DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A description of embodiments of the present invention will now be given with reference to the drawings. FIG. 1 illustrates a paraxial refractive power arrangement of an embodiment of a zoom lens in accordance with the present invention.

[0041] FIG. 2 is a sectional view of the lenses at a wide angle end of a zoom lens of a first numerical example (described later) of the embodiment of the present invention. FIGS. 3A to 3 D, FIGS. 4A to 4 D, and FIGS. 5A to 5 D show aberrations at the wide angle end, at an intermediate location, and at a telephoto end, respectively, of the zoom lens of the first numerical example.

[0042] FIG. 6 is a sectional view of lenses at a wide angle end of a zoom lens of a second numerical example (described later) of the embodiment of the present invention. FIGS. 7A to 7 D, FIGS. 8A to 8 D, and FIGS. 9A to 9 D show aberrations at the wide angle end, at an intermediate location, and at a telephoto end, respectively, of the zoom lens of the second numerical example.

[0043] FIG. 10 is a sectional view of lenses at a wide angle end of a zoom lens of a third numerical example (described later) of the embodiment of the present invention. FIGS. 11A to 11 D, FIGS. 12A to 12 D, and FIGS. 13A to 13 D show aberrations at the wide angle end, at an intermediate location, and at a telephoto end, respectively, of the zoom lens of the third numerical example.

[0044] FIG. 14 is a sectional view of lenses at a wide angle end of a zoom lens of a fourth numerical example (described later) of the embodiment of the present invention. FIGS. 15A to 15 D, FIGS. 16A to 16 D, and FIGS. 17A to 17 d show aberrations at the wide angle end, at an intermediate location, and at a telephoto end, respectively, of the zoom lens of the fourth numerical example.

[0045] FIG. 18 is a schematic view of the main portion of an optical apparatus using the zoom lens of the embodiment of the present invention.

[0046] In FIG. 1 and FIGS. 2, 6 , 10 , and 14 (which are sectional views of the lenses of the zoom lenses of the numerical examples), L 1 denotes a first lens unit having a positive refractive power (optical power=reciprocal of the focal length), L 2 denotes a second lens unit having a negative refractive power, L 3 denotes a third lens unit having a positive refractive power, and L 4 denotes a fourth lens unit having a positive refractive power.

[0047] SP denotes a stop which is disposed in front of the third lens unit L 3 , and which moves integrally with the third lens unit L 3 when magnification is being changed.

[0048] IP denotes an image plane where, for example, a silver film and an image pickup device, such as a CCD (charge-coupled device) or a CMOS, are disposed.

[0049] G denotes a glass block, which corresponds to a phase plate or an optical filter.

[0050] In the zoom lens of the embodiment, when zooming is performed from the wide angle end to the telephoto end, as indicated by the arrows shown in FIG. 1 , the first lens unit L 1 and the third lens unit L 3 are moved in the same way towards an object side, the fourth lens unit L 4 is moved so as to describe a convex path towards the object side, and the second lens unit L 2 is moved so as to describe a convex path towards an image plane side. As a result, at the telephoto end, the first lens unit L 1 and the third lens unit L 3 are disposed closer to the object side than to the wide angle end. On the other hand, at the telephoto end, the second lens unit L 2 is disposed closer to an image side than to the wide angle end.

[0051] A rear focusing system that performs focusing by moving the fourth lens unit L 4 on the optical axis is used. As shown by arrow 4 c , the fourth lens unit L 4 is moved forward towards the object side in order to change the focusing object distance from infinite distance to a close distance at the telephoto end. Solid curved line 4 a and dotted curved line 4 b for the fourth lens unit L 4 represent the paths of movement for correcting changes in the image plane IP caused by changes in magnification from the wide angle end to the telephoto end when an object at infinity and a nearby object are in focus, respectively.

[0052] In the embodiment, by moving the third lens unit L 3 towards the object side when performing zooming, the third lens unit L 3 is caused to provide the main magnification change effect. Further, by moving the first lens unit L 1 having a positive refractive power towards the object side, the second lens unit L 2 is also caused to provide a magnification change effect, so that a magnification change ratio of the order of 5 can be easily obtained without making the refractive powers of the first lens unit L 1 and the second lens unit L 2 very large.

[0053] In the embodiment, by providing a diffraction optical surface having a rotationally symmetrical grating structure with respect to an optical axis of one or more of the four lens units, and by properly setting the phase thereof, chromatic aberration that occurs at one or more of the lens units including a diffraction optical surface is reduced, whereby chromatic aberration is properly corrected over the entire magnification change range.

[0054] In the embodiment, the first lens unit L 1 having a large diameter is formed so as to consist of a single lens in order to make it light, thereby reducing the load of an actuator used to move the first lens unit L 1 . The third lens unit L 3 is formed so as to include, from the object side, two positive lenses and, at the image plane side, a negative lens having a highly concave surface. By making the principal point distance resulting from the combination of the second lens unit L 2 and the third lens unit L 3 small, the lengths of the lenses following those of the third lens unit L 3 are made small.

[0055] In order to correct changes in spherical aberration and other aberrations caused by focusing, the fourth lens unit L 4 is formed so that it includes an aspherical surface. In the first and second numerical examples, by forming the second lens unit L 2 so that it includes a diffraction optical surface, chromatic aberration that occurs at the second lens unit L 2 is reduced although it has a two-lens structure. Therefore, chromatic aberration is properly corrected over the entire magnification change range.

[0056] In the third numerical example, by forming the first lens unit L 1 so that it includes a diffraction optical surface, an even higher magnification change ratio of the order of 6 is achieved even though the first lens unit L 1 consists of a single lens.

[0057] It is advisable to form the diffraction optical surface of the first lens unit so that it has positive refractive power in order for the diffraction optical surface to possess a chromatic aberration correction effect.

[0058] Forming the base of the diffraction optical surface of the first lens unit L 1 into an aspherical surface is effective in correcting distortion at the wide angle end and spherical aberration and astigmatism at the telephoto end.

[0059] In the fourth numerical example, by forming the third lens unit L 3 so that it includes a diffraction optical surface, chromatic aberration is reduced while the refractive power of the third lens unit L 3 is increased, thereby reducing the size of the lens system.

[0060] By virtue of the above-described structure, there is realized a zoom lens which is suitable for use in video cameras, electronic still cameras, and cameras for silver-salt photography, which has a high magnification change ratio of the order equal to or greater than 5, an f-number of the order equal to or greater than 2.8, and a large aperture, and which is reduced in size while good optical performance is maintained.

[0061] The diffraction optical surface in the embodiment may be fabricated by a lithography technique used to produce a holographic optical element (HOE) or by binary optics used to produce an optical element by a binary technique. In these cases, in order to increase diffraction efficiency, it is desirable to form the diffraction optical surface into a saw-tooth-shaped surface. The diffraction optical surface may also be fabricated by molding using a die produced by either one of these methods.

[0062] Phase distribution φ(h) of the diffraction optical surface, which is described in the embodiment, is determined by the following formula:

φ( h )=2π/λ( C 2 ·h ² +C 4 ·h ⁴ + . . . C 2 I·h ²ⁱ )

[0063] where λ is the reference wavelength (d line), h is the distance from the optical axis, and C2i is the phase coefficient of the (2i) order term.

[0064] When the diffraction optical surface is formed so as to have one layer, sufficient diffraction efficiency can be obtained at wavelengths near a particular wavelength. Accordingly, in order to improve diffraction efficiency when what is called secondary spectrum becomes large with respect to wavelengths other that those near a particular wavelength, it is advisable to form the diffraction optical element into one having a layered structure that is formed by combining at least two diffraction gratings, as disclosed in Japanese Patent Laid-Open No. 10-133149. By forming the diffraction optical element into one having a layered structure including diffraction gratings, a high design-order diffraction efficiency equal to or greater than 95% over the entire wavelength range used is obtained.

[0065] The diffraction optical element having a layered structure may be formed of UV curable resin, plastic, or the like. Depending on the base material, it may be formed on a first direct base material. The thicknesses of the diffraction gratings do not have to be different. By combining materials, the thicknesses of the diffraction gratings can be made equal to each other.

[0066] In order for the zoom lens of the embodiment to have high optical performance in the entire magnification change range by further restricting changes in aberrations, caused by changes in magnification, at least one of the following conditions only need to be satisfied.

[0067] (Condition A)

[0068] When transverse magnifications at the wide angle end and the telephoto end of the third lens unit L 3 with respect to an object at infinity are β3w and β3t, respectively, and the focal lengths of the entire system at the wide angle end and the telephoto end are fw and ft, respectively, the following Condition (1) is satisfied:

0.4<(β3 t·fw )/(β3 w·ft )<0.9 (1)

[0069] Condition (1) is primarily provided in order not to make the sensitivities of the first lens unit L 1 and the second lens unit L 2 very large with the size of the entire lens system being reduced.

[0070] When the (β3t·fw)/(β3w·ft) value becomes less than the lower limit in Condition (1), so that the contribution of the third lens unit L 3 to changing magnification becomes small, it is necessary to increase the refractive powers of the first lens unit L 1 and the second lens unit L 2 in order to maintain the magnification change ratio of the entire system. Therefore, the sensitivities 5 become large, thereby increasing the effects of manufacturing errors. On the other hand, when the (β3t·fw)/(β3w·ft) value exceeds the upper limit in Condition (1), the amount of movement of the third lens unit L 3 resulting from changes in magnification becomes large. This is not good because the overall length at the wide angle end becomes large.

[0071] (Condition B)

[0072] When the amount of movement of the first lens unit L 1 and the amount of movement of the third lens unit L 3 , which are required for changing magnification from the wide angle end to the telephoto end, are m1 and m3, respectively, the following Condition (2) is satisfied:

0.35 <m 1 /m 3<0.9 (2)

[0073] When the m1/m3 value becomes less than the lower limit in Condition (2), so that the amount of movement of the first lens unit L 1 becomes too small, the amount of movement of the third lens unit L 3 required to change magnification becomes large, thereby causing the overall length at the wide angle end to become large. On the other hand, when the m1/m3 value exceeds the upper limit, so that the amount of movement of the first lens unit L 1 becomes large, the overall length at the telephoto end becomes too large. This is not good because, when the lenses are to be constructed so as to be collapsibly mountable, an increase in the number of rows that need to be collapsibly mounted and other factors affect the lens-barrel structure.

[0074] (Condition C)

[0075] When the focal length of the second lens unit L 2 is f2:

0.6 <|f 2/{square root}{square root over (( fw·ft ))}|<1.2 (3)

[0076] is satisfied.

[0077] In general, when the refractive power of the second lens unit L 2 is not properly set, the amount of movement of the second lens unit L 2 becomes too large, the performance thereof is decreased due to the effects of manufacturing errors caused by the sensitivity becoming too high, and jittering of an image becomes large when magnification changes are performed.

[0078] Condition (3) is provided to properly set the refractive power of the second lens unit L 2 , and to obtain a predetermined magnification change ratio with changes in aberrations caused by magnification changes being reduced.

[0079] When the |f2/{square root}{square root over ((fw·ft))}| value becomes less than the lower limit in Condition (3), so that the refractive power of the second lens unit L 2 becomes too small, the optical performance is deteriorated due to the effects of manufacturing errors, and jittering of an image becomes large when magnification changes are performed. On the other hand, when the |f2/{square root}{square root over ((fw·ft))}| value exceeds the upper limit in Condition (3), the amount of movement of the second lens unit L 2 becomes too large, so that the overall length cannot be reduced.

[0080] (Condition D)

[0081] When the second lens unit L 2 comprises a negative lens and a positive lens, and when the radius of curvature of an object-side lens surface and the radius of curvature of an image-side lens surface of the negative lens in the second lens unit L 2 (when aspherical surfaces are used, these are reference spherical surfaces determined by axial and effective diameters) are Ra and Rb, respectively, the following Condition (4) is satisfied:

−1.5<( Rb+Ra )/( Rb−Ra )<0.7 (4)

[0082] When the (Rb+Ra)/(Rb−Ra) value becomes less than the lower limit in Condition (4), distortion that occurs at the wide angle end becomes too negative. On the other hand, when the (Rb+Ra)/(Rb−Ra) value exceeds the upper limit, distortion at the telephoto end cannot be completely corrected.

[0083] (Condition E)

[0084] When the second lens unit L 2 comprises a diffraction optical surface, and when the second-order-term coefficient and the fourth-order-term coefficient of the diffraction optical surface of the second lens unit L 2 are C22 and C24, respectively, and one-half of the effective diameter of the diffraction optical surface is H, the following Condition (5) is satisfied:

5.0×10 ⁻³ <|C 22 ·H ² +C 24 ·H ⁴ <5.0×10 ⁻² (5)

[0085] When the |C22·H ² +C24·H ⁴ | value becomes less than the lower limit in Condition (5), chromatic aberration at the second lens unit L 2 is not sufficiently corrected, so that chromatic aberration of magnification at the wide angle end is not sufficiently corrected. On the other hand, when the |C22·H ² +C24·H ⁴ | value exceeds the upper limit in Condition (5), chromatic aberration is corrected excessively.

[0086] (Condition F)

[0087] Focusing of an object at finite distance is performed using the fourth lens unit L 4 .

[0088] (Condition G)

[0089] The first lens unit L 1 includes a diffraction optical surface.

[0090] (Condition H)

[0091] The third lens unit L 3 includes a diffraction optical surface.

[0092] (Condition I)

[0093] The diffraction optical surface has a layered structure including two or more layers.

[0094] (Condition J)

[0095] The second lens unit L 2 comprises two lenses, that is, a negative lens and a positive meniscus lens, and at least one surface is formed into a diffraction optical surface, with its phase properly set.

[0096] According to this structure, chromatic aberration that occurs at the second lens unit L2 is reduced, and chromatic aberration is easily properly corrected over the entire magnification change range.

[0097] When an attempt is made to correct chromatic aberration by forming only a refractive surface without the use of a diffraction optical surface, the refractive powers of the positive and negative lenses become large due to achromatization, so that it is difficult to construct the second lens unit L 2 with a small number of lenses, such as a single positive lens and a single negative lens, while the refractive powers are maintained.

[0098] In order to cause the diffraction optical surface to take over part of the achromatization by the second lens unit L 2 , it is desirable that the optical power of the diffraction optical surface be negative. When the optical power of the diffraction optical surface is positive, the direction in which chromatic aberration occurs (see symbol) is the same as that of an ordinary refractive optical system, so that an achromatizing effect is not produced at the diffraction optical surface, thereby preventing sufficient correction of chromatic aberration over the entire optical system.

[0099] (Condition K)

[0100] It is preferable that the second lens unit L 2 include an aspherical surface which is shaped so that the negative optical power thereof becomes weaker towards a peripheral portion of the surface.

[0101] By virtue of this structure, it is possible to effectively correct curvature of field and distortion that occur at the wide angle end due to a reduction in the number of lenses used. In the first and second numerical examples, the base of the diffraction optical surface is formed into an aspherical surface.

[0102] (Condition L)

[0103] It is preferable that the second lens unit comprise two lenses, which are, in order of lens units from the object side, a negative lens and a positive lens.

[0104] According to this structure, the location of an entrance pupil at the wide angle end can be brought towards a first surface, thereby making it possible to reduce the diameter of the first lens unit L 1 .

[0105] In order to properly correct astigmatism and distortion at the wide angle end, it is preferable to form the second lens unit L 2 so as to include, in order of lens units from the object side, a negative lens and a positive lens, the negative lens including an image-side surface having a higher optical power than the object-side surface and the positive lens including an object-side surface, which is a convex surface, having a higher optical power than the image-side surface.

[0106] An embodiment of a video camera (optical apparatus) using any one of the zoom lenses of the first to fourth numerical examples as a photographic optical system will be described with reference to FIG. 18 .

[0107] In FIG. 18 , reference numeral 10 denotes the body of the video camera, reference numeral 11 denotes a photographic optical system comprising any one of the zoom lenses of the first to fourth numerical examples, reference numeral 12 denotes an image pickup device, such as a charge-coupled device (CCD) or a CMOS, that receives a subject image, reference numeral 13 denotes recording means that records the subject image that has been received by the image pickup device 12 , and reference numeral 14 denotes a finder for observing the subject image displayed on a display device. The display device comprises, for example, a liquid crystal panel, and displays the subject image formed on the image-pickup device 12 .

[0108] In this way, by applying the zoom lens of the present invention to an optical apparatus, such as a video camera, a small optical apparatus having high optical performance is realized.

[0109] The data of the first to fourth numerical examples will be given. In the numerical examples, ri is the radius of curvature of an ith surface in order of surfaces from the object side, di is the thickness of an ith optical member in order of optical members from the object side or an air gap, and ni and vi denote, respectively, the refractive index and the Abbe constant of an ith optical member in order of optical members from the object side. The relationship between each of the aforementioned conditions and each of the numerical examples is given in Table 1.

[0110] In each of the numerical examples, a certain number of surfaces at the image side that do not possess refractive powers are parallel plates that correspond to optical filters or phase plates.

[0111] When the X-axis extends in the direction of the optical axis, the H-axis extends in a direction perpendicular to the optical axis, the direction of propagation of light is positive, R represents the paraxial radius of curvature, and k, B, C, and D each represent aspherical coefficients, the aspherical shape is defined by the following Formula (4): 1 $X=\frac{\left(1/R\right)H^2}{1+\sqrt{1-\left(1+k\right){\left(H/R\right)}^2}+{\mathrm{BH}}^4+{\mathrm{CH}}^6+{\mathrm{DH}}^8}$

[0112] D-ox represents x10 ^−x .

NUMERICAL EXAMPLE 1

[0113] 1

NUMERICAL EXAMPLE 2

[0114] 2

NUMERICAL EXAMPLE 3

[0115] 3

NUMERICAL EXAMPLE 4

[0116] 4

[0117] 5

[0118] It is possible to realize a zoom lens which comprises a diffraction optical surface in a portion of an optical system in order to reduce chromatic aberration that occurs in each lens unit by combining diffractive optical action and achromatization effect of a diffractive system, which makes it possible to reduce the overall length by reducing the number of lenses used while the refractive power at a magnification change portion is maintained, and which possesses good optical performance over the entire magnification change range from a wide angle and to a telephoto end. An optical apparatus using the zoom lens can also be realized.

[0119] It is possible to obtain a zoom lens that has reduced overall length while excellent optical performance is maintained by forming a diffraction optical surface that is rotationally symmetrical to at least one optical axis in this zoom lens which comprises four lens units, which are a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power. In the zoom lens, magnification changes are performed by moving the first and third lens units towards the object side, and the first lens unit consists of a single lens.

[0120] While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.