PORTABLE DEVICES

DESCRIPTION

Technical Field

This invention relates to a miniature telephoto lens assembly for electronic portable devices, such as mobile phones or tablets. The telephoto lens assembly is particularly useful when implemented in rear zoom cameras of mobile devices.

Background Art

Miniature lens camera designs have caused an increasing interest during the last 20 years due to an extensive application in the mobile phone industry. Many types of designs are required (from telephoto to wide angle designs) and the optical parameters of such designs (focal length, FOV, etc.) are pushed to further limits in each mobile phone generation.

Telephoto designs have become very popular in the mobile phone industry during the last five years as they allow to provide optical zoom while keeping the total track length of the camera small enough to fit in the mobile phone. The optical zoom enables a big number of applications in photography, such as portrait photography. Current telephoto designs show a telephoto ratio (Total Track Length/Effective Focal Length, or TTL/EFL) considerably smaller than 1 ; the smaller the ratio, the bigger is the optical zoom that can be reached. Some designs with telephoto ratios of 0.9 (document US2018/0088300-A1) and 0.84 (document US2017/0146777-A1) have been disclosed in the literature.

Also, lenses already incorporated in commercial mobile phones have shown thatx2 and x3 optical zooms can be reached within lenses of TTL<7.5 mm. Optical zooms x5 and even x10 can be reached if a folded design is used, and thus, the total track length of the camera is folded reducing its effective thickness.

Therefore, there is a need for a new design of a miniature telephoto lens assembly for mobile device cameras that improves the telephoto ratio while maintaining an optimal image quality. Summary of Invention

The present invention refers to a miniature telephoto lens assembly for mobile devices (i.e. electronic portable devices, such as smartphones). The lens assembly comprises five lens elements arranged along an optical axis passing through the center of curvature of each surface of the lenses.

The lens assembly comprises, in order from an object side to an image side, a first group of lenses, a second group of lenses and a third group of lenses. The first group of lenses comprises, from object side to image side, a first lens element and a second lens element which form a non-cemented achromatic doublet with positive refractive power. The second group of lenses comprises, from object to image, a third lens element and a fourth lens element. These lenses form a non-cemented achromatic doublet with negative refractive power. The third group of lenses comprising a fifth lens element, working as a field lens with positive or negative refractive power.

The lens assembly is provided with a determined spacing pattern between the lens elements. In particular, the following conditions are fulfilled:

T12

< 0.1;

T2

G34

< 0.1

G45 wherein T 12 is the axial distance between the first lens element and the second lens element, T23 is the axial distance between the second lens element and the third lens element, T34 is the axial distance between the third lens element and the fourth lens element, and T45 is the axial distance between the fourth lens element and the fifth lens element. The axial distance between adjacent lenses is the thickness of the air gap between said lenses, or the distance in between said lenses, measured at the optical axis.

Advantageously, this spacing pattern allows obtaining highly reduced telephoto ratios (e.g. telephoto ratios TTL/EFL smaller than 0.81) while keeping a high quality performance with low distortion (e.g. inferior to 3% for all fields) and corrected field aberrations. The relationship between the axial distances T12 and T34 may further satisfy the following condition, which also contributes for even better high-quality image and highly reduced telephoto ratios:

0.8 1.25

To obtain a reduced telephoto ratio with a small total track length (e.g. TTL inferior to 7 mm) and a good image quality, lens elements within the first and second group of lenses (i.e. the gap between first and second lens elements and the gap between third and fourth lens elements) may be separated by a small air gap, and groups of lenses (i.e. the gap between the second and the third lens element and the gap between the fourth and fifth lens elements) may be separated by a large air gap. The large air gap is preferably at least ten times the small air gap. The small air gap is preferably lower than 0.12 mm and the large air gap is preferably higher than 1 mm. All the embodiments described hereinafter show a total track length (TTL) lower than 7 mm, an image height (IH) lower than 3 mm, a telephoto ratio comprised within the interval (0.75-0.81), a controlled distortion lower than 3%, and a f-number (FNO) lower than 3. However, different constraints (e.g. TTL higher than 7 mm, distortion higher than 3%, telephoto ratios lower than 0.75 or higher than 0.81) may be applied in further embodiments, since the aim of the optical lens assembly of the present invention is to achieve a reduced telephoto ratio with an overall high image quality (e.g. low distortion).

There is a compromise between the telephoto ratio and the aberration balancing, concretely, with respect to field aberrations. The designer can choose to have worse image quality (worse compensated aberrations) but lower telephoto ratio. Distortion may be difficult to compensate in telephoto systems. There is a relation between the telephoto ratio, the image height (IH) and the distortion. In the embodiments described later, the telephoto ratio is kept within the interval (0.75-0.81) and the distortion is kept lower than 3%, and in an embodiment even lower than 2% for IH > 2 mm in all cases (in a case for IH > 2.8 mm); however, in other embodiments implementing the particular design of the present invention, telephoto ratios lower than 0.75 may be obtained at the expense of a greater distortion (e.g. between 3% and 4%) or, alternatively, an extremely high image quality (e.g. distortion lower than 1% for all fields) may be obtained at the expense of a higher telephoto ratio, e.g. a telephoto ratio higher than 0.81, such as within the interval (0.81-0.9). In the latter case, the telephoto ratio may be in the range of state of the art designs, but with an improved image quality and reduced distortion.

The design of the groups including a first positive group and a second negative group implemented as compact non-cemented achromatic doublets, and a third group aimed to correct field aberrations (distortion and field curvature) and residual lateral chromatic aberration, and the air gaps patterns between the lens elements of the groups allows for compact lenses that show big focal lengths, allowing to achieve telephoto designs with a telephoto ratio down up to 0.75, or even lower and with a good image quality and satisfactory aberration balancing.

Another aspect of the present invention refers to an optical lens system, or image capturing device, for electronic portable devices. The lens system comprises an image sensor along with the telephoto lens assembly as previously described. In an embodiment, the image capturing device is a light field camera comprising a microlens array arranged between the fifth lens element and the image sensor. The image capturing device may include further elements, such as an IR filter positioned between the fifth lens element and the image sensor.

Brief Description of Drawings

The drawings attached illustrate several embodiments. Together with the description, they allow to explain the principles of the embodiments of the present invention. Those skilled in the art would recognize the drawings as merely exemplary, and not limiting by any means the scope of the present invention.

Figure 1A shows the layout of the lens assembly according to a first embodiment. Figures 1B-1D depict some tables with lens parameters used in the first embodiment. Figure 1E shows the modulation transfer function (MTF) for the lens design of the first embodiment. Figure 1F shows the distortion suffered across the field by the lens design of the first embodiment.

Figure 2A shows the layout of the lens assembly according to a second embodiment. Figures 2B-2D depict some tables with lens parameters used in the second embodiment. Figure 2E shows the MTF curves for some representative fields of the second embodiment. Figure 2F shows the distortion suffered across the field by the lens assembly of the second embodiment.

Figure 3A shows the layout of the lens assembly according to a third embodiment. Figures 3B-3D depict some tables with lens parameters used in the third embodiment. Figure 3E shows the MTF curves for some representative fields of the third embodiment. Figure 3F shows the distortion across the field of the third embodiment.

Figure 4A shows the layout of the lens assembly according to a fourth embodiment. Figures 4B-4D depict some tables with lens parameters used in the fourth embodiment. Figure 4E shows the MTF curves for some representative fields of the fourth embodiment. Figure 4F shows the distortion across the field of the fourth embodiment.

Figure 5A shows the layout of the lens assembly according to a fifth embodiment. Figures 5B-5D depict some tables with lens parameters used in the fifth embodiment. Figure 5E shows the MTF curves for some representative fields of the fifth embodiment. Figure 5F shows the distortion across the field of the fifth embodiment.

Detailed description

The present invention refers to a telephoto lens assembly 100. Figures 1 -5 depict several embodiments of the lens assembly 100. In all the embodiments, the lens assembly 100 includes five lens elements (L1, L2, L3, L4, L5) arranged along an optical axis 112. In particular, the lens assembly 100 comprises, in order from an object side 114 to an image side 116:

A first group of lenses G1 comprising a first lens element L1 and a second lens element L2, forming a non-cemented achromatic doublet with positive refractive power. The group may also be comprised by three lenses, in the case that the first element L1 is split into two lenses to distribute its power.

A second group of lenses G2 comprising a third lens element L3 and a fourth lens element L4 forming a non-cemented achromatic doublet with negative refractive power.

- A third group of lenses G3 comprising a fifth lens element L5. In all the embodiments this group is composed of a single lens element, although additional lens elements may be added to the group. The fifth lens element L5 may be split into two thinner lenses, having more surfaces to correct for field aberrations. Alternatively, a sixth lens element may be included without making the fifth lens element L5 thinner, by making axial distance T57 or even axial distance T45 smaller. However, the inclusion of more lens elements makes the design more complex in terms of assembly, tolerances, and also in cost.

Two adjacent lens elements are separated from each other by an air gap. For instance, lens elements L1 and L2 are spaced by an air gap A12. Similarly, two adjacent groups of lenses are separated from each other by an air gap; for example, groups G1 and G2 are spaced by an air gap A23, corresponding to the air gap between the closest lens elements (L2 and L3) of the groups.

An axial distance between two adjacent lens elements can be defined as the thickness of the air gap separating said lens elements along the optical axis 112 (i.e. central thickness). The axial distance therefore refers to the thickness of the air gap between adjacent lenses, or the distance between adjacent lenses, measured at the optical axis 112. For example, in Figure 1A the axial distance between lens element L2 and lens element L3 is depicted as T23, and the axial distance between lens element L3 and lens element L4 is represented as T34. Similarly, an axial distance between two adjacent groups is defined as the thickness of the air gap that separated those groups along the optical axis 112; for instance, the axial distance between groups G2 and G3 is the axial distance between the closest lens elements of the groups, in this case the axial distance between lens elements L4 and L5 (i.e. axial distance T45).

In the figures, T12 is the axial distance between the first lens element L1 and the second lens element L2, T23 is the axial distance between the second lens element L2 and the third lens element L3, T34 is the axial distance between the third lens element L3 and the fourth lens element L4, and T45 is the axial distance between the fourth lens element L4 and the fifth lens element L5.

In all the embodiments, the five lens elements are spaced according to a determined pattern. In particular, the axial distances (T12, T23, T34, T45) between adjacent lens elements satisfy the following conditions: T 34

< 0.1 G45

The following condition is also fulfilled in all the embodiments:

T12

0.8 < - < 1.25

G34

The axial distances of this spacing pattern are measured at the optical axis 112. The following conditions are suggested to achieve an optimal design of the spacing pattern along the whole lens aperture:

0.8 1.2

R6

0.5 < — < 2 R7

R2, R3, R6 and R7 are respectively the radii of surfaces 2, 3, 6 and 7 of the lens assembly. In particular, R2 is the radius of the image-side surface (102b, 202b, 302b, 402b, 502b) of the first lens element L1, R3 is the radius of the object-side surface (104a, 204a, 304a, 404a, 504a) of the second lens element L2, R6 is the radius of the image- side surface (106b, 206b, 306b, 406b, 506b) of the third lens element L3, R7 is the radius of the object-side surface (108a, 208a, 308a, 408a, 508a) of the fourth lens element L4. This means that the radii of surfaces which are separated by small air gaps (i.e. surfaces adjacent between lens elements L1-L2 and surfaces adjacent between lens elements L3-L4) are alike, so that the spacing between said lenses is sufficient along the whole aperture of the lenses and a small separation is maintained along the whole lens aperture. All the embodiments depicted in the figures fulfil these suggested relationships between the radii R2, R3, R6 and R7. In the embodiments, the surfaces of the lens elements are aspheric and are defined by a certain radius (R1, R2, etc.) and aspheric coefficients.

Advantageously, the design of the lens assembly 100 including a positive non-cemented achromatic doublet, a negative non-cemented achromatic doublet and a fifth lens spaced according to a determined pattern (with the aforementioned particular relations between the axial distances T12, T23, T34 and T45) allows obtaining compact lenses with large focal lengths, so that when applied to a camera (optical lens system 120 in Figures 1-5) with an image sensor 124 (having an image surface 126 for receiving incoming light rays from the object side 114) and an optional IR filter 122, a highly reduced telephoto ratio TTL/EFL can be achieved while maintaining an optimal image quality with low distortion and field aberrations. For instance, in the embodiments shown in Figures 1-5, telephoto ratios TTL/EFL smaller than 0.81 are achieved with this design, while keeping a distortion across the field smaller than 3%; although other combinations of telephoto ratios TTL/EFL and image quality (e.g. distortion across the field and aberrations) can be obtained (for instance, if image quality is an absolute priority, the parameters of the lens elements may be adjusted to obtain an outstanding image quality with a TTL/EFL than can be in that case higher than 0.81).

The design of the lens assembly 100 also allows obtaining a small f-number (the smaller the f-number, the better the optical quality obtained by the lens assembly). For example, in all the embodiments of the figures the lens assembly 100 has a f-number smaller than 3.

In addition, this particular design of the lens assembly 100 allows achieving the goal of obtaining a reduced telephoto ratio TTL/EFL with optimal image quality while maintaining lax manufacturing requirements. For instance, the surfaces of all the lens elements of the lens assembly 100 have preferably a maximum slope of 55°, which is a factor that improves manufacturability.

Also, the form factor of the lenses is preferably kept within comfortable numbers for the optical manufacturer. To that end, for all the lens elements of the lens assembly the center thickness (CT) of a lens element (i.e. thickness measured at the optical axis 112) is preferably four times or less than the edge thickness (ET) of the lens (i.e. thickness measured at the edge of the lens), that is:

CT

— < 4 ET ^~

All the embodiments shown in the figures satisfy this condition, and in one embodiment the following condition is fulfilled:

CT

^~ ET ^{£ 3} which further increases manufacturability. The embodiment satisfying the latter condition is however the one with less exigent telephoto ratio. The combination of the positive and negative achromatic doublets prevents from spherical aberration, coma, astigmatism, and from chromatic aberrations and also allows for a telephoto configuration. The arrangement of such doublets (both compact with a large air gap between them), together with the afore-mentioned radii and thickness constraints, allow for easily manufacturable small telephoto ratios at the same time that the main aberrations are corrected following, to a certain extent, the “thin lens approach” to telephoto designs. According to the thin lens approach, which is well-known in the art, if the lens assembly design includes a positive lens and a negative lens separated by a space d, by considering the paraxial model and using the paraxial equations of the lenses the designer can use the results for obtaining the focal lengths, the spaces between lenses, the TTL and telephoto ratio as a kind of pre-design, in which some aberrations are basically easier to correct. The arrangement of a fifth lens at the end of the lens assembly 100, separated from the previous group of lenses (negative achromatic doublet, G2) by an axial distance T45 at least 10 times the axial distance T34 between the lens elements (L3, L4) of the negative achromatic doublet, can be employed to correct field aberrations and residual lateral color, acting as a field lens. Concretely, this field lens corrects for field curvature, distortion and also some residual astigmatism and coma.

In all the embodiments, all the adjacent groups of lenses are separated from each other by a relatively large air gap (A23, A45), when compared with the relatively small air gap (A12, A34) that separates adjacent lens elements within the first G1 and second G2 group of lenses. The axial distances (T12, T34) of the small air gaps (A12, A34) are at least 10 times smaller than the corresponding axial distances (T23, T45) of the large air gaps (A23, A45). The spacing between the different lens elements, as shown in the figures, proves the group structure, where two achromatic doublets are combined resulting in an apochromatic telephoto lens. According to an embodiment of the lens assembly 100, the large air gaps (A23, A45) have an axial distance (T23, T45) greater than 1 mm, and the small air gaps (A12, A34) have an axial distance (T12, T34) smaller than 0.12 mm. These optional size requirements are useful to achieve a very short total track length (TTL) as they ease the telephoto design configuration. For instance, all the embodiments shown in the figures comply with these size requirements (large air gaps higher than 1mm, small air gaps lower than 0.12mm), which allow obtaining a total track length (TTL) smaller than 7 mm. In all these embodiments, when the lens assembly 100 is installed on the optical lens system 120, the axial distance T57 between the fifth lens element L5 and an image surface 126 of the image sensor 124 is smaller than 0.71 mm; however, this additional size requirement is also optional (the selected axial distance T57 may be greater or shorter than 0.71 mm).

When a goal is set to obtain a reduced total track length, such as lower than 7 mm, the value of the axial distance T57 between the fifth lens element L5 and the image sensor 124 is not entirely free, since it depends on the design of the doublets (G1, G2). Arranging the fifth lens element L5 close to the image sensor 124 allows better correction of field aberrations. Likewise, it is normally necessary to leave sufficient space for arranging other additional elements between the fifth lens element L5 close to the image sensor 124 and, if necessary, also leave room for alignment or autofocus; for instance, a gap of at least 0.5 mm is recommended for intercalating an I R filter 122 and a microlens array (when the lens assembly is applied to a plenoptic camera). For instance, in all the embodiments the axial distance T57 is higher than 0.6 mm and lower than 0.71 mm.

Each one of the lens elements may be made of plastic or glass, or any other material employed in the manufacturing of lenses. In an embodiment, all the lens elements are made of plastic. The materials employed in the lens elements of each achromatic doublet have a significant difference in Abbe number. In order to improve the correction of chromatic aberrations, the difference in Abbe number is preferably higher than 31 , so that the following conditions are fulfilled:

|V1- V2 |> 31

|V3- V4 |> 31 wherein V1, V2, V3 and V4 are, respectively, the Abbe number of the first (L1), second (L2), third (L3) and fourth (L4) lens element. In an embodiment, the material of the lens elements of the lens assembly 100 are selected from among only two materials, wherein these two materials have a difference in Abbe number higher than 31. However, three or more different materials with different Abbe numbers may be used. The surfaces of all the lens elements of the lens assembly 100 are preferably, but not necessarily, aspheric. In the embodiments shown in Figures 1-5, all the lenses are aspheric. As previously explained, the particular design of the lens assembly 100 allows achieving a reduced ratio between the total track length and the total effective focal length (known as telephoto ratio, TTL/EFL) of the optical lens system 120 that includes the lens assembly 100. In an embodiment, the design of the lens assembly 100 achieves, when incorporated into an optical lens system 120, a telephoto ratio smaller than 0.81. For instance, in all the embodiments of Figures 1-5 the telephoto ratio fulfils the following condition:

TTL

0.75 < < 0.81

~EFL

The lens assembly preferably satisfies one or more of the following conditions:

The ratio between the total effective focal length, EFL, and an effective focal length of the first group of lenses, EFL12, fulfils:

EFL

1.5 < - < 2

EFL12

The ratio between the total track length, TTL, and an effective focal length of the first group of lenses, EFL12, fulfils:

TTL

1.25 < < 1.45.

EFL12

The ratio between the total track length, TLL, and a maximum image height on an image surface 126 of the image sensor 124, IH, fulfils:

TTL

2 < ΊΪG < 3.5

All the embodiments shown in Figures 1-5 comply with these three optional requirements. It is common in telephoto configurations that the positive element is heavily loaded in terms of focal length. When following the first two conditions above mentioned, a small telephoto ratio can be maintained (if one follows the thin lens reasoning). Also, it is a good practice to keep the relation TTL/IFI within the margins of the third condition, so the image sensor 124 is not too small nor too big when compared to the size of the lens.

Another aspect of the present invention refers to an optical lens system 120 (or image capturing device) for electronic portable devices, comprising an image sensor 124 and a telephoto lens assembly 100 as previously described. The optical lens system 120 may also comprise an IR filter 122 arranged between the fifth lens element L5 and the image sensor 124, as depicted for instance in the embodiments of Figures 1-5. In an embodiment, the optical lens system is a light field camera comprising a microlens array arranged between the fifth lens element L5 and the image sensor 124.

A telephoto lens assembly 100 is herein described through five lens embodiments which intend to be non-limiting examples. Each embodiment is defined by an arrangement of five lens elements (L1 , L2, L3, L4, L5), which are in turn defined by two surfaces, an object-side surface (for instance, 102a for the first lens element L1 of the first embodiment) facing toward the object side 114 and an image-side surface (e.g. 110b for the fifth lens element L5 of the first embodiment) facing toward the image side 116. A surface is defined convex or concave as seen from the object side 114 if such surface is acting as an interface between air and plastic/glass (in the direction of the light path), and it is defined convex or concave as seen from the image side 116 when such surface is an interface between plastic/glass and air (in the direction of the light path). All or some of the surfaces may be aspheric. For the description of the aspheric surfaces in the embodiments below, which are rotationally symmetric, the following even asphere surface equation is used (although other known asphere surface equations may be employed): where z(r) is the surface sag for a certain radius r. The a coefficients are the aspheric coefficients that define the aspheric part of the surface, which is a polynomial. The coefficient c refers to the surface curvature, and the coefficient k refers to the conic constant.

Starting from a lens arrangement design satisfying the spacing pattern, and optionally the radii and the thickness conditions previously defined, the rest of the lens parameters (such as the description of the aspheric surfaces according to equation [1], the lens thickness and the Abbe number material of the five lens elements) can be adjusted such that a certain condition (or conditions) is satisfied. For instance, the conditions may

TTL include — < 0.81 and distortion across the field being smaller than 3%.

EFL

In all the embodiments, the lens assembly structure has a total track length (TTL) below 7 mm and a total effective focal length (EFL) greater than 8 mm, which leads to telephoto ratios (TTL/EFL) below 0.81 in all lens embodiments herein described.

As previously explained, the lens assembly structure is formed by five lens elements arranged, from object to image as: a first lens L1 and a second lens L2 which form a first non-cemented achromatic doublet (first group of lenses G1) with an effective focal length EFL12; a second non-cemented achromatic doublet (second group of lenses G2) formed by third L3 and fourth L4 lenses with an air separation in the optical axis (axial distance T34) and with an effective focal length EFL34; and a third group of lenses G3 formed by a single lens (fifth lens L5). First and second doublets are spaced by a considerably big air gap A23, the central thickness of which is defined by the axial distance T23. The first group of lenses G1 is a positive group, whereas the second group of lenses G2 is a negative group. The fifth lens element L5, which may be positive or negative, acts as a field lens. The air gap A45 between the second G2 and third G3 group of lenses, the central thickness of which is defined by axial distance T45, is also considerably big. In contrast, the air gaps between the lens elements within the first and second groups, air gaps A12 and A34, are considerably smaller (the corresponding axial distances T12 and T34 are at least one order of magnitude smaller). In the lens structure of all the embodiments depicted in the figures, the ratios T12/T23 and T34/T45 are smaller than 0.1, the axial distances T12 and T34 are smaller than 0.12 mm, and the axial distances T23 and T45 are greater than 1 mm. Also, the ratio EFLVEFL12 is greater than 1.5 and below 2, and the ratio TTL/EFL12 is greater than 1.25 and below 1.45.

The structure of the design with a first compact (with a small air gap in between the lenses L1 and L2) positive group G1, a big air gap A23, a second compact (with a small air gap in between the lenses L3 and L4) negative group G2, another big air gap A45 and a field lens L5 allows for designing a large effective focal length within a short TTL (i.e. a reduced telephoto ratio TTL/EFL). Figure 1 A shows a 2D layout of a first embodiment of the telephoto lens assembly 100. Three representative fields 101, 103 and 105 are drawn, respectively corresponding to 0% (0°), 75% (9.7°) and 100% (13°) of the field. According to the criteria previously defined, this lens assembly 100 is comprised, in order from the object side 114 to the image side 116, by a first convex surface 102a and a second convex surface 102b, which together form the first lens element L1 , with a positive refractive power. This lens is made of an optical plastic with a high Abbe number. The first lens aperture is the stop aperture of the system. Optionally, a mechanical aperture can be introduced.

After the first lens element L1 there is a small air gap A12 with a central thickness of 0.105 mm, followed by the second lens element L2, which is formed by a first concave surface 104a and a second concave surface 104b. The second lens element L2 shows a negative refractive power and is designed with a material that shows a small Abbe number. Lens elements L1 and L2 form a first achromatic doublet G1, which helps correcting the axial color.

After the first achromatic doublet there is an air gap A23 with a central thickness of 1.3 mm (axial distance T23), which is followed by a second achromatic doublet G2 formed by two lenses, third lens element L3 and fourth lens element L4. The third lens element L3 is made of a material having a small Abbe number and comprised by a concave surface 106a and a convex surface 106b. The fourth lens element L4, made of a material with a high Abbe number, is composed by a concave surface 108a that has an inflection point and gets convex by the end of the aperture, and a concave surface 108b. This second doublet G2 is also achromatic, formed with the combination of a material with low dispersion for third lens element L3 and a material with high dispersion for fourth lens element L4. The combination of the first and second doublets prevent from spherical aberration, coma, and astigmatism; also, from chromatic aberrations. The design is followed by an air gap A45 of central thickness equal to 1.8 mm (axial distance T45).

The last lens, fifth lens element L5 is formed by a concave surface 110a which turns into a convex surface at the edge of its aperture, and a convex surface 110b. This last lens is useful to correct for field aberrations. Also, it helps correcting for residual lateral color. Finally, there is an air gap A56, an optional IR filter 112, an air gap A67 and then the image sensor 114 with a diagonal of 4 mm. Axial distance T57 represents the distance, measured along the optical axis 112, between the fifth lens element L5 and the image sensor 124 (i.e. the distance between the image-side surface 110b of the fifth lens element L5 and an image surface 126 of the image sensor 124).

The lens basic parameters for the first embodiment are shown in the table of Figure 1B. The detailed surface description is addressed in the tables of Figures 1C and 1 D.

The table of Figure 1B shows that the telephoto ratio TTL/EFL of this first embodiment is 0.786, which is much smaller than regular telephoto designs for miniature lenses. This design maximizes the focal length, enabling to obtain big focal lengths, and thus, big optical zooms. As previously described, the first embodiment complies with the basic condition of T12/T23 and T34/T45 being lower than <0.1. In this embodiment, axial distances T23 and T45 are greater than 1 mm, and the total track length, TTL, is kept smaller than 7 mm (in this example, 6.71 mm).

Advantageously, the ratio EFL/ EFL12 between the total effective focal length, EFL, of the whole lens assembly and the effective focal length of the first doublet, EFL12, is greater than 1.5 and smaller than 2, as previously stated. Advantageously, the ratio TTL/EFL12 between the total track length, TTL, and the effective focal length, EFL12, of the first doublet is greater than 1.25 and smaller than 1.45. These two conditions allow for small telephoto ratios if the thin lens reasoning is used.

Figure 1C shows the radii, thicknesses, materials and conic constants, and Figure 1D the aspheric coefficients used in the first embodiment. The optical plastic materials used in the lens elements of the achromatic doublets have a difference in Abbe number of 32.74, which is very useful to correct for chromatic aberrations. The slopes of all surfaces (i.e. the angle of the tangent to the surface with respect to the vertical) are smaller than 55 degrees, which increases manufacturability.

Figure 1E shows the modulation transfer function of the lens described in the first embodiment for three representative fields (101, 103, 105) across the spatial frequency Tangential and sagittal curves (TS) for those fields are shown in solid and dashed lines, respectively. Figure 1F shows the distortion across the field, which is kept lower than 3% for all fields. Figure 2A represents a second embodiment of the telephoto lens assembly 100. The lens structure is very similar to that of the first embodiment. From object side 114 to image side 116, there is a positive first lens element L1, formed by a convex surface 202a and a convex surface 202b (with a point of inflection at the edge of its aperture).

Next, there is a small air gap A12 the central thickness of which (axial distance T12) is 0.119 mm and then a negative second lens element L2, formed by two concave surfaces 204a and 204b. Lenses L1 and L2 form the first achromatic doublet G1 and they are made of high and low Abbe number materials, respectively. Air gap A23 is about 1.24 mm.

Next, the second doublet G2, formed by lenses L3 and L4, which are in turn formed by a concave 206a and a convex 206b surface in the case of lens L3, and two concave surfaces (208a and 208b) in the case of lens L4. These two lenses L3 and L4 are made of materials of low and high Abbe number, respectively. The central thickness of the air gap A45 between lenses L4 and L5, axial distance T45, is 1.9 mm.

Next, there is a positive fifth lens L5. As with the first embodiment, this lens L5 design prevents from field aberrations. These kind of lenses or field flatteners which work as a field lens, are usually negative lenses; however, in this embodiment fifth lens element L5 shows a positive refractive power with few power, showing that all the power can be carried by the two first doublets and aberrations can be satisfactorily corrected when using this lens structure.

Finally, there is an air gap A56, an IR filter 122, an air gap A67 and an image sensor 224. The IR filter 122, of 110 microns, is an optional element, so that it may be included between the fifth lens element L5 and the image sensor 224 or not.

The lens basic parameters for the second embodiment are shown in the table of Figure 2B. Detailed surface description is addressed in the tables of Figures 2C and 2D.

The table of Figure 2B shows that the telephoto ratio TTL/EFL of this second embodiment is 0.795. Again, advantageously, the ratio EFL / EFL12 between the total effective focal length, EFL, of the whole lens embodiment and the effective focal length EFL12 of the first doublet is comprised within the interval (1.5, 2). Also, the ratio TTL/EFL12 between the total track length TTL and the effective focal length of the first doublet EFL12 is within the interval (1.25, 1.45). The ratio between the central thickness of air gap A12 and air gap A23, (T12/T23), and the ratio between the central thickness of air gap A34 and air gap A45, (T34/T45), are smaller than 0.1 , and central thickness of air gaps A23 and A45 (i.e. axial distances T23 and T45) are higher than 1 mm. All surfaces show a maximum slope of 55°, which increases manufacturability.

Figure 2C shows the radii, thicknesses, materials and conic constants, and Figure 2D shows the aspheric coefficients used in the second embodiment.

Figure 2E shows the modulation transfer function of the lens described in the second embodiment for three representative fields (201 , 203, 205) across the spatial frequency. Figure 2F shows the distortion across the field, which is lower than 3% for all fields.

Figure 3A shows a third embodiment of the telephoto lens assembly 100, in which the telephoto ratio TTL/EFL is 0.75. The lens structure is very similar to that of the first embodiment, including the same elements (although with different parameters and axial distances) in the same order.

The lens basic parameters for the third embodiment are shown in the table of Figure 3B. Detailed surface description is addressed in the tables of Figures 3C and 3D.

As it is shown in the table of Figure 3B, in this third embodiment the ratio EFL/EFL12 between the total effective focal length, EFL, of the whole lens embodiment and the effective focal length EFL12 of the first doublet is 1.78, and thus comprised within the interval (1.5, 2). The ratio TTL/ EFL12 between the total track length TTL and the effective focal length EFL12 of the first doublet is 1.35, also between 1.25 and 1.45. In this case, the focal length has been increased and the air gaps A12 and A34 have been reduced, leading to thicknesses T12 and T34 lower than 0.08 mm. Ratios T12/T23and T34/T45 are smaller than 0.1, as described in the previous embodiments. Central thicknesses of air gaps A23 and A45 (i.e. axial distances T23 and T45) are also higher than 1 mm. All surfaces show a maximum slope of 55°, which increases manufacturability.

Figure 3C shows the radii, thicknesses, materials and conic constants, and Figure 3D shows the aspheric coefficients used in the third embodiment. Figure 3E shows the modulation transfer function of the lens described in the third embodiment for three representative fields (301, 303, 305) across the spatial frequency. Figure 3F shows the distortion across the field, which is again smaller than 3% for all fields.

Figure 4A shows a fourth embodiment the telephoto lens assembly 100, wherein the structure of the lens assembly, which was thoroughly described in the first embodiment, has been slightly modified. The lens structure is as follows: a positive first lens element L1, which is formed by two lens surfaces 402a and 402b, followed by an air gap A12 whose central thickness (i.e. axial distance T12) is 0.089 mm; a second lens element L2, which is a negative lens formed by surfaces 404a and 404b. These two lenses L1 and L2 form a first achromatic doublet G1, and are made of materials with high and low Abbe number, respectively.

Then, there is an air gap A23 with a central thickness (i.e. axial distance T23) of 1.455 mm. Next, a second doublet G2 formed by a biconcave lens L3 and a convex-concave lens L4 with an air gap between them whose thickness (i.e. axial distance T34) is 0.077 mm. In this fourth embodiment, the second doublet G2 is formed by a high and low Abbe number materials, respectively.

Next, an air gap A45. Final lens group G3 is formed by a negative lens L5. And lastly, there is an air gap A56, an optional IR filter 122, air gap A67 and image sensor 124.

The lens basic parameters for the fourth embodiment are shown in the table of Figure 4B. Detailed surface description is addressed in the tables of Figures 4C and 4D.

The table of Figure 4B shows that the telephoto ratio TTL/EFL of this fourth embodiment is 0.788. Again, advantageously, the ratio EFL/EFL12 between the total effective focal length, EFL, of the whole lens embodiment and the effective focal length EFL12 of the first doublet G1 is higher than 1.5 and lower than 2. Also, again, the ratio TTL/ EFL12 between the total track length TTL and the effective focal length EFL12 of the first doublet is higher than 1.25 and lower than 1.45. The ratios T12/T23 and T34/T45 are lower than 0.1 , and the central thickness of air gap A23 and air gap A45 are greater than 1 mm. All surfaces show a maximum slope of 55°. Figure 4C shows the radii, thicknesses, materials and conic constants, and Figure 4D shows the aspheric coefficients used in the fourth embodiment.

The modulation transfer function of the lens described in the fourth embodiment is described in Figure 4E for three representative fields (401, 403, 405) across the spatial frequency. Figure 4F shows the distortion across the field, which in this case is lower than 2% for all fields. The lens structure of this fourth embodiment helps correcting the distortion.

Figure 5A shows a fifth embodiment wherein the image height (IH) of the lens assembly, and thus the diagonal of the image sensor 124, has been increased up to an IH equal to 2.88 mm. The imaging plane of the lens assembly is a fictitious plane in which the image generated by the lens assembly is projected, and where the image sensor is typically placed. The lens assembly is corrected so that in this plane the lens assembly produces a quality image up to the image height IH, and beyond this image height IH the quality of the image cannot be guaranteed. Ideally, the height of the image sensor corresponds to the image height IH of the lens assembly. Structurally, the lens assembly is very similar to that of the first embodiment. However, the telephoto ratio TTL/EFL has been increased up to 0.81 to be able to increase the field of view (FOV) and thus, the IH of the lens assembly. It is known that ^{= S<} T when increasing the FOV, also the IH increases.

The lens basic parameters for the fifth embodiment are shown in the table of Figure 5B. Detailed surface description is addressed in the tables of Figures 5C and 5D.

The table of Figure 5B shows that the telephoto ratio TTL/EFL for this fifth embodiment is 0.807, which is the highest of all embodiments, but still smaller than any other found in the prior art of miniature telephoto lens assemblies for mobile devices. Advantageously, the ratio EFL/EFL12 between the total effective focal length, EFL, of the whole lens embodiment and the effective focal length EFL12 of the first doublet G1 is 1.57 (higher than 1.5 and lower than 2). Also, the ratio TTL/ EFL12 between the total track length TTL and the effective focal length EFL12of the first doublet G1 is 1.27 (higher than 1.25 and lower than 1.45). The ratios T12/T23 and T34/T45 are smaller than 0.1. Central thicknesses of air gaps A23 and A45 (i.e. axial distances T23 and T45) are higher than 1 mm.

Figure 5C shows the radii, thicknesses, materials and conic constants, and Figure 5D shows the aspheric coefficients used in the fifth embodiment.

Figure 5E describes the modulation transfer function (MTF) for three representative fields (501, 503, 505) across the spatial frequency, and Figure 5F shows the distortion, which is kept smaller than 3% for all fields.