JPH0567930B2 - - 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, as a focusing method for a zoom lens system, a so-called front lens extension method is generally used, in which a focusing lens group is disposed 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. . Focusing methods that are different from this front lens extension method include a focusing method in which the entire lens system is extended, and a focusing method in which a focusing lens group is placed inside or at the rear of the lens system, but 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 larger the extension amount must be. For example, in the case of the entire feeding method, the amount of feeding is approximately the square of the zoom ratio. Therefore, a correction mechanism is required to correct this 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 control of the extension amount is performed electrically, so it is possible to electrically correct the difference in the extension amount due to the focal length, but in this case as well, the difference in the extension amount Too large size 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 using a feeding method other than front lens feeding. 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 object, the present invention is characterized by a focusing lens group F having a negative refractive power that performs focusing by movement in the optical axis direction, and a focusing lens group F that is disposed closer to the object side than the focusing lens group F, at least two front lens groups A having positive refractive power, and at least one rear lens group B having positive refractive power disposed on the image side of the focusing lens group F; The lateral magnification β _F of the focusing lens group F always satisfies β _F <0, and during zooming, the combined focal length of the front lens group A and the lateral magnification β F of the focusing lens group _F changes, and the direction of change in the lateral magnification β _F of the focusing lens group F due to zooming from the short focal length side to the long focal length side is the same as the direction of the change in the lateral magnification β F of the focusing lens group F when zooming from the short focal length side to the long focal length side. This zoom lens system is characterized in that the lateral magnification β _F of the group F always changes in the same direction. Furthermore, the lateral magnification β _FW of the focusing lens group F at the shortest focal length during infinity focusing is β _FW ≒−1
(Including β _FW =−1) is advantageous as will be described later. In the present invention, since the focusing lens group F is negative and β _F <0, the front lens group A is positive and a positive rear lens group B is required. The present invention will be explained in detail below. FIG. 1 shows a configuration diagram of a zoom lens according to the present invention based on thin wall approximation at a certain focal length. f _F is the focal length of the negative focusing lens group F, f _A is the focal length of the positive front lens group A that is closer to the object side than the focusing lens group, and f _B is the focal length of the positive front lens group A that is closer to the image plane than the focusing lens group. It is assumed that the focal length of the positive rear lens group B is 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 respectively, the lens bag is LB, the focal length of the entire system is f, and the lateral magnifications of lens groups F and B are β _F and β _B , respectively, the following equations are obtained. To establish. f=f _A β _F β _B (1) e ₁ = f _A + (1-1/β _F ) f _F (2) e ₂ = (1-β _F ) f _F + (1-1/β _F ) f _B (3) LB = (1-β _B ) f _B (4) From equation (1), at least one of f _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 .DELTA.X to achieve focusing at the position _S1 from the A group. At this time, if the lateral magnifications of the A group and the F group are β' _A and β' _F , respectively, the following equation holds true. S ₁ = (1-1/β' _A )f _A (5) e ₁ +ΔX = (1-β' _A ) f _A + (1-1/β' _F ) f _F (6) e ₂ -ΔX= (1-β' _F )f _F + (1-1/β _F )f _B (7) Regarding the lens bag LB, see Figure 2.
The same equation as equation (4) holds true. Here, we will discuss the signs in equations (1) to (7). (1)
Since f in the equation is the focal length of the entire system, it must obviously be positive. Also, f _A represents the focal length,
Naturally, f _F and f _B are positive, negative, and positive, respectively. Furthermore, regarding the lateral magnification, since β _F and β′ _F are negative, β′ _A and β _B are also negative. Note that the positive and negative lateral magnifications are positive when the object point and image point are in the same direction relative to the lens, and negative when the object point and image point are in opposite directions relative to the lens. There is. Thin wall spacing e ₁ , e ₂
can also take a negative value due to the relationship between principal points. As can be seen from FIG. 2, the amount of movement ΔX 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), ΔX = (β′ _F −β _F )_・f _FHere , if we set Δβ _F =β′ _F −β F, then ΔX=Δβ _F・f _F (8) Next, by subtracting equation (2) from equation (6) and using equation (8), we get β _A ′=−β _F (β _F +Δβ _F )/β _F (β _F +Δβ _F )・
ΔX/f _A (9) is obtained. Here, since β _A ′・f _A <0, ΔX
>0 then β _F β _F ′>1, and when ΔX<0 then β _F β _F ′
<1. Therefore, the focusing lens group moves toward the image side when Fβ _F β _F ′>1, and moves toward the object side when β _F β _F ′<1 to focus on a close object. 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)ΔX=P (10) Here, P=f _A ²・β _F (β _F +Δβ _F )/β _F (β _F +Δβ
_F )−1(11) Q=f _A +e ₁ +e ₂ +LB (12). Equation (10) above shows the relationship between the photographing distance D and the amount of movement ΔX of the focusing lens group F. This equation (10) is a basic equation showing the relationship between the movement amount ΔX 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 ΔX with respect to the photographing distance D will also take on a different value. Therefore, in order to reduce the difference in the focusing movement amount ΔX due to the focal length, it is necessary to reduce the changes in P and Q due to zooming. If we pay attention to equations (11) and (12), Q is an amount that changes on the order of the first power of the focal length of each lens group, but P is an amount that changes on the order of the square of the focal length of lens group A. It can be seen that the change in P due to zooming has a particularly large influence as a factor that causes a difference in the focusing extension amount ΔX depending on the focal length. 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), first, when zooming, f _A , β _F
It can _be seen that there is no change in the value of P if both of .
This method applies to cases in which the number is changed, and it cannot be adopted because it corresponds to the front ball feeding method. Therefore, it is necessary to consider other methods to reduce the change in P during zooming. In this case, either f _A or β _F will change, but even if either one changes, the change in P during zooming cannot be reduced unless the other changes accordingly. . 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 front lens group should be arranged closer to the object side than this focusing lens group. The conclusion is that it is necessary to change the composite focal length f _A of A. To this end, to change β _F , move the focusing lens group F or move the rear lens group B during zooming, and to change f _A , move the front lens group A at least twice It is necessary to configure the camera with two lens groups, and to change the air distance between the two lens groups during zooming. Therefore, the zoom lens system of the present invention has at least four lens groups. Furthermore, considering the value of the numerator f _A ²・β _F (β _F +Δβ _F ) in equation (11), we can see from equation (1) that as f _A・β _F increases, f
Since it is natural to increase f _A ² ·β _F (β _F +Δβ _F ) when f increases. Therefore, in order to reduce the change in the value of P with respect to an increase in f, the absolute value of the denominator β _F (β _F +Δβ _F )−1 in equation (11) must also increase with respect to an increase in f. To this end, if β _F (β _F +Δβ _F )>1, |β _F | needs to increase as f increases, and if β _F (β _F +Δβ _F )<1, |β _F | needs to decrease as f increases. Also, if we look at the change in the lateral magnification of the focusing lens group due to focusing from the infinity side to the near side, since f _F <0, from equation (8), ΔX>
0, then Δβ _F <0. β _F , β _F ′
Since both are negative, this means that |β _F ′| increases with focusing toward the near side. On the other hand, when ΔX<0, Δβ _F >
Since it is 0, |β _F ′| decreases as the focus moves toward the near side. Summarizing the above, we reach the following conclusion. (i) When β _F β _F ′>1 If the focusing lens group F is moved to the image side, focusing to the near side can be performed. (ΔX>0) When zooming, |β _F | is increased as f increases. Following focusing to the near side |β _F ′|
increases. (ii) When β _F β _F ′<1 By moving the focusing lens group F toward the object side, focusing to the near side can be performed. (ΔX<0) When zooming, |β _F | decreases as f increases. Following focusing to the near side |β _F ′
| decreases. In both cases (i) and (ii) above, the direction of change in the lateral magnification of the focusing lens group is the same when f increases and when focusing toward the near side. Next, we will consider the range of variation of |β _F |. First, in case (i) above, |β _F | increases as f increases. Considering this regarding equation (1), we get
The change in |β _F | contributes to the change in f, and |β _F |
This means that the larger the rate of change of f, that is, the ratio of the maximum value to the minimum value of |β _F |, the larger the ratio of the maximum value to the minimum value of f, that is, the zoom ratio. On the other hand |
As already mentioned, the change in β _F | is achieved by moving the focusing lens group F or the rear lens group B in the optical axis direction, but the amount of this movement depends on the range of change in |β _F | dependent. Therefore |β _F |
The smaller the width of change in |β _F | and the larger the rate of change in |β F |, the more effectively a large zoom ratio can be obtained with a small amount of movement. For example, considering the case where |β _F | changes from 1 to 2 and from 2 to 4,
Both rates of change are 2x and their contribution to the zoom ratio is the same, but the range of change is 1 for the former and 2 for the latter, indicating that the former, which has a smaller amount of movement, is more advantageous. Therefore, _in case (i) where β _F β _F ′ > 1, β _FW , which is the minimum value of |β _F The closer the focus is, the more advantageous it is, and since |β _F | increases as the focus moves closer, β _F β _F ′
>1 is satisfied, it follows that it is most advantageous to set β _FW =−1. On the other hand, in the case (ii) above where β _F β _F ′<1, |β _F | decreases as f increases from the above.
Therefore, from equation (1), the change in |β _F | itself is f
It is inappropriate to examine the change range of |β _F | alone. Therefore
Consider the relationship between |f _A · β _F | and |β _F | in equation (1). The relationship between these two is such that the closer |β _F | is to 1, the more effectively |f _A ·β _F | increases with a slight decrease in |β _F |. That is, |f _A ·β _F | When viewed as a whole, a decrease in |β _F | contributes to an increase in f. Here, it is desirable to obtain the desired zoom ratio by minimizing the amount of movement of the lens group for changing |β _F |, as in case (i) above, so β _F β _F ′<1. In the case of (ii), it is more advantageous to set β _FW , which is the maximum value of |β _F |, close to −1 and to make the range of variation of β _F as close to −1 as possible as a whole. Also,
Since |β _F | decreases as the focus moves closer, β _F β _F ′<1 is always satisfied, so it is most advantageous to set β _FW =−1. As mentioned above, in both cases (i) and (ii), the conclusion is that the range of variation of β _F as a whole should be set as close to -1 as possible, in other words, β _FW should be set as close to -1 as possible. be done. Note that the above discussion does not take into account the depth of focus of the zoom lens system, and the relationship between the magnitude of β _F β・_F ′ and 1 and the direction of movement of the focusing lens group for close focusing is Due to the derived restriction, the limit of the range of variation is -1 both when β _F < -1 and when β _F < -1, and if the range of variation of β _F straddles both sides of -1, then I couldn't think of anything. Here, for example, in the case (i) above, it is assumed that β _FW T<−1<β _FW where β _FW ≈−1. (However, β _F T is the lateral magnification of the focusing lens group F at the longest focal length when focusing at infinity.) In the above case, in the range from the focal length where β _F = -1 to the longest focal length, β _F
β _F ′>1, and the configuration is as defined in case (i). However, since |β _F |<1 in the range from the shortest focal length to the focal length where β _F =−1, there is always a region near infinity where β _F β _F ′<1. Therefore, in this area (i)
Focusing lens group F according to the definition in the case of
Moving toward the image side produces an effect that is contrary to focusing toward the near side. However, (i)
In this case, |β _F | increases due to the movement of the focusing lens group F toward the image side, so β _F β _F ' approaches 1 due to focusing toward the near side.
Moreover, since β _FW ≒-1, when focusing slightly from the infinity side to the near side, β _F β _F ′＞
It will enter the area where it becomes 1. Furthermore, since |β _F | becomes larger, β _FW becomes closer to −1 as it approaches the long focus side, and during focusing, it enters the region where β _F β _F ′>1 near infinity. .
Therefore, the above-mentioned effect contrary to focusing occurs only in a relatively limited range where the photographing magnification is small and the photographing distance is large. This range is a region with a deep depth of focus, and it can be seen that the above-mentioned effects that go against focusing are within the depth of focus and are not substantially harmful. If the depth of focus is taken into account as described above, it is possible to set the range of variation of β _F to span both sides of −1 with the condition that β _FW ≈−1. This means that the range of variation of β _F as a whole can be set closer to -1, resulting in a more desirable design. The above discussion also applies to the case (ii) above, and this applies to the case where βW _F T<-1<β _FT However, β _FW ≈-1. In this case as well, focusing is substantially possible if the depth of focus is taken into account. In particular, in case (ii), the direction of change of |β _F | itself is disadvantageous to the change of f, so the range of change of β _F is set to span both sides of -1, and the range of change of β _F is set as a whole. It is particularly meaningful to set the value near -1 to effectively obtain the desired zoom ratio. Next, examples of the present invention will be shown. Example 1 is the above (i)
This applies to the above case, and Example 2 corresponds to the above case (ii). Further, in both of these embodiments, the variation range of β _F spans both sides of −1, but the present invention is not limited to such implementation, and for example, when β _FW ≒ −1
It is clear from the above explanation that it is also possible to carry out an implementation in which the variation range of β _F is only on one side of β _F =-1, provided that β _FW =-1 is also included.

【table】

[Table] In the first embodiment, r16 to r18 are the focusing lens group F that is movable during zooming, and its focal length f _F is -30.125. The lateral magnification β _F of the focusing lens group and the focal length f _A of the front lens group at each focal length are as follows.

[Table] The configuration diagram and aberration diagram of Example 1 are shown in FIGS. 3 and 4, respectively. Moreover, when the focusing lens group F is moved to the image plane side in Example 1,
Table 1 shows the amount of image point movement on the short focus side and intermediate focus, based on the amount of movement on the long focus side.

【table】

[Table] As is clear from Table 1, the allowable circle of confusion diameter is 0.04
Considering the degree, the depth of focus is 0.12 at f=12.8,
When f=25.5, it is about 0.13, and all the values in the table above are within this range, so it is considered to be equivalent to the conventional front ball feeding method.

【table】

[Table] In the second embodiment, r15 to r17 are the focusing lens group F that is fixed during zooming, and the focal length f _F is -26.667. The lateral magnification β _F of the focusing lens group and the focal length f _A of the front lens group at each focal length are as follows.

[Table] A configuration diagram and an aberration diagram of this example are shown in FIGS. 5 and 6, respectively. When the focusing lens group F is moved toward the object side in Example 2, Table 2 shows the amount of image point movement on the short focus side and the intermediate focus, based on the movement amount on the long focus side.

【table】

[Table] From Table 2, if the allowable circle of confusion diameter is 0.033, the depth of focus is 0.24 at f=28.8, 0.26 at f=35.0,
The amount of image point movement is f=28.8, which is a little large, but it is still considered to be within the depth of focus. As is clear from the above embodiments, the present invention has the same effect as the front lens extension, but there is a difference in the amount of focusing movement between varifocal zoom lenses and autofocus zoom lenses. is not a big problem, so
Relaxing the constraint that equation (11) remains almost constant during zooming,
A slight difference in focusing movement amount due to zooming can be tolerated. This is an advantageous factor for aberration correction, but the lateral magnification β _FW on the short focus side is β _FW ≒
By setting -1.0, the ratio of the amount of movement from the short focus side to the long focus side can be easily set. 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 movement amount due to a change in focal length is extremely small, regardless of the front lens extension method. That is, the present invention requires a positive front lens group and a positive rear lens group in front and behind the focusing lens group by making the focusing lens group negative and its lateral magnification β _F always negative. By changing both the composite focal length f _A of the group and the lateral magnification β _F of the focusing lens group, the difference in focusing movement due to zooming can be reduced, and the focusing lens group can be adjusted as the focal length increases during zooming. The direction of change in the lateral magnification is made to match the direction of change in the lateral magnification of the focusing lens group accompanying focusing toward the near side. Matching the directions of the lateral magnification changes described above has the meaning of effectively reducing the difference in focusing movement, and in addition to matching the directions of the lateral magnification changes described above, by further setting β _FW ≈-1, it is possible to achieve the desired result. A zoom ratio can be effectively obtained.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of the zoom lens system of the present invention at a certain focal length using thin-wall approximation, Figure 2 is a configuration diagram of the zoom lens system in Figure 1 during focusing, and Figures 3 and 5 are thick-wall approximations. FIGS. 4 and 6 are diagrams showing the aberrations of the lenses of each embodiment of the present invention, which are constructed in a compact manner. F...Negative focusing lens group, A...
At least two front lens groups closer to the object side than the focusing lens group, B...at least one rear lens group closer to the image side than the focusing lens group.

Claims (1)

[Claims] 1. A focusing lens group with a negative refractive power that performs focusing by movement in the optical axis direction; at least two groups arranged on the object side of the focusing lens group and having a positive refractive power as a whole; It has a front lens group and at least one rear lens group with positive refractive power disposed on the image side of the focusing lens group, and the lateral magnification β _F of the focusing lens group always satisfies β _F <0. In addition, the combined focal length of the front lens group and the lateral magnification of the focusing lens group change during zooming, and the focusing lens group changes as zooming from the short focal length side to the long focal length side. A zoom lens system characterized in that the direction of change in lateral magnification always coincides with the direction of change in lateral magnification of the focusing lens group accompanying focusing from an infinity side to a near side. 2 The lateral magnification β _F of the above focusing lens group when focusing at infinity changes from near -1 at the shortest focal length to -1 as the focal length increases.
2. The zoom lens system according to claim 1, wherein the zoom lens system is set to change so as to pass through 1 and move toward the value at the longest focal length. 3. The zoom lens system according to claim 1, further characterized in that the lateral magnification β _FW of the focusing lens group at the shortest focal length during infinity focusing is set to β _FW ≈-1. .