JPH0477291B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a zoom lens system, and more particularly to its focusing method. Conventionally, the most common focusing method for zoom lens systems is the so-called front lens extension method, in which a focusing lens group is placed closest to the object side of the lens system, and this focusing lens group is moved in the optical axis direction. . This method enables focusing by giving the focusing lens group almost the same amount of extension for the same shooting distance over the entire focal length range, so most zoom lens systems use this method. There is. however,
Due to the relationship between the refractive power and the amount of movement of the focusing lens group, it is difficult to use this front lens extension method in zoom lens systems that include wide-angle lenses. There is no choice but to adopt a focusing method in which a focusing lens group is arranged inside or at the rear of the lens system. However, in these focusing methods, the amount of focusing extension for the same photographing distance differs depending on the focal length, and the longer the focal length is, the greater the amount of focusing has to be. For example, in the case of the entire feeding method, the ratio of the feeding amount is approximately the square of the zoom ratio. Therefore, a correction mechanism is required to correct the difference in the amount of extension during zooming, and the structure of the lens barrel becomes complicated. In addition, in the case of the autofocus method, the amount of extension is controlled electrically, so it is possible to electrically correct the difference in the amount of extension due to the focal length.
In this case as well, the difference in the amount of feed is too large.
This is undesirable in terms of focusing speed, driving energy, driving space, etc. An object of the present invention is to provide a new focusing method in which the ratio of the amount of focusing movement between the shortest focal length end and the longest focal length end is extremely small, approximately 2 to 3 times or less, using a feeding method other than front lens feeding. There is a particular thing. Another object of the present invention is that by using a feeding method other than front lens feeding, the difference in the feeding amount depending on the focal length is extremely small, and even if the same feeding amount is adopted, the deviation in the image plane position due to the focal length is within the depth of focus. The object of the present invention is to provide a new focusing method that is comparable to the front lens feeding method. In order to achieve the above first object, the present invention is characterized by a focusing lens group that performs focusing by movement in the optical axis direction, and a focusing lens group that is arranged closer to the object side than the focusing lens group. It has at least two lens groups, and when zooming, the combined focal length of the two lens groups changes due to a change in the air distance between the two lens groups, and when zooming, the focusing The lateral magnification β _F of the lens group changes while always satisfying |β _F | > 1, and the change is such that |β _F | increases with zooming from the short focal length side to the long focal length side. This is a zoom lens system that satisfies the following conditions. 1/2<P _T /D-Q _T P _W /D-Q _W <2 However, P=f _A ²・β _F ² /β _F ² −1 Q=β _F /β _F 2-1・f _A ² /f _F +2f _A + (2-β _F -1/β _F ) f _F + (2-β _B -1/β _B ) f _B D: Shortest shooting distance, f _A : Than focusing lens group The composite focal length of the lens group on the object side, f _F : The composite focal length of the focusing lens group, f _B : The composite focal length of the lens group on the image plane side than the focusing lens group, and P _T and Q _T are the values of the above P and Q at the longest focal length, respectively, and P _W and Q _W are the values of the above P and Q, respectively.
This is the value of Q at the shortest focal length. Furthermore, in order to achieve the second object, the present invention is characterized by a focusing lens group having a positive refractive power that performs focusing by movement in the optical axis direction, and a focusing lens group having a positive refractive power. has at least two lens groups disposed on the object side, and when zooming, the combined focal length of the two lens groups changes due to a change in the air distance between the two lens groups, and The lateral magnification β _F of the focusing lens group satisfies 1<|β _F |<2 at the shortest focal length, and |β _F | increases with zooming from the short focal length side to the long focal length side. It is in the zoom lens system. The present invention will be explained in detail below. FIG. 1 shows a configuration diagram of a general zoom lens based on thin-wall approximation at a certain focal length. _F is the focal length of the focusing lens group F, _A is the focal length of the lens group A that is closer to the object side than the focusing lens group, and _B is the focal length of the lens group B that is closer to the image plane than the focusing lens group. It is assumed that each focal length may change during zooming.
From the configuration in the same figure, if the thin wall spacing is e ₁ and e ₂ , the lens back is LB, the focal length of the entire system is β _F and β _B is the lateral magnification of lens groups F and B, respectively, the following equations hold true. do. = _A β _F β _B (1) e ₁ = _A + (1-1/β _F ) _F (2) e ₂ = (1-β _F ) _F + (1-1/β _B ) _B (3) LB = (1-β _B ) _B (4) From equation (1), at least one of _A , β _F , and β _B must change during zooming. FIG. 2 shows that, at the focal length represented by the configuration of FIG. 1, the focusing lens group F was moved by ΔΧ and focusing was achieved from the A group to the position _S1 . At this time, if the lateral magnifications of the A group and the F group are respectively β' _A and β' _F , the following equation holds true. S ₁ = (1-1/β' _A ) _A (5) e ₁ +ΔΧ= (1-β' _A ) _A + (1-1/β' _F ) _F (6) e ₂ −ΔΧ= (1- β′ _F ) _F + (1-1/β _B ) _B (7) Regarding the lens back LB, see Figure 2 as well.
The same equation as equation (4) holds true. Here, we will discuss the signs in equations (1) to (7). (1)
Since the value in the equation is the focal length of the entire system, it must obviously be positive. _A , _F , _B representing focal length
β _A ′, β _F , β _B etc. representing the lateral magnification can be positive or negative, but have the following restrictions. That is, from equation (5), S ₁ must be positive, so in practice β _A ′· _A <0. Furthermore, since LB must also be positive, (1-β _B ) _B > 0 from equation (4). still,
The positive or negative lateral magnification is positive when the object point and image point are located in the same direction relative to the lens, and negative when the object point and image point are located in opposite directions relative to the lens.
The thin wall spacings e ₁ and e ₂ can also take negative values due to the relationship between the principal point spacings. As can be seen from FIG. 2, the movement amount ΔΧ of the focusing lens group F is positive when the focusing lens group F moves toward the image side, and negative when it moves toward the object side. In the above, subtracting equation (7) from equation (3), ΔΧ=(β′ _F −β _F )・_FHere , if we set Δβ _F =β′ _F −β _F , ΔΧ=Δβ _F・_F (8 ) Next, by subtracting equation (2) from equation (6) and using equation (8), we get β _A ′=1−β _F (β _F +Δβ _F )/β _F (β _F +Δβ _F )
・ΔΧ／ _A (9) is obtained. Here, since β _A ′・_A < 0, ΔΧ
> 0, β _F β _F ′> 1, and ΔΧ < 0, β _F β _F ′
<
It is 1. Therefore, focusing lens group F
moves toward the image side when |β _F |>1, and moves toward the object side when |β _F |<1 to focus on a nearby subject. The photographing distance D when focusing at close range is expressed by the following equation. D=S ₁ +e ₁ +e ₂ +LB By substituting each of the above equations into this and rearranging, the following equation is obtained. (D-Q)ΔΧ=P (10) Here, P= _A ²・β _F ² /β _F ² −1 (11) Q=β _F ／β _F ² −1 _A ² / _F +2 _A + (2− β _F −1/
β _F ) _F + (2−β _B −1/β _B ) _B (12). Here, when finding equation (10), the following approximation was performed with Δβ _F <<β _F. β _F (β _F +Δβ _F )/β _F (β _F +Δβ _F )−1≒β _F ² +Δ
β _F ·β _F /β _F ² −1 (13) Equation (10) is the relation between the photographing distance D and the amount of movement ΔΧ of the focusing lens group F. However, β _F ² =1
Since it is indeterminate when , β _F ² ≠1 is assumed hereafter. Equation (10) is a basic equation showing the relationship between the amount of movement ΔΧ of the focusing lens group F with respect to the change in the photographing distance D, and this relationship is a hyperbola with P and Q as parameters. If P and Q change during zooming, the focusing movement amount ΔΧ for the same photographing distance D will take on different values. Therefore,
In order to reduce the difference in the focusing movement amount ΔΧ due to the focal length, it is necessary to reduce the changes in P and Q due to zooming. Here (11), (1
Paying attention to equation 2), Q is an amount that changes on the order of the first power of the focal length of each lens group, and P is an amount that changes on the order of the square of the focal length of lens group A, so focusing is dependent on the focal length. Quantity ΔΧ
It can be seen that the change in P due to zooming has a particularly large effect as a factor that causes a difference in . Therefore, in order to reduce the difference in the focusing amount depending on the focal length, it is effective to suppress the change in P during zooming. Therefore, consider conditions for reducing the change in P. Paying attention to equation (11), it can be seen that if both _A and β _F do not change during zooming, the value of P will not change.As is clear from equation (1), this means that This method applies when changing the focal length by changing only the time β _B , and cannot be adopted because it corresponds to the front lens extension method. Therefore, it is necessary to consider other methods to reduce the change in P during zooming. In this case, _A ,
One of β _F will change, but even if one changes, unless the other changes correspondingly, the change in P during zooming cannot be reduced. Therefore, in order to reduce the change in P during zooming, the lateral magnification β _F of the focusing lens group F should change during zooming, and the lens group A disposed closer to the object side than this focusing lens group. composite focal length _A
The conclusion is that it is necessary to change. For this purpose, focusing lens group F
move during zooming, or arrange at least one lens group behind the focusing lens group F, and move this lens group during zooming, and at least two lens groups closer to the object than the focusing lens group. It is necessary to arrange a lens group of , and to change the combined focal length of the two lens groups during zooming by changing the air distance between the two lens groups during zooming. Therefore, the zoom lens system of the present invention has at least three lens groups. Furthermore, considering the value of the numerator _A ² · β _F ² in formula (11), it is natural to increase as _A · β _F increases from formula (1), so when increases _A
²・β _F ² increases. Therefore, in order to reduce the change in the value of P with respect to the increase, the absolute value of the denominator β _F ² −1 in equation (11) must also increase with respect to the increase in . For this purpose, when |β _F |>1, |β _F | must increase as |β F | increases, and when |β _F |<1, |β _F | decreases as |β F | increases. There is a need.
Here, in the latter case, it means that the focusing lens group reduces the magnification as the focal length increases, which is unnatural when increasing the zoom ratio due to the design of the zoom lens. From the above, in order to suppress the change in P during zooming, the lateral magnification β _F of the focusing lens group must change while always satisfying |β _F | > 1, and the change must be It is concluded that |β _F | needs to increase as zooming from the distance side to the long focal length side. Furthermore, as is clear from equation (11) and the above discussion, in order to reduce the difference in focusing amount depending on the focal length, more directly, It can be seen that it is necessary to have a condition that the value is approximately constant during zooming. Next, from equations (10), (11), and (12), consider the conditions from the viewpoint of giving the same amount of focusing movement for the same photographing distance regardless of changes in focal length. In order to provide the same amount of movement for the same photographing distance over the entire focal length range of the zoom lens, the following two conditions must be satisfied during zooming. P=constant (14) Q=constant (15) When both equations (14) and (15) are not satisfied, the shape of the hyperbola expressed by equation (10) changes as already mentioned, and the amount of movement changes. It will be different. P can take a positive or negative sign depending on the magnitude relationship between |β _F | and 1. Next, check whether equations (14) and (15) hold true during zooming. As already mentioned, it is important not to change P, so we will start with equation (14). First, from equations (14) and (11), P = _A ² · β _F ² / β _F ² −1 = constant (16) When only β _B changes during zooming in equation (1), _A and β Both _F are constant and equation (16) holds, but this cannot be used because it corresponds to the front ball being fed out. Also, when _A is constant during zooming, (16)
From the equation, there is no solution other than β _F being constant, so the front ball will be fed out in the same way. This is the same as what we have already considered. That is, _A must change when zooming. From this it can be seen that β _F must also change during zooming. In other words, as already mentioned, the focusing lens group needs to change its lateral magnification β _F during zooming. As mentioned earlier, this does not necessarily mean that the focusing lens group F moves during zooming. This is because by moving the B group during zooming, β _F can be changed without moving the F group. Assuming that equation (16) holds true on the short focal length side and the long focal length side, let the respective focal lengths and lateral magnifications be _AW and β _FW .
Assuming _AT and β _FT , _AW ²・β _FW ² /β _FW ² −1= _AT ²・β _F ² _T /β _FT ² −
1 must hold true. This formula is transformed as follows. ( _AT / _AW ) ² = 1-(1/β _FT ) ² /1-(1/
β _FW ) ² (17) In equation (17), it is obviously practical to consider that group A multiplies when zooming from a short focus, so _AT > _AW and from equation (17), 1 - (1/ β _FT ) ² /1−(1/β _FW ) ² >1. From this, when β _FW ² <1, 1>|β _FW |>|β _FT |(18) and when β _FW ² >1, 1<|β _FW |<|β _FT |(19) are obtained. However, the signs of β _FW and β _FT are the same. Equation (18) expresses that the focusing lens group F reduces the magnification when zooming from a short focal point, and when designing a zoom lens, it is necessary to take into account that the zoom ratio is larger than the desired one. It is not practical except for zoom lenses with small zoom ratios.
Equation (19) indicates that the focusing lens group F multiplies the magnification when zooming from a short focal point, and is a very effective condition for designing a zoom lens. Therefore, equation (19) will be considered from now on. This equation (19) has the same meaning and content as previously discussed as a requirement of the present invention. From the above, we assume that equation (14) holds true under the conditions of equation (19), and then consider equation (15). (15) to (14), (1
9) If we rearrange β _F using equations, we get the following equation. Q=-1/ _F β _F { _F ² (β _F ² +1)+2√・_F
√ _F ² −1−P}+2 _F + (2−β _B −1/β _B ) _B =− _F β _F {β _F ² +2(√P/ _F )√ _F ² −1+1
−(√P/ _F ) ² }+2 _F +(2−β _B −1/β _B ) _B (
20) The sign + in { } corresponds to the + of _A・β _F. The conditions for formula (20) to be constant for any β _{F are F} ,
Considering _B and β _B as variables is complicated, so now,
To make the equation easier to understand, if we assume that _F , _B , and β _B are constant during zooming, then Q becomes a function of β _F only, and
Letting it be G(β _F ), the following equation is obtained. G (β _F )=− _F β _F {β _F ² +2(√P _F )√ _F ² −
1+1−(√P _F ) ² }+2 _F +(2−β _B −1/β _B )
_B (21) In equation (21), G(β _F ) is clearly not constant for any β _F . Therefore, if _F , _B , and _βB are constant during zooming, equation (15) will not hold. In order to examine equation (21) in more detail, we set Χ ² =β _F ² −1 (22) and substitute it into equation (21), resulting in the following equation. However, since equation (23) is symmetrical with respect to Χ=0, it is set as Χ>0. The sign + in front of _F is β sign + of _F
corresponds to Equation (23) has two + signs and is complicated, so for the sake of simplicity, the term (2-β _B -1/β _B ) _B is excluded and classified as follows. () When _A β _F >0, β _F <-1 () When _A β _F >0, β _F >1 () When _A β _F <0, β _F <-1 () When _A β _F <0, β _F >1 First, consider the sign of _F in the above four classifications. In the case of (), since _A < 0, β _F <-1 cannot be realized paraxially when _F < 0. Yotsute _F ＞
0. In the case of (), since _A > 0, when _F > 0, β _F > 1 cannot be achieved unless the image is formed once within the lens system. Therefore, _F <0. In the case of () and (), since _A β _F <0, in order to make the focal length of the entire system positive, the lateral magnification β _B of the B group is required, and from equation (1), its sign is β _B < Must be 0. Considering the sign of _F in the same way as in the case of () and (), in the case of (), _F < 0, and in the case of (), _F
>0. Next, consider each case. G(Χ)/ in case of ()
The graph of _F is shown in Figure 3. In the case of Figure 3, Χ=
1, that is, it has an extreme value near β _F = −√2,
When the horizontal magnification β _F is changed including this magnification, G(β _F )
The fluctuation in the value of can be made small. From this, considering that the above discussion is based on approximation, it is appropriate to set β _FW , which is the minimum value of β _F , to 1<|β _FW |<2. That is, it is not appropriate to make the value of |β _F | too large. As the value of the parameter (√P/ _F ) increases, the fluctuation range around the extreme value of G(Χ)/ _F increases, but G(Χ)
is proportional to _F , and for the same value of P, the larger the value of the parameter -(√P/ _F ), the smaller _F becomes, and as a result, the fluctuation range of G(Χ) does not become so large. However, when correcting zoom lens aberrations, _F
It is not desirable to make it too small. Therefore, an example of a paraxial solution when the parameter -(√P/ _F ) is set to, for example, √P/ _F = 1 (24) will be shown in the case where there is no B group. Let the focal length and lateral magnification on the short focal length side and long focal length side be _W , _T , ^AW , _AT , and β _FW and β _FT , respectively. Example 1 If _W = 28, _T = 135 β _FW = −1.2, then from equation (1), _AW = −23.333 From (23), (11), and (13), _F = 42.212 β _FT = −3.35 ( From equation 1) _{, AT} = -40.299. Therefore, the paraxial solution when _W = 28 is as follows. _AW −23.333 e _1W 54.056 _F 42.212 LB _W 92.866 When _T = 135, _AT −40.299 e _1T 14.514 f _F 42.212 LB _T 183.618 Similarly, using the subscript M as the intermediate focal length, when _M = 60, _AM −34.522 e _1M 31.978 _F 42.212 LB _M 115.576 In the above example, the value of the photographing distance when the focusing lens group F is moved toward the image plane side is shown.

[Table] Regarding accuracy, since P is constant, a fairly good accuracy can be achieved, although the error due to Q and the approximation error due to equation (13) are particularly large on the short focus side. Next, consider the case of (). The graph of G(Χ)/ _F in this case is shown in Figure 4. From the same figure, since the graph of G(Χ)/ _F monotonically decreases, G(Χ) increases monotonically from _F < 0. If the lateral magnification on the short focus side is set close to the same magnification, the approximation error by equation (13) will increase, and in practice it is impossible to make a highly accurate judgment from the graph of G (Χ), but in the case of () Similarly, when √P/ _F = -1 (24) and there is no B group, the following paraxial solution is obtained from β _FW = 1.2. Example 2 _W =28 _AW 23.333 e _1W 16.298 _F −42.212 LB _W 8.442 _M =60 _AM 34.522 e _1M 16.598 _F −42.212 LB _M 31.152 _T =135 _AT 40.299 e _1T 10.688 _F −42.212 LB _T 99.194 In the above example, The value of the photographing distance when the focusing lens group F is moved toward the image plane side is shown.

[Table] This also shows quite good accuracy. In the case of () and (), by determining β _B (<0) and _B , a paraxial solution can be obtained as in the case of () and (). However, as is clear from the results of () and (), when _F , β _B , and _B are constant, there is an inherent error. Of course, whether or not this error can be tolerated depends on the purpose of use of the zoom lens. For example, for varifocal or autofocus, a good focusing method that has never been available before is characterized by almost all the difference from the short focus side to the long focus side. It has many advantages, such as focusing at the same photographing distance with the same amount of extension, the amount of movement is small, and the focusing lens group is close to the image plane. Next, we will consider a method to further reduce errors during focusing. From the explanation so far, it is clear that it is possible to reduce the error by making _F , β _B , and _B variable instead of constant during zooming. This is possible by changing _F in a 3-group configuration, and by changing at least one of _F , β _B , and _B in a 4-group configuration, but as an extension of the 3-group configuration, Since this is possible, we will represent the case where _F changes in a three-group configuration. In this case, consider the graph in FIG. 3 using the paraxial solution of Example 1. In the same graph, as Χ or β _F increases, G(Χ)/ _F increases, but
It can be seen that as Χ increases, _F gradually decreases, and the G(Χ)/ _F curve moves to the lower curve. This shows that errors can be reduced by reducing the focal length of the focusing lens group when zooming from the short focus side. However, on the short focal length side, the lateral magnification is close to the same magnification, so the error due to approximation formula (13) is large.
The above explanation cannot be applied as is. Therefore, in Example 1, the shooting distance of _T = 135 is set to _M
Consider adjusting the shooting distance to =60. Some calculations show that the focal length _F of the focusing lens group F at _T = 135 should be 0.85 times that at _M = 60. Example 3 The paraxial solution of _M = 60 in Example 1 and the next paraxial solution of _T = 135 When _T = 135, _AT −40.299 e ₁ ′ _T 6.291 _F ′ 35.88 (0.85× _F ) LB _T ′ 156.083 This The amount of movement of the focusing lens group F and its photographing distance all match the values of _M = 60 in Table 1. By making the focal length of the focusing lens group F variable during zooming in this manner, it is possible to achieve exactly the same photographing distance paraxially. In order to design an actual zoom lens, the thin approximation equation described above must be made thicker and changes in the distance between principal points must also be taken into consideration, and approximation equation (13) also does not hold. However, the basic content described above is the same. In order to realize a focusing method with fewer errors, the focal length of the focusing lens group F is
I think _you can understand that this is possible by introducing some of the lateral magnification β _B of the lens group B located on the image side than the F group and its focal length _B for adjustment purposes. Next, an example of a zoom lens system of the present invention with increased thickness will be shown.

【table】

[Table] In the above embodiment, the lens group from _r14 to _r26 is the focusing lens group F. Focal length _F and parameters of focusing lens group F

[Formula] and the value of √/ _F are as follows.

[Table] As shown in the table above, the value of the parameter √/ _F is
For zoom lenses with a zoom ratio of about 3x,
Approximately 0.5 to 2.5 is appropriate. The configuration diagram and aberration diagram of this example are shown in FIGS. 5 and 6. When the focusing lens group in the above zoom lens is moved toward the image plane side, Table 3 shows the amount of image point movement on the short focus side and intermediate focus, based on the movement amount on the long focus side.

[Table] As is clear from Table 3, the amount of image point movement is well within the depth of focus, and is considered to be equivalent to the conventional front lens extension method. As is clear from the above examples, the present invention has the same effect as the front lens extension, but in the case of a varifocal zoom lens or an autofocus zoom lens, there is a difference in the amount of focusing movement. Since this is not a very big problem, it is acceptable even if the ratio of the focusing movement amount between the short focal length side and the long focal length side is 2 to 3 times.
If the allowable ratio is doubled, this can be expressed as follows. 1/2<ΔΧ _T /ΔΧ _W <2 (25) However, ΔΧ _T and ΔΧ _W are the focusing movement amounts at the longest focal length and the shortest focal length, respectively. According to equation (10), the focusing movement amount ΔΧ is expressed as follows. ΔΧ=P/D−Q Therefore, according to the present invention, equation (25) can be expressed as follows. 1/2<P _T /D-Q _T /P _W /D-Q _W <2 However, P _T and Q _T are P defined by equations (11) and (12),
The value of Q at the longest focal length, P _W , Q _W is (11),
This is the value at the shortest focal length of P and Q defined by equation (12). When a zoom lens with a high zoom ratio is fully extended, the amount of movement changes according to the square of the zoom ratio.
By selecting P _T , Q _T , P _W , and Q _W , the ratio of the amount of focusing movement between the shortest focal length and the longest focal length can be suppressed to within 3 times at the photographing distance D. Note that Example 3 is a zoom lens with a zoom ratio of about 3 times, but if the same configuration as Example 3 is applied to a zoom lens with a higher magnification ratio, the amount of movement of focusing lens group F due to zooming will be Depending on the specifications, it may become difficult to design. In such cases,
For example, as shown in Fig. 7, by providing a group B after the focusing lens group F, which multiplies the magnification during zooming from the short focal length side, changes in the lateral magnification of the focusing lens group F can be minimized. It is clear from the explanation so far that this is advantageous. As is clear from the above, the present invention provides a new focusing method for a zoom lens system in which the difference in focusing amount due to changes in focal length is extremely small, regardless of the front lens extending method.
Furthermore, the present invention also provides a focusing method for a new zoom lens system that allows focusing to be performed at the same focusing distance by the same focusing amount regardless of changes in focal length, regardless of the front lens extension method. .

[Brief explanation of drawings]

Figure 1 is a configuration diagram using a thin-wall approximation at a certain focal length of a general zoom lens to explain the focusing method of the present invention. 3
The figure and Figure 4 show the focusing lens group F when the horizontal axis is Χ (=β _F ² -1), which is related to the lateral magnification β _F of the focusing lens group F with respect to the parameter (√/ _F ). G representing the movement amount error
(Χ), FIG. 5 is a lens configuration diagram of an embodiment of the present invention configured with a thicker wall, FIG. 6 is its aberration diagram, and FIG. 7 is a lens movement of another embodiment of the present invention. It is a figure showing a format. F... Focusing lens group, A... At least two lens groups closer to the object than the focusing lens group.

Claims (1)

[Claims] 1. A focusing lens group with a positive refractive power that performs focusing by movement in the optical axis direction, and at least two lens groups arranged closer to the object side than this focusing lens group. During zooming, the combined focal length of the two lens groups changes due to a change in the air distance between the two lens groups, and the lateral magnification β _F of the focusing lens group changes at the shortest focal length. A zoom lens system that satisfies 1<|β _F |<2 and is characterized in that |β _F | increases with zooming from a short focal length side to a long focal length side. 2. The focusing lens group moves in the optical axis direction during zooming, whereby the lateral magnification β _F of the focusing lens group changes during zooming. zoom lens system. 3 When the composite focal length of the above two lens groups located closer to the object side than the above focusing lens group is f _A , 2. A zoom lens system according to claim 1, wherein: is substantially constant during zooming. 4. A patent claim characterized in that the focusing lens group consists of at least two or more lens groups, and the combined focal length of the focusing lens group changes during zooming due to a change in the air gap between them. The zoom lens system according to the range 1 or 3. 5 A focusing lens group that performs focusing by movement in the optical axis direction, and at least two lens groups arranged closer to the object side than this focusing lens group, and when zooming, the above two groups The combined focal length of the two lens groups changes due to a change in the air distance between the lens groups, and during zooming, the lateral magnification β _F of the focusing lens group becomes |β _F |>1 at the shortest focal length. ｜β _F ｜
The zoom lens system is characterized by increasing the amount of the change and satisfying the following conditional expression: 1/2<P _T /D-Q _T P _W /D-Q _W <2 However, P=f _A ²・β _F ² /β _F ² −1 Q=β _F /β _F 2−1・f _A ² /f _F +2f _A +(2−β _F −1/β _F )f _F +(2−β _B -1/β _B ) f _B D: Minimum object distance, f _A : Combined focal length of the lens group closer to the object side than the focusing lens group, f _F : Combined focal length of the focusing lens group, f _B : The composite focal length of the lens group located closer to the image plane than the focusing lens group, where P _T and Q _T are the values of the above P and Q at the longest focal length, respectively, and P _W and Q _W are the values of the above P and Q, respectively. ，
This is the value of Q at the shortest focal length.