JPH03225310A - Rear focus zoom lens - Google Patents

Abstract

PURPOSE:To obtain the rear focus zoom lens which has a high power variation ratio and is compact by providing a front group which has positive refracting power on the whole and a rear group which has negative refracting power on the whole in order from the object side, and varying lens intervals for zooming. CONSTITUTION:This rear focus lens is equipped with the front group composed of lens groups I, II, and III having the positive refracting power on the whole and the rear group composed of lens groups IV and V which include a positive and a negative lens in order and have the negative refracting power on the whole in order from the object side. This lens is put in zooming operation from the wide-angle end to the telephoto end by varying lens intervals so as to increase the image power of the front group and varying the interval between the positive lens IV and negative lens V so as to increase the image power of the rear group, and this zoom lens is put in focus by moving the negative lens V. Consequently, the rear focus lens which is compact and has the high power variation ratio and excellent optical performance over a wide range up to a short-distant body can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a zoom lens suitable for 35 mm film cameras, video cameras, etc., and in particular, a zoom lens that is compact, has a high zoom ratio, and uses a rear focus system. This invention relates to a rear focus zoom lens that has good optical performance over a wide range of objects.

[Prior art]

Traditionally, the focusing method for zoom lenses is
This is generally done by moving the lens group that makes up the lens closest to the object, and the mechanical structure of the lens barrel is relatively simple because the amount of extension of the focus lens relative to the same object is constant regardless of the focal length. It has the advantage of being

However, it is necessary to make the image large in order to prevent vignetting of the light beam when focusing on a close object. In addition, in recent years, autofocus cameras that use a motor to automatically move the focusing lens to perform focusing have become mainstream, but it is also necessary to move the aforementioned focusing lens group, which is heavy in the middle of the photographic lens. This is disadvantageous in terms of motor energy consumption and focus speed.

In order to alleviate this problem, various so-called rear focus methods have been proposed in which focusing is performed by moving lenses other than the above-mentioned lenses. for example,
JP-A-59-33418, JP-A-59-5221
5, Japanese Unexamined Patent Application Publication No. 195618/1983, etc.

[Problem that the invention seeks to solve]

In Japanese Patent Application Laid-Open No. 59-33418, focusing is performed by moving a part of the fixed imaging lens during zooming, but the focus sensitivity E (focus movement amount/focus lens movement amount) is Since the distance is the same in all areas, the amount of extension becomes large, especially at the telephoto end, and it is not sufficient in terms of compactness.

Japanese Unexamined Patent Publication No. 59-52215 discloses a zoom lens having a four-group structure, and focusing is performed by moving the third lens group toward the relay lens group. However, the overall length is relatively long, and sufficient miniaturization has not been achieved.

JP-A-63-195618 discloses a standard rear focus zoom lens that includes a wide-angle range, and also has a large number of movable lens groups during focusing, so that each lens group can be moved during autofocus. If you move the
It is difficult to focus quickly.

[Means for solving problems]

In view of these problems, the present invention is designed to be compact when using a rear focus method in a telephoto zoom lens with a high variable power ratio, and to take into account lens controllability of the focus lens when applied to an autofocus camera. The purpose of the present invention is to provide a rear focus zoom lens that allows quick focusing.

For this purpose, the zoom lens of the present invention includes, in order from the object side, a front group (11th, 2nd, and 3rd lens groups) that has a plurality of lenses and has positive refractive power as a whole, and a positive lens and a negative lens in that order. It has a rear group (fourth and fifth lens groups) that includes a lens and has a negative refractive power as a whole, and zooming from the wide-angle end to the telephoto end is controlled by the image magnification b of the front group (the lens spacing increases). In addition, the distance between the positive lens and the negative lens is changed so that the image magnification of the rear group is increased, and focusing is performed by moving the negative lens.

〔Example〕

Hereinafter, the present invention will be explained in detail based on the drawings.

1 to 3 are cross-sectional views of lenses of numerical examples 1 to 3 related to the present invention.

In order from the object side, ■ is the Luns group with positive refractive power;
■ is the second lens group with negative refractive power, ■ is the third lens group with positive refractive power, ■ is the fourth lens group with positive refractive power, and ■ is the fifth lens group with negative refractive power. It is a group.

Under such a refractive power arrangement, the distance between the lenses of the second lens group I and the second lens group ■ is widened, and the distance between the lenses of the second lens group I and the third lens group The lens spacing of the lens group (2) is narrowed, the lens spacing of the third lens group (2) and the fourth lens group (2) is widened, and the lens spacing of the fourth lens group (2) and the fifth lens group V is narrowed. In other words, when zooming from the wide-angle end to the telephoto end, the lateral magnification of the entire front lens group, which consists of the first, second, and third lens groups and has positive refractive power as a whole, increases; The air distance between each lens group is changed so that the lateral magnification of the entire rear group, which is composed of four lens groups and has a negative refractive power as a whole, is increased, thereby efficiently increasing the variable power ratio. Furthermore, by making the refractive powers of the front group and rear group positive and negative, respectively, the lens is made more compact as a telephoto type.

In the embodiment of the present invention, focusing from an object at infinity to a nearby object is performed by the fifth point, as shown by a straight solid line.
The lens group is moved toward the image side to improve controllability and optical performance over a wide range.

Next, more preferable conditions for achieving the object of the present invention will be shown. That is, the focal length and lateral magnification of the fifth lens group are f
5, β5, the sensitivity of the 5th lens group at the telephoto end and the wide-angle end are respectively E 5T % E 5W % The focal length of the entire system at the telephoto end is fT, and the sensitivity of the 5th lens group when zooming from the wide-angle end to the telephoto end is When the amount of movement is M5 (however, the direction from the object side to the image plane side is positive) and the zooming is Z, 0.07< If51 /f, <0.45
・ (2) 0.095< 1M51/(Z-lfs
l ) < 0.5 (3).

Next, the conditional expression will be explained.

Conditional expression (1) defines the ratio of the amount of feed to the same object at the wide-angle end and the telephoto end, and exceeding the lower limit of conditional expression (1) indicates whether the sensitivity at the wide-angle end is too large. , which means that the sensitivity at the telephoto end is too small,
In the former case, it becomes difficult to mechanically control the focus group, and in the latter case, the amount of extension at the telephoto end becomes too large, which is not preferable. Exceeding the upper limit of conditional expression (1) means that the sensitivity at the wide-angle end is too small or the sensitivity at the telephoto end is too large. In the former case, the space for focusing is widened at the wide-angle end. In the latter case, the overall length of the lens becomes longer, and in the latter case, the performance deterioration due to manufacturing errors increases (which is not desirable. Therefore, by setting Eya and E6 as in conditional expression (1), the amount of extension for the same object can be increased. is made smaller at the wide-angle end,
By making it larger at the telephoto end, there is less space for focusing at the wide-angle end, and
The overall length of the lens at the wide-angle end is shortened. On the other hand, at the telephoto end, focusing can be performed using the space expanded by zooming, making it possible to make the overall length of the lens more compact.

Conditional expression (2) is a conditional expression for setting the sensitivity of the fifth lens group to an appropriate level. If the lower limit of conditional expression (2) is exceeded and the focal length of the fifth lens group becomes shorter than the focal length of the entire system at the telephoto end, it is advantageous to make the overall lens length compact; The sensitivity of the lens becomes too large, which is undesirable in terms of manufacturing and control of the focus lens, and if the upper limit of (2) is exceeded and the focal length of the fifth lens group becomes longer than the focal length of the entire system at the telephoto end. The overall length of the lens cannot be made sufficiently compact.

Conditional expression (3) is based on conditional expression (2) and is intended to set the ratio of the sensitivity at the wide-angle end to the sensitivity at the telephoto end to an appropriate value, and is the lower limit of conditional expression (3). If the amount of movement of the fifth lens group exceeds , the amount of movement of the fifth lens group is small compared to the zoom ratio and the focal length of the fifth lens group. If the amount of movement is too small (too much), and if the amount of movement of the fifth lens group increases beyond the upper limit in (3), the amount of focus movement at the wide-angle end will become large (the total length of the lens will have to be increased to secure this space). It becomes long, which is not desirable.

Next, the zoom lens consisting of five groups of this embodiment has a focal length of fwsfT at the wide-angle end and telephoto end of the entire system.
The focal length of the first i-th lens group is f3, and the combined focal length of the first lens group and the second lens group at the wide-angle end is f12W.
N is the amount of change in principal point spacing at the telephoto end with respect to the wide-angle end of the third and fourth lens groups, and Δe34 is the length from the first lens surface to the final lens surface of the entire system at the wide-angle end.
When w, 0.05 f T / f w <Δe
34/D w < 0, 15 f T/f
w...(4) 3.5<f+zw/fw<-1,3-(5)0.05<
fs/ft2w<0.6 +++ (6
)0.35<f3/f4<2.5・
...The conditional expression (7) is satisfied.

In particular, in this example, in order to reduce the size of the entire lens system and to avoid an unnecessarily long back focus, the entire lens system is made not to have a very strong reverse telephoto type at the wide-angle end; The refractive power and paraxial arrangement of each lens group are set to provide a weak reverse telephoto type.

Next, the technical meaning of each of the above conditional expressions will be explained.

Conditional expression (4) defines the amount of change in the principal point distance between the third lens group and the fourth lens group when zooming from the wide-angle end to the telephoto end, with respect to the total lens length at the wide-angle end. . Conditional expression (4
), the third lens group and the fourth lens group at the telephoto end exceed the upper limit of
If the distance between the principal points and the lens group becomes too large, it becomes necessary to secure a large movement space for zooming so that the lens does not interfere at the wide-angle end, and as a result, the overall length of the lens at the wide-angle end becomes It gets bigger and I don't like it.

Also, if the lower limit of conditional expression (4) is exceeded, the third
If the distance between the principal points of the lens group and the fourth lens group becomes too small, the zoom strokes of the lens group and the fifth lens group must be increased in order to obtain the desired zoom ratio and shorten the total optical length at the wide-angle end. I have to (I don't like it.

Conditional expression (5) defines the ratio of the focal length of the fifth lens group to the combined focal length of the lens group and the second lens group at the wide-angle end. If the negative refractive power of the fifth lens group becomes too weak by exceeding the upper limit of conditional expression (5), the distortion will become small (in the direction of becoming small), but in order to obtain a predetermined zoom ratio, the fourth lens group It becomes necessary to widen the distance between the fifth lens group at the wide-angle end, and the back focus at the wide-angle end also becomes unnecessarily long, making it difficult to make the entire lens system compact.

Furthermore, if the lower limit of conditional expression (5) is exceeded and the negative refractive power of the fifth lens group becomes too strong, the thread-wound distortion becomes large and the Petzval sum increases in the negative direction, causing the image surface characteristics to deteriorate. This is undesirable because it becomes difficult to properly correct the difference.

Conditional expression (6) defines the ratio of the combined focal length of the first lens group and the second lens group at the wide-angle end to the focal length of the entire system at the wide-angle end, and is mainly used to compensate for aberration fluctuations due to zooming. This is to make the entire lens system more compact, especially by optimizing the length of the back focus.

If the upper limit of conditional expression (6) is exceeded and the combined negative refractive power of the lens group and the second lens group becomes too weak, it is necessary to reduce the outer diameter of the lenses of the lens groups after the third lens group and increase the aperture. Second
When placed closer to the image plane than the lens group, if it is advantageous to reduce the outer diameter of the lens barrel by reducing the aperture diameter, it is possible to secure the necessary back focus and make the entire lens system more compact. In addition, it is necessary to increase the negative refractive power of the fifth lens group, and as a result, thread-wound distortion becomes large.

Furthermore, if the lower limit of conditional expression (6) is exceeded and the combined negative refractive power of the first lens group and the second lens group becomes too strong, the back focus becomes too long, making the entire lens system larger and reducing various aberrations. Fluctuations, especially fluctuations in spherical aberration and field curvature, become large. Furthermore, the diameter of the aperture for securing the necessary F number also increases, which is undesirable because the outer diameter of the lens barrel becomes larger.

Conditional expression (7) defines the ratio of the focal length of the third lens group to the focal length of the fourth lens group.

If the upper limit of conditional expression (7) is exceeded and the positive refractive power of the third lens group becomes too weak, the distance between the fourth lens group and the fifth lens group at the wide-angle end must be adjusted to obtain a predetermined zoom ratio. It needs to be large, making it difficult to make the lens system more compact. Furthermore, in order to maintain the compactness of the lens system, the refractive power of the second lens group or the fifth lens group must be strengthened, and as a result, it becomes difficult to properly correct various aberrations.

Furthermore, if the lower limit of conditional expression (7) is exceeded and the refractive power of the third lens group becomes too strong, the aberrations generated in the third lens group, especially spherical aberrations, become thick (and this can be corrected by using other lens groups). Good balance (it becomes difficult to correct).

Next, numerical examples of the present invention will be shown. In numerical examples R
i is the radius of curvature of the i-th lens surface in order from the object side, D
i is the i-th lens thickness and air distance from the object side, Ni
and νi are the refractive index and Abbe number of the glass of the i-th lens, respectively, in order from the object side.

〔effect〕

As described above, according to the present invention, it is possible to provide a rear focus zoom lens that is compact, has a high variable power ratio, and has good optical performance over a wide range up to close objects.

[Brief explanation of drawings]

FIGS. 1 to 3 are diagrams of various aberrations of the zoom lens according to the present invention. In the aberration diagram (A) -1° (B) -1
, (C)-1 are various aberration diagrams at the wide-angle end, intermediate, and telephoto end, respectively, and (A)-2, (B)-2, and (C)-2 are the wide-angle end when focusing at an object distance of 3 m. Various aberration diagrams at the intermediate and telephoto end are shown. d is the d-line, g is the g-line, S and C are the sine conditions, ΔS is the sagittal image plane, ΔM is the meridional image plane, and y is the image height. ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ F No. ,5'w=/? , 5゜10. # 6 collection turtle 1 difference bone leaf θ, 44 (, φ0 Hai, collection σ, 9 1 tooth 5) B (7 粁′のl sine cloud case (sled 2) upper string rabbit leaf C (part 1) L string Eika No. 4 (Sotsu 2) Upper 31 橡 Kano FNo/4.6 ni/=llF' w=n, y' ?: String Tetsu case FNt/ 4.'7 11/= 1?5' W 2/2.5゜F/10/ 6./ W=75゜Sine solid W=75' FNo/ 5./ W-4,?'W24.2# Sine full FNo/ 6.g it/=4.2゜W=4.?'16# Hayashi/Electricity 'E Kashuha ρ144 10もO Hai P Collection σ, 4-0 1 meeting σ0 FNo/6.7 VJ = 5.5' W = 5.5' -0,9 Slope charge sine Eiyo 4# I, -O Haisai, aberration θ, 40 -30t) 1 meeting σ0 j, pO sword (so/) -ty, 4o mlN1 (1 & 0.40 -
DA) Hai Teishichi, Han ty, m -3, ao
1. ICF-no, f, o. Sine Rabbit Shibrow (Sotsudo) ``E Kaden Kano B (So/) Sine Bare Kano Diagram FNo/6 W= and 4' W2♂, 4' Kamikamori Kano FNo. 15.g v=42" Upper Gen Rabbit Part W=4y2' FNo/6.7 W=S, SゝW265' ″E string row part

Claims (1)

[Claims] (1) In order from the object side, there is a front group that includes a plurality of lenses and has a positive refractive power as a whole, and a rear group that includes a positive lens and a negative lens and has a negative refractive power as a whole. When zooming from the wide-angle end to the telephoto end, the lens interval is changed so that the image magnification of the front group increases, and the lens interval between the positive lens and the negative lens is changed so that the image magnification of the rear group increases. A rear focus zoom lens characterized in that focusing is performed by changing the negative lens. (2) The focal length and lateral magnification of the negative lens are f_
5, β_5, sensitivity at telephoto end and wide-angle end are each E_5
_T, E_5_W, the focal length of the entire system at the telephoto end is f
_T, the amount of movement of the negative lens when zooming from the wide-angle end to the telephoto end is M_5 (however, the direction from the object side to the image plane side is positive), and the zoom ratio is 0. 01<E_5_T/E_5_W・Z^2<0.70
．． 07<|f_5|/f_T<0.45 0.095<|M_5|/(Z・|f_5|)<0.5
A rear focus zoom lens according to claim 1, which satisfies the following conditional expression.