal behavior (abnormal glasses). The abnormal glasses can further be classified according to the magnitude of their relative partial dispersion in the short wavelength region of the spectrum. So, there are lenses with a high dispersion in the short wavelength spectrum like the fluorite crowns (e.g. FK51A from SHOTT or SFPL51 from OHARA) and glasses with a low dispersion in the short wavelength spectrum like the dense or special short flints (see KzFS in
Further advantages can be achieved by using the following ranges of focal lengths for the lenses:
The numerical data corresponding to this first embodiment are given in Tab. 6A. The exemplary glass types are taken by way of example only. The used abbreviations correspond to tradenames well-known to one skilled in the art. The glass types are offered and the tradenames are used by Schott AG, Mainz, Germany, or Ohara Corporation, Japan.
The aspherical constants for the aspheres 114 used in the first embodiment are given in Tab. 6B.
Surface profiles of aspheric surfaces are governed by the following conventional equation:
where the optic axis is presumed to lie in the z direction, and z(r) is the sag, i.e. the z-component of the displacement of the surface from the vertex (pole) of the surface, at distance r from the axis. The coefficients C1, C2, . . . describe the deviation of the surface from the axially symmetric quadric surface specified by R (the radius of curvature of the spherical surface) and K (the conic constant).
The correction means used to correct the most important aberrations are positioned in the field proximity space 120 and the aperture stop proximity space 118. According to
A common design principle for all embodiments of this invention is that there are no aspheres in doublet lenses, as these are very costly in production, requiring difficult centering of the lenses.
Correspondingly the influence of these two aspherical surfaces is large on distortion and astigmatism as shown in Tab. 7.
Tab. 7 lists all third-order (Seidel) aberration contributions for the given surfaces as given by CODEV optical design software. There are two contributions listed. The first is the spherical surface contribution, i.e. the contribution of a spherical lens. And the second, listed as second row only in case that the surface is aspherical, is the contribution of the aspherical shape.
Also from Tab. 7 we can see that the aspheric surfaces 21 and 24 positioned in the aperture stop proximity space 118 have the strongest influence on spherical aberration and coma (influence in bold type).
In
In this first embodiment the objective lens has in this order from the object side toward the image side
This corresponds to the general optical structure of a retro-focus lens or inversed telephoto lens, which is able to cover large fields of view.
It is also one of the features of this first embodiment that in the objective lens more than one optical element is moved independently for focusing. More precisely there are two groups of lenses LG21, LG22 moved axially for focusing. This assures a focusing process maintaining the low aberration level in the image. The first subgroup LG21 of these two subgroups has a positive refractive power, and the second LG22 also has a positive refractive power. The remaining lenses L11 to L15 between the second subgroup LG22 and the image plane taken as a group have a positive refractive power.
The group structure of the first embodiment can thus be summarized as
N-P-P-stop-P,
where N denotes a negative refractive power and P a positive one.
The aspherical surfaces are indicated by a black dot. It is also a feature of this first embodiment that two correcting aspherical surfaces 1, 3 are positioned in the field proximity space 120 and two aspherical optical surfaces 21, 24 are positioned in the aperture stop proximity space 118. In this way four main aberrations are corrected: spherical aberration, coma, astigmatism and distortion.
There is another feature of this first embodiment that the aspherical elements within one group are positioned adjacent to each other. This increases the correction effect on aberrations and enables a selective correction effect on higher order aberrations. For example since the first aspherical term is influencing third order aberrations, an opposite contribution of the two aspheres could cause a reduced effect on this order of aberration, maintaining the large effect on the 5th and higher order aberrations. In this first embodiment the aspherical lens elements are positioned adjacently in both groups.
The correction level in terms of the MTF (modulation transfer function) vs. spatial frequency in line pairs/mm is indicated in
These are the common Fraunhofer wavelengths used for calculation wherein the weight for the g and h spectral lines are a factor of 3 respectively 13 times lower than the weight of the other wavelength. This spectral distribution corresponds to the spectral sensitivity of common sensors.
In
There is another advantageous feature of this first embodiment that the aspherical elements 1, 3, 21, 24 within one group are positioned adjacent to each other. This increases the correction effect on aberrations and enables a selective correction effect on higher order aberrations. For example since the first aspherical term is influencing third order aberrations, an opposite contribution of the two aspheres could cause an reduced effect on this order of aberration, maintaining the large effect on the 5th and higher order aberrations. In this first embodiment the aspherical lens elements 124, 126, 128, 130 are positioned adjacent to one another in both groups.
The chromatic aberrations are very important to be corrected. Therefore a plurality of low and anomalous dispersion glasses have been used in the lens designs.
As a result, the transverse ray aberration for different relative field heights and wavelengths across the pupil typically remains below 30 μm for tangential and sagittal rays, even at a relative field height of 1 (corresponding of 41.22° of the chief ray angel on the image side). This matches also with the MTF values given in
There are two kinds of chromatic aberrations: axial and lateral. In order to correct these aberrations low dispersion glasses are to be involved. According to the Schott glass catalog, the departure from the normal line of the relative partial dispersion ΔPgF of a chosen glass type for the g and F Fraunhofer wavelengths is given by the equation:
In this equation ng, nF, nC are the refractive indices of the chosen glass at the Fraunhofer wavelengths g (422.670 nm), F (486.134 nm) and C (656.281 nm) correspondingly. νd is the Abbe number of the glass type at the Fraunhofer wavelengths d (466.814 nm).
PgF denotes the relative partial dispersion for the above mentioned Fraunhofer wavelengths g and F:
The straight line in
ΔPgF as the departure from the normal line, is an indicator of anomalous behavior of glass dispersion. Larger absolute values indicate a glass with a stronger anomalous behavior (anomalous dispersion glasses) and thus a better option for correcting chromatic aberrations. On the other hand, low and anomalous dispersion glasses have physical and chemical proprieties, which make them hard to manufacture.
If we build the sum of all departures from the normal line of the relative partial dispersion of all lenses and divide it by the number of lenses we get an indicator of the number of lenses with anomalous dispersion and call it Glass anomalous ratio (GAR):
If this number is too large, then there are too many lenses made of anomalous dispersion glasses used. If the number is too small, than there is not enough potential for correcting chromatic aberrations. A ratio between 125<GAR<175 would ensure a good potential for correcting chromatic aberrations.
Using the lens data as given in Tab. 6A and B, the different transversal aberration curves for one field point and different wavelengths have a very small departure from one to another, indicating a very low level of chromatic aberration.
The best correction of chromatic aberration can be achieved when abnormal glasses of the type fluorite crown are used in positive powered lenses and dense or special short flints are used in negative powered lenses, when the lenses are positioned in the aperture stop proximity space. In the field proximity spaces low dispersion abnormal glasses are to be used in order to reduce the chromatic aberration contributions from these lenses.
In this first embodiment there are five lenses with anomalous dispersion behavior in the aperture stop proximity space 118: L7, L8, L9, L10, and L12. The low and anomalous dispersion glasses are used in positive powered lenses L8 (NPK51), L10 (SFPL53) and L13 (SFPM2) and a high and anomalous dispersion glass is used in the negative powered lens L9 (NKZFS8).
In the field proximity spaces 120, 122 there are low and anomalous dispersion glasses in L1 (SFPM3) and L15 (SFPL53), reducing their influence on chromatic aberrations. All other glasses have also a significant departure from the normal line.
For the first embodiment GAR=157.
The second embodiment can be summarized as follows:
The * denotes the aspheres. There are no other lens elements in this objective lens.
With this general setup, the major advantages of the invention can be achieved.
The aspherical surfaces are preferably positioned as follows:
As explained above, at these positions, they have a very strong influence on correcting different aberrations.
An advantageous realization of the first embodiment can be achieved with the following forms of the lenses:
Further advantages can be achieved by using the following orientations of the lenses:
Further advantages can be achieved by using the following glass types for the lenses:
The definitions of the glass types are given in the glossary.
Further advantages can be achieved by using the following ranges of focal lengths for the lenses:
The numerical data of the objective lens of the second embodiment according to
The aspherical constants for the aspheres used in the second embodiment are given in Tab. 13B.
The influence of the different aspherical surfaces used in the second embodiment on different aberrations is given in Tab. 14.
It can be clearly seen that the contribution (in bold type) on astigmatism and distortion of the aspherical surface 1 included in the field proximity group of lenses is by a large factor greater than the contribution of the same surface 1 on spherical aberration and coma. Corresponding to the position in the aperture proximity group of lenses, the two aspherical surfaces on lenses L11 and L12 have a large effect (in bold type) on spherical aberration and coma and less on astigmatism and distortion. Using this distribution of aspherical lens elements, an optimal correction of image aberrations is achieved.
The group separation of the lenses of the objective lens of
In this second embodiment the lens has in this order from the object side toward the image side
It is also one of the features of this second embodiment that in the objective lens more than one optical element is moved independently for focusing. More precisely there are two groups of lenses LG21, LG22 moved axially for focusing. This assures a focusing process maintaining the low aberration level in the image. The first subgroup LG21 of these two subgroups has a positive refractive power, and the second LG22 also has a positive refractive power. The remaining lenses L11 to L14 between the second subgroup LG22 and the image plane taken as a group have a positive refractive power.
The group structure of the second embodiment can thus be summarized as
N-P-P-Stop-P,
where N denotes a negative refractive power and P a positive one.
The aspherical surfaces are indicated by a black dot. One correcting aspherical surface 1 is positioned in the field proximity space 520 and two aspherical optical surfaces 22, 24 are positioned in the aperture stop proximity space 518. In this way again four main aberrations are corrected: spherical aberration, coma, astigmatism and distortion.
The first two lenses L1, L2 in the first group are of meniscus type and the first lens L1 has an aspherical shape 1 on the object side. This is optimal for correction and manufacturability.
The
Also for this embodiment, the transverse ray aberrations are typically below 30 μm both for tangential and sagittal rays.
From the lens data in Tab. 13A it can be seen that a lot of glasses with anomalous dispersion have been used.
In this second embodiment there are seven lenses with anomalous dispersion behavior in the aperture stop proximity space 518: L5, L6, L8, L9, L10. L11 and L12. The low and anomalous dispersion glasses are used in the positive powered lenses L8 (SFPL51), L10 (SFPL51), and L12 (SFPM2) and high and anomalous dispersion glasses are used in the negative powered lens L9 (NKZFS5).
In the field proximity spaces 520, 522 there are low and anomalous dispersion glasses in L1 (SFPM3) and L14 (SFPL51), reducing their influence on chromatic aberrations. All other glasses have also a significant departure from the normal line.
The GAR for the second embodiment is 146.
The third embodiment can be summarized as follows:
The * denotes the aspheres. There are no other lens elements in this objective lens.
With this general setup, the major advantages of the invention can be achieved.
The aspherical surfaces are preferably positioned as follows:
As explained above, at these positions, they have a very strong influence on correcting different aberrations.
An advantageous realization of the first embodiment can be achieved with the following forms of the lenses:
Further advantages can be achieved by using the following orientations of the lenses:
Further advantages can be achieved by using the following glass types for the lenses:
The definitions of the glass types are given in the glossary.
Further advantages can be achieved by using the following ranges of focal lengths for the lenses:
The numerical data of the objective lens of the third embodiment according to
The aspherical constants for the aspheres used in the third embodiment are given in Tab. 20B.
The influence of the different aspherical surfaces on aberration correction is given in Tab. 21. The biggest influences of the aspherical surfaces are again in bold type. It is evident from the data in Tab. 21 that the aspherical surfaces 1, 3 in the object side field proximity space 820 are mainly correcting astigmatism and distortion and the aspherical surface 21 in the aperture stop proximity space 818 is mainly controlling spherical aberration and coma.
The glass material selection includes a plurality of glasses with anomalous dispersion but keeping the GAR at a value of 157 thus between the optimum limits.
From the lens data in Tab. 20A it can be seen that a lot of glasses with anomalous dispersion have been used.
In this third embodiment there are five lenses with anomalous dispersion behavior in the aperture stop proximity space: L8, L9, L10, L11, and L12. The low and anomalous dispersion glasses are used in the positive powered lenses L8 (SFPM3), L10 (SFPL51) and L12 (SFPM2) and high and anomalous dispersion glasses are used in negative powered lens L9 (NKZFS8) and L11 (SLAH58).
In the field proximity spaces there are low and anomalous dispersion glasses in L1 (SFPL51) and L14 (SFPL53), reducing their influence on chromatic aberrations. All other glasses have also a significant departure from the normal line.
The MTF is shown in
For this embodiment, the transverse ray aberrations are typically below 30 μm both for tangential and sagittal rays.
The group distribution for the lens in this third embodiment is represented in
The first subgroup LG21 of these two subgroups has a positive refractive power, and the second LG22 also has positive refractive power. The remaining lenses L11 to L14 between the second subgroup LG22 and the image plane taken as a group have a positive refractive power.
The group structure of the third embodiment can thus be summarized as
N-P-P-Stop-P,
where N denotes a negative refractive power and P a positive one.
The second and third embodiment are two objective lenses with the same focal length of 35 mm and aperture value f-number of 1.7. The two configurations have also similar correction means but distributed in two different ways. In the second embodiment according to
The fourth embodiment can be summarized as follows:
The * denotes the aspheres. There are no other lens elements in this objective lens.
With this general setup, the major advantages of the invention can be achieved.
The aspherical surfaces are preferably positioned as follows:
As explained above, at these positions, they have a very strong influence on correcting different aberrations.
An advantageous realization of the first embodiment can be achieved with the following forms of the lenses:
Further advantages can be achieved by using the following orientations of the lenses:
Further advantages can be achieved by using the following glass types for the lenses:
The definitions of the glass types are given in the glossary.
Further advantages can be achieved by using the following ranges of focal lengths for the lenses:
The numerical data of the objective lens of the fourth embodiment according to
The aspherical constants for the aspheres used in the fourth embodiment are given in Tab. 27B.
Since the focal length of the lens is larger, the field proximity space is reduced as compared with the corresponding spaces within the 25 mm and 35 mm focal length objective lenses. As a consequence, the contribution of the aspherical surface is increased for the aberration depending on the marginal ray height, as can be seen in Tab. 28.
Tab. 28 shows the contribution of the aspherical surfaces to the correction of the different aberrations. The biggest influences of the aspherical surfaces are again in bold type.
It can be clearly seen that the impact of the aspherical surfaces 3, 26 in the field proximity space 1120, 1122 on distortion is by a large factor greater than the impact of the aspherical surface 18 in the aperture stop proximity space 1118 on the same aberration. The action of the aperture proximity aspherical surface 18 on spherical aberration is also by a large factor stronger than the action of the two field proximity aspherical surfaces 3, 26 on the same aberration.
The lens comprises a plurality of lens elements which can be separated into groups as indicated in
As shown in
The two subgroups LG21, LG22 have a positive refractive power. The remaining lenses L11 to L14 between the second subgroup LG22 and the image plane taken as a group also have a positive refractive power.
The group structure of the fourth embodiment can thus be summarized as
N-P-P-Stop-P, with the aperture stop 1114 being part of LG22,
where N denotes a negative refractive power and P a positive one.
The performance of this lens is shown in
As could be seen from this diagram, there is an outstanding performance due to the optimal distribution of aspherical lens elements within the objective lens.
The glass materials used are listed in Tab. 27A. It could be easily calculated for one skilled in the art that the GAR ratio has a value of 151 for this lens, thus between the optimum limits.
While the present invention has been described and illustrated in conjunction with a number of specific embodiments, those skilled in the art will appreciate that variations and modifications may be made without departing from the principles of the inventions as herein illustrated, as described and claimed. The present invention may be embodied in other specific forms without departing from their spirit or essential characteristics. The described embodiments are considered in all respects to be illustrative and not restrictive. The scope of the inventions are, therefore, indicated by the appended claims, rather than by the foregoing description.
Many objective lenses with diverse focal lengths can be made based on the types of objective lenses disclosed in this application. Not only the described embodiments can be realized, but a whole series of objective lenses can be realized based on the teaching of the invention. This is at least possible by simply scaling all distances and radii by the ratio of the desired focal length and the focal length of a disclosed embodiment.
An objective lens or—in short—an objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image, typically on an image sensor or film. Objective lenses are also called object lenses or simply lenses.
In this specification, an optical element denotes a single lens or doublet lens or a lens group.
A lens means a single lens or an objective lens.
A lens elements designates a single lens or a lens doublet.
A lens group is a group of lens elements comprising one or more lens elements.
The f-number of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil. The entrance pupil being the optical image of the physical aperture stop, as ‘seen’ through the front of the lens system.
The term full frame is used as a shorthand for an image sensor format which is the same size as a 35 mm format film, i.e. 36 mm×24 mm.
According to M. J. Kidger: “Fundamental Optical Design”, SPIE Press, Bellingham, W A 2001, the marginal ray is defined to be the ray which passes through the center of the object and the edge of the aperture stop.
The marginal ray has at every intersection point with an optical surface a distance HM to the optical axis.
For a centered optical system the plane formed by the optical axis and the marginal ray is called by convention the meridional plane.
According to the M. J. Kidger: “Fundamental Optical Design”, SPIE Press, Bellingham, W A 2001, the chief ray is defined to be the ray from an off axis point in the object plane, passing through the center of the aperture stop. In the description used in this document, the chief ray of the outmost object field point is considered.
The chief ray from the outmost object field point has at every intersection point with an optical surface a distance HC to the optical axis.
The space around the aperture stop satisfying the relation HC/HM<0.5 is defined to be the aperture stop proximity space. A surface is said to lie within the aperture stop proximity space if HC/HM<0.5 for this particular surface. A lens is said to lie within the aperture stop proximity space if both surfaces of the lens lie within the aperture stop proximity space. Sometimes, only one of the surfaces of a lens lies within the aperture stop proximity space.
The space in front and beyond the aperture stop proximity space is called to be the field proximity space. In other words, the field proximity space is the space satisfying the relation HC/HM>=0.5. Typically, there are field proximity spaces in an objective, one on the object side of the objective lens and one on the image side. A surface is said to lie within the field proximity space if HC/HM>=0.5 for this particular surface. A lens is said to lie within the field proximity space if both surfaces of the lens lie within the field proximity space. Sometimes, only one of the surfaces of a lens lies within the field proximity space, while the other may lie within the aperture stop proximity space.
see anomalous dispersion glasses
The names of the glass material classes are given in
Low dispersion glasses are glasses with an Abbe number νd of 62 or higher.
Glasses with anomalous dispersion are defined as glasses whose departure from the normal line of the relative partial dispersion ΔPgF is at least 0.005, in terms of absolute value.
The relative partial dispersion PgF of an optical glass is defined for the Fraunhofer wavelengths g and F as:
In this equation ng, nF, nC are the refractive indices of the chosen glass at the Fraunhofer wavelengths g (422.670 nm), F (486.134 nm) and C (656.281 nm) correspondingly.
Departure from the Normal Line of the Relative Partial Dispersion ΔPgF
The departure of the relative partial dispersion ΔPgF from the normal line of a chosen glass for the g and F Fraunhofer wavelengths is given by the equation:
In this equation ng, nF, nC are the refractive indices of the chosen glass at the Fraunhofer wavelengths g (422.670 nm), F (486.134 nm) and C (656.281 nm) correspondingly. νd is the Abbe number of the glass type at the Fraunhofer wavelengths d (466.814 nm).
The sum of the absolute values of all departures from the normal line of the relative partial dispersion of all lenses divided by the number of lenses and multiplied by 10{circumflex over ( )}4 is called glass anomalous ratio (GAR):
It serves as indicator of the number of lenses with anomalous dispersion.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 102018106236.2 | Mar 2018 | DE | national |
