JPH03225307A - Rear focus zoom lens - Google Patents

Abstract

PURPOSE:To obtain the compact rear focus zoom lens with high variable power by composing the zoom lens of a front group which has positive refracting power on the whole and a rear group which has negative refracting power on the whole, and moving the rearmost lens which in the rear group to the object side for zooming. CONSTITUTION:This zoom lens has the front group which has the positive refracting power on the whole and consists of lens groups I, II, and III and the rear group which consists of a lens group IV with positive refracting power and a lens group V with negative refracting power in order from the object side. When the zoom lens is put in zooming operation from the wide-angle end to the telephoto side, the lens interval is varied so as to increase the image power of the front group and the lens interval between the lens groups IV and V is varied so as to increase the image power of the rear group, thereby performing high-power zooming. Then the lens is put in focus by moving the lens group IV and an inequality holes for the lateral power beta4 of the lens. Consequently, the rear focus zoom lens which is compact and has a high power variation ratio and excellent optical performance over a wise range to a short- distant body is obtained.

Description

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

[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 increase the diameter in order to prevent vignetting of the light beam when focusing on objects at close range. In addition, in recent years, autofocus cameras that use a motor to automatically move the focusing lens to perform focusing have become mainstream, but the middle weight of the photographic lens also moves the heavy front focusing lens group. This is disadvantageous in terms of motor energy consumption and focusing 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 front lens. for example,
JP-A-59-33418, JP-A-59-5221
5-, Japanese Patent Application Laid-open 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 lens is the same in all areas, the amount of extension is large (particularly at the telephoto end), and it was not sufficiently compact.

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 (as expected, sufficient miniaturization has not been seen).

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, making it difficult to move each lens group during autofocus. When moving, it is difficult to quickly focus.

[Means for solving problems]

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

In the rear focus zoom of the present invention, the front group (first, second, and third lens groups) has a plurality of lenses in order from the object side and has positive refractive power as a whole; and a rear group (fourth and fifth lens groups) which includes lenses with negative refractive power and has negative refractive power as a whole, and when zooming from the wide-angle end to the telephoto side, the front group Further, the lens spacing is changed so that the image magnification is multiplied, and the lens spacing of the lenses having positive refractive power and negative refractive power is changed so that the image magnification of the rear group is multiplied. When zooming is performed and focusing is performed by moving the lens having positive refractive power, and the magnification of the positive lens is β4, the conditional expression β41<l...(1) is obtained. It is characterized by satisfaction.

〔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 refraction 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 group, which consists of the 11th second and third lens groups and has positive refractive power as a whole, increases, and also the lateral magnification of the third and fourth lens groups increases. The air distance between each lens group is changed so that the lateral magnification of the entire rear group, which has a negative refractive power as a whole, increases, thereby effectively 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 close object is performed by moving the fourth lens group immediately after the convergent beam emitted from the front group toward the object side as shown by the straight solid line. There is. In addition, the zoom lens of this example is
When zooming from the wide-angle side to the telephoto side, the third lens group is moved toward the object side in order to increase the zoom ratio.
During focusing, the distance between the lenses created by the third and fourth lens groups on the telephoto side is used to allow the fourth lens group to fit into this space, making it possible to downsize the entire photographic lens and focus on close objects. The lens movement is designed to allow for sufficient focusing even when compared to other lenses.

In the present invention, by satisfying conditional expression (1), the fourth lens group as a focus lens is set on the object side in order to perform quick and accurate focusing when focusing from a distant object to a close object. I'm trying to move it to. Further, in this embodiment, the entire lens system is made smaller by moving to the space behind the third lens group side, that is, toward the object side.

Next, further preferred conditions for achieving the object of the present invention will be shown.

That is, when zooming from the wide-angle side to the telephoto side, the amount of movement of the fifth lens having negative refractive power is M5 (however, the direction from the object side to the image side is positive), and the focal length is f6.
, the focal length of the entire system at the telephoto end is fT, and the zoom ratio is Z
When, M5〈0...(2)0.0
7<l f51 /f□<0.45...
(3) 0.095< l M51/(Z-1f51)
<0.5... (4) This is the conditional expression.

By the way, it is generally necessary to consider the following points as conditions for realizing rear focus in a zoom lens.

(a) There is no point where the magnification β2 of the focus lens group becomes βF]−1 at each focal length.

(b) The sensitivity ESF of the focus lens group has an appropriate value.

etc. First, the meaning of condition (a) is 1β.

When =1, the focus position does not change even if the focus lens is moved, making focusing impossible. Furthermore, when 1βF1>1, the focus lens is moved toward the image side when focusing from infinity to close range, and when iβF1>1, the opposite is true. Regarding condition (b), if the sensitivity ESF is too small, the focus position will only move a little even if the focus lens is moved a lot, so the focusing time will be longer, or the total length of the lens will be longer to secure the movement space of the focus lens. Increased and undesirable. On the other hand, if the sensitivity ESF is too large, the focus position will change greatly even if the focus lens is moved a little, making it extremely difficult to control the focus lens position, which is impractical. In addition, when zooming from the wide-angle end to the telephoto end, if the ESF increases in proportion to the square of the zoom ratio, the amount of extension for the same object distance will be constant at each focal length, but it will change at the wide-angle end and the telephoto end. It is necessary to provide a space to secure the amount of extension, which increases the size of the lens system and reduces the degree of freedom in design.

Here, the sensitivity of the fifth lens group V is E5, and the focal length is f.
5. The sensitivity of the 4th lens group is E4, the lateral magnification is β4, the 5th
When the distance from the rear principal point of the lens group to the image plane is e5, E4=(1-β4')・β5'
...(A) e5 β5-1-1 ...(B
)5 The following formula holds true.

Here, in the present invention, since e5 takes a positive value and f5 takes a negative value, it can be seen from equation (B) that the lateral magnification β5 of the fifth lens group always takes a value of 1 or more. And conditional expression (2)
However, since the distance e5 from the image plane increases when the fifth lens group zooms from the wide-angle side to the telephoto side,
From equation (B), the lateral magnification β5 of the fifth lens group also increases. Therefore, from equation (A), the sensitivity E4 of the fourth lens group increases with zooming from the wide-angle side to the telephoto side.

In other words, conditional expression (2) means that an appropriate amount of focus movement is given on the wide-angle side, and that the amount of focus movement is appropriately suppressed on the telephoto side.

Then, by setting the focal length of the fifth lens group according to conditional expression (3) and giving a suitable focus sensitivity E4 of the fourth lens group, it is possible to improve controllability, especially when adjusting the lens position with autofocus. And what's more, it allows for quick autofocus.

In other words, if the focal length of the fifth lens group becomes too short exceeding the upper limit of conditional expression (3), the lateral magnification β5 of the fifth lens group becomes too large, as is clear from equation (B), and (A)
According to the equation, as the focus sensitivity E4 of the fourth lens group increases, the amount of focusing movement decreases, but it becomes difficult to control the lens position by autofocus.

On the other hand, if the upper limit of conditional expression (3) is exceeded and the focal length of the fifth lens group becomes too long, as is clear from equations (A) and (B), the focus sensitivity E4 becomes too small and the lens Although positional controllability is improved, space must be secured especially at the wide-angle end to move the focus, resulting in an increase in the overall length of the lens. Furthermore, the amount of focus movement is large, which is undesirable because autofocus lacks quickness.

Conditional expression (4) is the focus sensitivity, which is the fourth
This is a condition for setting the sensitivity E4 of the lens group to an appropriate level. If the lower limit of conditional expression (4) is exceeded and the amount of movement of the fifth lens group becomes small compared to the product of the zoom ratio and the focal length of the fifth lens group (then the focus sensitivity of the fourth lens group becomes small, In particular, the amount of focus movement on the telephoto side becomes too large, resulting in an increase in the size of the lens system and a lack of speed, which is also undesirable.

On the other hand, if the amount of movement of the fifth lens group becomes too large beyond the upper limit, the amount of focus movement becomes too small, and the controllability of the lens position similarly deteriorates.

Furthermore, in the present invention, it is desirable to satisfy the following conditional expression.

0.5<1f51/e5w<1.4...(5)0.3
5 <β4. /(β4w-Z) < 0.95
- (6) However, esw: Principal point interval β4w between the fifth lens group and the image plane at the wide-angle end: Lateral magnification β4T of the fourth lens group at the wide-angle end:
This is the lateral magnification of the fourth lens group at the telephoto end.

Conditional expression (5) is a conditional expression for setting the sensitivity of the fourth lens group to an appropriate level at the wide-angle end, and the size of esw is almost determined by the back focus condition. Therefore, if the focal length of the fifth lens group exceeds the upper limit of conditional expression (5), the lateral magnification of the fifth lens group becomes small, and therefore the sensitivity of the fourth lens group becomes small. It is necessary to provide a large space for focusing at the wide-angle end, which increases the overall length of the lens.

When the focal length of the fifth lens group becomes short by exceeding the lower limit of conditional expression (5), the lateral magnification of the fifth lens group becomes thick (and therefore, the sensitivity of the fourth lens group becomes thick). This is unfavorable in terms of aberration correction, and particularly causes a large string-shaped distortion.

Conditional expression (6) is for setting the sensitivity ratio between the telephoto end and the wide-angle end of the fourth lens group to an appropriate value.If the upper limit of conditional expression (6) is exceeded, the fourth lens group The ratio of sensitivity between the wide-angle end and the telephoto end of the group becomes small, that is, the difference in the amount of extension for the same object between the wide-angle end and the telephoto end becomes too large.

If the lower limit of conditional expression (6) is exceeded, the ratio of the sensitivity at the wide-angle end and the telephoto end becomes too large, which means that the focus space needs to be widened at the wide-angle end, or the sensitivity becomes too large at the telephoto end. This is undesirable because it becomes too much and mechanical control becomes difficult.

Next, in the zoom lens of this embodiment, which has a five-group configuration, the focal lengths at the wide-angle end and telephoto end of the entire system are fw and f, respectively.
T, the focal length of the fifth lens group is f+, and the combined focal length of the lens group and the second lens group at the wide-angle end is f12
W % When the amount of change in principal point spacing at the telephoto end relative to the wide-angle end of the third and fourth lens groups is Δe34, and the length from the first lens surface to the final lens surface of the entire system at the wide-angle end is Dw. 0.05f completed/fw<Δe 34/
D w < 0, 15 f T / f w...
(7) 3.5<f+2w/fw<-1,3-(8)0.05<
fs/f+2w<0.6...+ (9)
0.35<f3/f4<2.5...
The conditional expression (lO) 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 (7) defines the amount of change in the distance between the principal points of 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 (7
), the third lens group and the fourth lens group at the telephoto end exceed the upper limit of
If the principal point distance between the lens group 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 with each other at the wide-angle end, and as a result, the overall length of the lens at the wide-angle end becomes thick. Undesirable.

Also, if the lower limit of conditional expression (7) 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. This is not desirable.

Conditional expression (8) 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 (8), the distortion tends to decrease. It becomes necessary to widen the distance between the five lens groups at the wide-angle end, and the back focus at the wide-angle end also becomes longer than necessary, making it difficult to make the entire lens system compact.

Furthermore, if the lower limit of conditional expression (8) is exceeded and the negative refractive power of the fifth lens group becomes too strong, 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 (9) 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 combined negative refractive power of the lens group and the second lens group exceeds the upper limit of conditional expression (9), the negative refractive power of the lens group and the second lens group becomes too weak. the 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. Moreover, it is necessary to strengthen the negative refractive power of the fifth lens group, which results in an increase in thread-wound distortion.

Furthermore, if the lower limit of conditional expression (9) 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 causing 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 mirror becomes large.

Conditional expression (lO) is the third value for the focal length of the fourth lens group.
This defines the ratio of focal lengths of lens groups. If the upper limit of conditional expression (10) 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 satisfactorily correct various aberrations.

Also, the refractive power of the third lens group is strong beyond the lower limit of conditional expression (lO) (if it becomes too strong, aberrations will occur in the third lens group,
In particular, spherical aberration becomes large, and it becomes difficult to balance (correct) it with other lens groups.

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 υi are the glass refractive index and Atsube number of the i-th lens, respectively, in order from the object side.

Furthermore, Table 1 shows the relationship between each of the above-mentioned conditional expressions and the word values in the numerical examples.

Table 1 [Effects] As explained 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 zoom lens aberrations related 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, and (A)-2, (B)-2, and (C)-2 are the wide-angle end, intermediate, and telephoto when focusing at an object distance of 3 m. Diagrams of various aberrations at the edges 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. ■ ■ ■ ■ ■ ■ ■ ■ '' ■ ■ ■ ■ ■ Fig. 4 FNo/4(,.K/l-)/86' W2//J(supersine sequence FNo/4π to/= /2'4 'W Near 24 @ Sine Kano FNo/Ni, / 11J=752'W=7n' First Singing Case No. 4 Figure FNo/ 5.14 V2243+ W2843+ One θ40 Match?t1 Shukame Kamijou Kano Hai 2427' -040 Spherical convergence sine convergence θ, 4σ 10も0 hai p4 θ, 4゜koto f!l7 (-≦) Fig. 4 FNo/ 5,74 W=655″ W2, s, ss' 1040 Ball fI collection sine dead leaf θ, 4゜10ヂ0 Haibi, d θ, 4-0 Tsujizu slope'ni i;ノ!, Del FNo/4otvl/=8-/1Pa W2//,?' 1040 Ball #J collection Wax sine series 0.40 HaiQx difference Glue -3θθ Rii Tsuru Iko5≦No Fig. 5 F No / 4.74 VJ= /2.r” W2/23' -64ρ Kimono harvest'iI: Aberration 1 6) Fig. 5 F/Io/ 5゜/ v = 'zg W near 4'' Sine condition FNo/S, ff'' ``E string 11J = 8.4゜W2 2.4 FNo/, 5.7 w=4.2'. W24,2゜10,40 Ball #J collection sine rabbit leaf θ140 i, ヂQ induced aberration θ, 40 -3θO 1 會σρ, o. F No./5. lriv=,ss'W26. S' Upper string row FNo./4 v=, 7' W 2, 7m -0,9 pitch collection 1-string instrument θ, 4° Haitou collection σ, 40 -3, θO Kushizu tier (Moxibustion ≦) FNo/ 4 IAI=I71'W2/7/' Jōgen Koji FNo/6 14/= g, ri 0 w=g, y' 10, 40 KōgJ Shukajō Genfox Kano Hai, d θ, 40 raw + d17↓J! , p. FNo/ 1 v='zs' W=first C Upper string leaf FNo/ 5.7 Stop string rabbit leaf/= 6゜W2 radius [second; FNo/-567 11/=6. f (1 - IIQ - figure?) W = 5, 2' θ, 4L17 -to. Kutozu Town'(7;)

Claims (1)

[Scope of Claims] (1) A front group having a plurality of lenses in order from the object side and having positive refractive power as a whole, including a lens having positive refractive power and a lens having negative refractive power in order; and a rear group having negative refractive power, and when zooming from the wide-angle end to the telephoto side, the lens interval is changed so that the image magnification of the front group is multiplied, and the image magnification of the rear group is multiplied. The distance between the lenses having the positive refractive power and the negative refractive power was changed so that the focusing was performed by moving the lens having the positive refractive power, and the lateral magnification of the positive lens was set to β_4. A rear focus zoom lens that satisfies the following conditional expression: |β_4|<1. (2) The rear focus zoom lens according to claim 1, wherein the zooming is performed by moving the rearmost lens of the front group at least toward the object side. (3) The amount of movement of the lens having negative refractive power during zooming from the wide-angle side to the telephoto side is M_5 (however, the direction from the object side to the image side is positive). When the focal length of the lens is f_5, the focal length of the entire system at the telephoto end is f_T, and the zoom ratio is Z, then M_5<0 0.07<|f_5|/f_T<0.45 0.095<|M_5| /(Z・|f_5|)<0.5
A rear focus zoom lens according to claim 1 or 2, characterized in that the following conditional expression is satisfied: