The present invention is directed to a macro lens assembly and camera objectives especially as claimed in the independent claims .

Macro lens systems, capable of close up photographing, as for example disclosed in US 8681435 usually have a plurality of lens groups, wherein typically two or more lens groups are moveable.

EP3226054 discloses endoscope lens systems for proximity magni fying observation. An optical system may comprise (in order from an object side) a first lens group having a positive refractive power, a second lens group having a negative refractive power and third lens group having a positive refractive power, wherein focusing is carried out by moving the second lens group.

It is an object of the present invention to overcome the draw backs of the state of the art, in particular it is an object to provide a lens assembly which can be produced cost efficiently and which nevertheless has a stable performance, preferably at an 1:1 magnification.

The objects are solved with the invention as defined in the claims .

A macro lens assembly for a camera comprises a first group of lenses having a positive refractive power, a second group of lenses having a positive refractive power, a third group of lenses having a negative refractive power and a fourth group of lenses having a positive refractive power, in order from an ob ject side to an image plane side. The lens assembly is contained in a housing and has a mechanical and optionally an electric and electronic interface for connec tion with a camera.

Each group of lenses comprises at least one lens.

The first group, the second group and the fourth group are fixed. Thus the distance between the respective groups remains constant. The third group is moveable with respect to the other groups, such that a focus is adjustable by moving the third lens group .

The distance between the second and third group and thereby also the distance between the third group and the fourth group can be changed .

Hence only the third lens group is used for focusing which al lows a construction of the lens assembly with reasonable costs.

Preferably the macro lens assembly enables taking pictures from infinity to a 1:1 magnification.

Preferably the macro lens assembly is a telephotographic macro lens assembly.

For a lens assembly of the telephoto type, the macro lens system has an overall positive refractive power. Usually a lens group with a negative refraction power is used as a field flattener to correct the curvature of the image field.

In this case the performance is maximized by placing a lens group with a positive refraction power close to the image plane. The macro lens system may satisfy the expression

r

wherein f ₄ denotes the focal length of the fourth group of lens, which is placed close to the image plane and wherein f denotes the overall focal length of the assembly at in infinite object distance .

Preferably the fourth group of lens comprises more than one lens .

In particular the macro lens system may satisfy the expression

r

wherein fi _ast denotes the focal length of the lens of the fourth group which is placed most closely to the image plane.

The above mentioned ratio may be achieved by a specific curva ture of the last lens.

In a beneficial embodiment of the invention the macro lens sys tem satisfies the expression

wherein Ri _ast denotes the curvature of the lens of the fourth group which is placed most closely to the image plane.

Advantageously the assembly is configured for forming an image of an object an infinite distance when the third group is placed most closely to the second group, and for acting as a macro lens, preferably with magnification of -1, when the third group is placed most closely to the fourth group. The moveable third group may change the magnification of the macro lens system according to a changed object distance.

In particular the assembly satisfies the expression

wherein EP denotes the entrance pupil of the widest aperture at infinity and EP _macro denotes the entrance pupil of the widest aperture at minimum focal distance. Regularly apertures or stops move in macro systems. For the as sembly according to the present invention, preferable the aper ture or stop, in particular arranged between the first and the second group of lens, does not move. Hence, there is no change in the entrance pupil and EP is more or less equal to EP _macro .

Advantageously the assembly satisfies the expression

wherein f3 denotes the focal length of the third lens group.

Advantageously the assembly satisfies the expression

wherein fi denotes the focal length of the first lens group.

Advantageously the assembly satisfies the expression

wherein f2 denotes the focal length of the second lens group. The conditions provide for minimizing a performance change when the focal distance is changed from infinity to a minimum focal distance .

In a preferred embodiment of the invention the assembly does not comprise any aspherical lens. Hence the assembly may be con structed without cost intensive elements.

Preferably the macro lens assembly has an overall focal length at in infinite object distance of f greater than 50mm, prefer ably greater or equal 150mm.

In a beneficial embodiment of the macro lens assembly the aper ture is arranged between the first lens group and second lens group. In particular the aperture is not moveable and hence the first lens group, the aperture and the second lens group form a fixed unit.

In an advantageous embodiment of the macro lens assembly the first lens group includes two cemented lens, each comprising two lens .

At least one of the two cemented lens may comprise a negative lens .

The first lens group may comprise two further single lens, wherein preferably one is arranged between the cemented lens.

The second lens group may consist only of one lens.

In an advantageous embodiment of the macro lens assembly the third lens group includes a cemented lens, the cemented lens comprising two lens. The third lens group may comprise one further lens.

The object according to the inventions is also solved by a cam era objective comprising a macro lens assembly as described above .

The objective with a macro lens assembly as described herein provides the advantages of the assemblies described herein.

The description is in the following described with reference to schematic drawings, which show non-limiting examples of the in vention .

The figures schematically show:

Fig. 1: A macro lens assembly according to the invention in a first position;

Fig. 2: The macro lens assembly according to the invention in a second position;

Fig. 3: The macro lens assembly according to the invention in a third position;

Fig. 4: A spot diagram of the embodiment of Figure 1 at infini ty;

Fig. 5: A spot diagram of the embodiment of Figure 2 with a magnification of -0.5;

Fig. 6: A spot diagram of the embodiment of Figure 3 with a magnification of -1;

Fig. 7: A Ray Fan of the embodiment of Figure 1 at infinity; Fig. 8: A Ray Fan of the embodiment of Figure 2 with a magnifi cation of -0.5;

Fig. 9: A Ray Fan of the embodiment of Figure 3 with a magnifi cation of -1; Fig. 10a: A schematic representation of a camera comprising a camera objective with a macro lens assembly in a side view;

Fig. 10b: A schematic representation of the camera according to Fig. 10a in a perspective view.

Figure 1 shows a macro lens assembly 10 according to the inven tion in a first position.

The macro lens assembly 10 comprises a first group 1 of lenses having a positive refractive power, a second group 2 of lenses having a positive refractive power, a third group 3 of lenses having a negative refractive power and a fourth group 4 of lenses having a positive refractive power, in order from an ob ject side 5 to the side 6 of the image plane 8.

The first group 1, the second group 2 and the fourth group 4 are fixed and the third group 3 is moveable with respect to the oth er groups, such that a focus is adjustable by moving the third lens group 3.

An aperture 9 is arranged between the first lens group 1 and second lens group 2. The aperture 9 may comprise 11 rounded blades .

In a first position the third group 3 is placed most closely to the second group 2, such that an image of an object an infinite distance may be formed.

A camera with a macro lens assembly 10 in the first position has an angle of view of 16.02°. Figure 2 shows a macro lens assembly 10 according to the inven tion in a second position, wherein the third group 3 has an in termediate position between the second group 2 and the forth group 4, such that a magnification of -0.5 is achieved.

The first lens group 1 includes two cemented lens 11, 12, each comprising two lens 13, 14, 15, 16.

One of the cemented lenses 12 comprises a negative lens 16.

A camera with a macro lens assembly 10 in the second position has an angle of view of 10.134°.

Figure 3 shows a macro lens assembly 10 according to the inven tion in a third position, wherein the third group 3 is posi tioned close to the forth group 4.

The first group 1 and the second group 2 remain at a fixed posi tion.

The third lens group 3 includes a cemented lens 17. The cemented lens 17 comprises two lens 18, 19.

A camera with a macro lens assembly 10 in the third position has an angle of view of 12.4°.

Typical specifications representing a first embodiment of the assembly of Figures 1-3 are disclosed in the following tables.

[Table 1] Specifications of embodiment 1

f=150mm Fno=2.8 [Table 2] Results for embodiment 1

Figures 4-6 show spot diagrams of the embodiment of Figure 1 at different magnifications. Figure 4 (third column) is related to the embodiment with the third lens group (3) in a first position as shown in Figure 1. Figure 5 (third column) is related to the embodiment with the third lens group (3) in a second position as shown in Figure 2. Figure 6 (third column) is related to the em bodiment with the third lens group (3) in a third position as shown in Figure 3.

The spot diagrams represent the intersections of rays from a given object point at a field position (y-axis) with an image plane at a "defocusing position" (x-axis) of the macro lens as sembly. The spot diagrams provide an immediate visual indication of the imaging quality at that points.

At least for magnifications m=-0.5 (figure 5) and m=-1.0 (figure 6) the macro lens assembly shows excellent imaging characteris tics for most of the positions.

Figure 7 shows a Ray Fan of the embodiment of Figure 1 at infin ity. Fig. 8 shows a Ray Fan of the embodiment of Figure 2 with a magnification of -0.5. Fig. 9 shows a Ray Fan of the embodiment of Figure 3 with a magnification of -1.

Figure 10a shows a schematic representation of a camera 100 com prising a camera body 30 and a camera objective 20 with a macro lens assembly 10 in a side view. The camera 100 in particular is a digital photo camera, wherein the camera objective 20 can be exchanged.

The third lens group 3 is in a first position according to fig ure 1.

By turning a part of the camera objective 20 around the optical axis 21 the third lens group may be moved along the axis 21.

The camera objective 20 may have a length 24 of 100mm-150mm, in particular 128mm, and a diameter 25 of 50mm-110mm, in particular 87mm.

Figure 10b shows a schematic representation of the camera 100 as shown in Figure 10a in a perspective view.

The camera 100 may be a high resolution digital single lens re flex (DSLR) camera with am 35mm "full image" format. The camera objective 20 may have an aperture range of f/2.8mm -f/32mm, which facilitates short exposure times.

A 1:1 macro reproduction ratio may be available at a focusing distance of 0.3m-0.4m, in particular 0.345m.

High precision of focusing across the entire range may be en sured by the objective 20 comprising a wide focus ring 22 with an anti-slip coating, for example a rubber coating, having a ro tational angle of 210° . An additional focus ring lock (not ex plicitly shown in the figures) may ensure that the focus posi tion remains fixed when needed.

The front filter thread 23 may have diameter of 50mm-90mm, in particular 77mm.