JP2901066B2 - Zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a small, lightweight, low-cost zoom lens suitable for a camera, particularly a compact camera using a silver halide film.

[Prior Art] In recent years, zooming of a photographing lens has been progressing in a silver halide compact camera, and a camera using a zoom lens is expected to become a mainstream in the future due to an advantage of increasing photographing variations. You.

An example of a small zoom lens having a zoom ratio of about 2 and being described in Japanese Patent Application Laid-Open No. Sho 62-264019 is known. This zoom lens is composed of a first lens unit having a positive refractive power and a second lens unit having a negative refractive power in order from the object side, and performs zooming by changing the distance between the two units. It is a compact lens system. However, the number of lens components is as large as eight, which is expensive, and the overall length is not sufficiently short because the telephoto ratio at the wide-angle end is about 1.3.

Therefore, in the existing compact camera with zoom lens, the proportion of the zoom lens occupied by
Since both weight and cost are large, it is very important to reduce the size, weight and cost of this zoom lens.

Plasticizing the lens is a very effective means for reducing the weight and cost of the lens system. However, the types of plastics that can be used for optics at present are extremely limited, and those that can be used have a low refractive index of about 1.5 to 1.6. Therefore, it is not easy to adopt a plastic lens in lens design.
That is, when a plastic lens is simply replaced with a conventional type lens system, the performance is greatly deteriorated.

For example, as a small zoom lens system having a small number of lenses, there is one disclosed in Japanese Patent Application Laid-Open No. 57-201213. This conventional zoom lens includes, in order from the object side, a positive first unit and a negative second unit, where the first unit is positive,
The negative and positive three lenses and the second group are composed of a total of five lenses including two positive and negative lenses, which is a small number of lens systems. The lens configuration as in this conventional example is a very advantageous lens configuration for downsizing the lens system. However, this conventional zoom lens has a variable magnification ratio of 1.5, which is small. Further, the refractive index of each lens is about 1.7, and it is not possible to replace a plastic lens having a low refractive index as described above.

When the zoom ratio is set to 2 and the telephoto ratio at the wide-angle end is set to about 1.2 with the same lens configuration as the above-mentioned conventional example, the power of the first and second units becomes extremely large, and spherical aberration and distortion are caused. , Astigmatism, and coma are all large, and the imaging performance is significantly reduced.

[Problems to be Solved by the Invention] The present invention is to provide a zoom lens for a compact camera having a zoom ratio of about 2, which simultaneously achieves a reduction in size, weight, and cost and has good optical performance. It is intended for.

Means for Solving the Problems In order to achieve the above object, a zoom lens according to the present invention has a first lens of a positive lens having a convex surface facing the object side in order from the object side and a concave surface facing the object side. A first lens unit composed of a second lens of a negative lens and a third lens of a biconvex lens and having a positive refractive power as a whole, and a fourth lens of a positive meniscus lens having a concave surface facing the object side and a concave surface facing the object side. In a lens system comprising a second lens unit having a fifth negative lens and having a negative refractive power as a whole and performing zooming by changing an interval between the first unit and the second unit, the first lens to the fifth lens unit may be used. All of the lenses are homogeneous, at least three of which are plastic, have a diaphragm between the first and second groups, and each group has at least one surface. Having an aspheric surface, especially The third
The lens has an aspherical surface.

The zoom lens according to the present invention has the first
The first group and the second group are composed of a total of five lenses including three positive, negative and positive lenses and two positive and negative two lenses, and using at least three plastic lenses. At least one for each
By appropriately arranging the aspherical surfaces, a lens system which is more excellent in zoom ratio and telephoto ratio at the wide-angle end than in the conventional example and in which various aberrations are excellent can be obtained.

Among these aspheric surfaces, the aspheric surface used for the first lens group is mainly effective for correcting spherical aberration. In particular, when used on a surface close to the stop, only the spherical aberration can be controlled. The shape of the aspheric surface needs to be such that spherical aberration is corrected, that is, the positive refractive index decreases as the distance from the optical axis increases.

The aspherical surface used for the second lens group is effective mainly for correcting off-axis aberrations such as astigmatism and distortion. The shape of the aspherical surface used here needs to be such that negative distortion is corrected, that is, the negative refractive power becomes weaker as the distance from the optical axis increases.

It is desirable that these aspheric surfaces satisfy the following conditions (1) and (2).

(1) 5 × 10 ⁻⁴ <(ΣΔx _I ) / f _W <1 × 10 ⁻² (y = y _3.7 ) (2) 5 × 10 ⁻³ <(ΣΔx _II ) / h <1 × 10 ⁻¹ ( y = y _EC ) where Δx _I is the amount of deviation of the aspheric surface used for the first unit from the reference sphere, Δx _II is the amount of deviation of the aspheric surface used for the second unit from the reference sphere, and f _W is the total amount of the lens at the wide-angle end. The focal length of the system,
h is the maximum image height, y is the height from the optical axis, y _3.7 is the marginal ray height of F / 3.7 at the wide-angle end of the first group of aspheric surfaces, y
_EC is the principal ray height at the maximum angle of view at the wide-angle end in the aspheric surface of the second lens unit. The signs of Δx _I and Δx _II are positive for Δx _{I in the} direction in which the positive refractive power is weakened, and positive for Δx _{II in the} direction in which the negative refractive power is weak.

If the lower limit of the above condition (1) is exceeded, the spherical aberration will be undercorrected. If the upper limit is exceeded, the correction will be overcorrected.

If the lower limit of the condition (2) is exceeded, the negative distortion becomes excessive and the astigmatism worsens on the wide-angle side. If the upper limit is exceeded, the negative distortion becomes overcorrected on the wide-angle side and the astigmatism and coma Aberration worsens.

Next, in the zoom lens of the present invention, many plastic lenses are used. Currently, plastic materials that can be used for optics, such as plastic lenses, have low refractive index and low dispersion (typically acrylic) and medium refractive index (typically polycarbonate) with low refractive index and low dispersion (about 1.5 and Abbe number about 60). , High dispersion (refractive index about 1.6, Abbe number about 30).

When a plastic lens is applied to the zoom lens of the present invention, a lens having a low refractive index and a low dispersion is suitable for the first lens, the third lens, the fourth lens, and the fifth lens in consideration of correction of chromatic aberration. A medium-refractive-index, high-dispersion lens is suitable for the two lenses. If a high-dispersion lens other than the second lens is used, the chromatic aberration becomes excessive, and if a low-dispersion lens is used for the second lens, the chromatic aberration is insufficiently corrected.

From the above, it is desirable to satisfy the following condition (3).

(3) ν ₂ <45 where ν ₂ is the Abbe number of the second lens.

If the condition (3) is further deviated, chromatic aberration will be insufficiently corrected.

As described above, when a plastic lens is used in the lens system of the present invention, each of the first lens, the third lens, and the fourth lens has a low refractive index and a low dispersion. In this case, since the positive lens has a low refractive index, the Petzval sum tends to have a large positive value, and it is difficult to maintain good image surface properties. This is difficult to correct even if an aspherical surface is used.

In order to correct the Petzval sum and maintain the image surface property, it is necessary to use a material having a low refractive index for the fifth lens, which is a negative lens. Refractive index n ₅ of the fifth lens For that purpose it is preferable to satisfy the following condition (4).

(4) When n ₅ exceeds the range of <1.6 Condition (4), when intensive plastic lens, the Petzval sum image surface resistance becomes a large value of the positive deteriorates.

In the lens system of the present invention, it is desirable to satisfy the following conditions (5) and (6) in order to better correct various aberrations.

(5) −0.3 <f ₅ / f ₄ <0 (6) −6 <f _W / r ₁₀ <−3 where f ₄ and f ₅ are the fourth lens and the focal length of the fifth lens, respectively, and r ₁₀ is the fourth lens. The radius of curvature of the object-side surface of the five lenses.

Condition (5) defines the ratio of the focal lengths of the fourth lens and the fifth lens forming the second group.

In order to obtain a zoom lens having a zoom ratio of about 2 and a telephoto ratio at the wide-angle end of about 1.2, which is the object of the present invention, a large negative power is required for the second lens unit. When calculating this power in the paraxial theory -1 / f _w becomes longitudinal, it made it to be shared by the fourth lens and the fifth lens. Here, when the positive power of the fourth lens is increased, the negative power of the fifth lens is increased accordingly, and the generation of higher-order aberrations becomes excessive. Conversely, when the positive power of the fourth lens is weakened, the ability to correct the aberration generated in the fifth lens is lost. Therefore the fourth
The power distribution between the lens and the fifth lens is important, and it is preferable to satisfy the condition (5).

If the lower limit of the condition (5) is exceeded, especially the astigmatism will worsen, and the decentering of both the fourth lens and the fifth lens will tend to be more pronounced.

Condition (6) is a condition for defining the radius of curvature of the object-side surface of the fifth lens.

In the zoom lens of the present invention, when correcting aberrations, it is important to balance off-axis aberrations on the wide-angle side with spherical aberrations and coma on the telephoto side. Condition (6).

When the value exceeds the lower limit of the condition (6), astigmatism particularly at the wide-angle end deteriorates. When the value exceeds the condition, spherical aberration and coma at the telephoto end deteriorate, which is not preferable.

With the zoom lens according to the present invention, all five lenses can be made of plastic by adopting the above-described configuration. If all five lenses are made of plastic lenses, they are very low cost and lightweight, but are susceptible to temperature and humidity. In other words, a change in temperature or humidity changes the refractive index or shape of the lens, resulting in defocus. This is not preferable because a compact camera of a type in which focusing is performed by driving a lens to a preset position does not achieve focusing. However, the correction can be made in combination with an autofocus mechanism as shown in, for example, JP-A-62-111223, so that it is not fatal.

When one or two glass lenses are used in the lens system of the present invention, it is desirable to satisfy the following conditions (7) and (8).

(7) n _i <1.75 (8) v _i > 45 where n _i and v _i are the refractive index and Abbe number of the ith lens, respectively.

Exceeding the range of the condition (7) is not preferable because the Petzval sum takes a large negative value and the image surface is deteriorated. When the value exceeds the range of the condition (8), chromatic aberration becomes excessively large, which is not preferable.

When one or two of the five lenses are replaced with glass lenses in the lens system of the present invention, the effects of temperature and humidity can be made very small by appropriate selection of this lens. However, if three or more lenses are glass lenses, low cost,
The benefits of weight reduction are reduced.

As described above, the zoom lens of the present invention is based on changing the distance between the first group and the second group to change the magnification. However, aberrations can be more favorably corrected by changing the lens interval in each group minutely during zooming. This is because the degree of freedom in aberration correction is increased, and particularly, astigmatism and the like can be favorably corrected.

Focusing in the zoom lens of the present invention is as follows.
Normally, the whole operation is performed by extending the first group, but it is also possible to perform the operation by moving the second group.

EXAMPLES Next, examples of the zoom lens of the present invention will be described.

Example 1 f = 36.05 to 67.9, F / 3.64 to F / 5.2 Maximum image height 21.6, telephoto ratio at wide-angle end 1.28 r ₁ = 20.9531 d ₁ = 3.0000 n ₁ = 1.49216 ν ₁ = 57.50 r ₂ = 69.4838 d ₂ = 2.0000 _{r 3 = -21.4411 d 3 = 4.0129} n 2 = 1.58362 ν 2 = 30.37 r 4 = 145.2381 d 4 = 3.0820 r 5 = 21.2730 d 5 = 4.2000 n 3 = 1.49216 ν 3 = 57.50 r 6 = -18.0087 ( aspherical) d ₆ = 1.0000 r ₇ = ∽ (aperture) d ₇ = D ₁ (variable) r ₈ = -30.5685 (aspherical surface) d ₈ = 2.6000 n ₄ = 1.49216 ν ₄ = 57.50 r ₉ = -22.8150 d ₉ = 5.2513 r _{10 = -9.7569 d 10 = 2.0000 n} 5 = 1.49216 ν 5 = 57.50 r 11 = -72.1631 aspherical coefficients (sixth _{surface) P = 0.8842, A 2 =} 0, A 4 = 0.67468 × 10 -4 A 6 = - 0.51333 × 10 ^-8 , A ₈ = 0.19994 × 10 ^-8 (8th surface) P = 1.9017, A ₂ = 0, A ₄ = 0.66594 × 10 ^-4 A ₆ = 0.20332 × 10 ^-8 , A ₈ = 0.64631 × 10 ^-8 f 36.05 50 67.9 D ₁ 11.854 6.112 2.200 (ΣΔx _I ) / f _W = 1.0 × 10 ^-3 (ΣΔx _II ) /h=1.3×10 ^-2 f ₅ / f ₄ = −0.141, f _W / r ₁₀ = −3.69 Example 2 f = 36.05 to 67.9, F / 3.64 to F / 6.9 Maximum image height 21.6, Telephoto ratio at wide-angle end 1.22 r ₁ = 21.1119 d ₁ = 3.0000 n ₁ = 1.65844 v ₁ = 50.86 r ₂ = 39.3452 d ₂ = 2.0000 r ₃ = -17.7024 d ₃ = 30.3041 n ₂ = 1.58362 ν ₂ = 30.37 r ₄ = 211.2865 d ₄ = D ₁ (variable) r ₅ = 21.9567 d ₅ = 3.4000 n ₃ = 1.49216 ν ₃ = 57.50 r ₆ = -15.1014 (aspherical surface) d ₆ = 1.0000 r ₇ = ∽ (aperture) d ₇ = D ₂ (variable) r ₈ = −26.6032 (aspherical surface) d ₈ = 2.6000 n ₄ = 1.49216 ν ₄ = 57.50 r _{9 = -20.7714 d 9 = 4.9909 r} 10 = -9.5334 d 10 = 2.0000 n 5 = 1.49216 ν 5 = 57.50 r 11 = -42.7519 aspherical coefficients (sixth _{surface) P = 0.4066, A 2 =} 0, A 4 = 0.75646 × 10 ⁻⁴ A ₆ = −0.17438 × 10 ⁻⁶ , A ₈ = 0.26160 × 10 ⁻⁸ (8th surface) P = 1.4686, A ₂ = 0, A ₄ = 0.85086 × 10 ⁻⁴ A ₆ = −0.36793 × 10 ^-7 , A ₈ = 0.91682 × 10 ^-8 f 36.05 50 67.9 D ₁ 1.862 2.504 2.222 D ₂ 12.960 6.143 2.000 (ΣΔx _I ) / f _W = 1.7 × 10 ^-3 (ΣΔ ^{_{x II) /h=1.3×10 -2 f 5 /}} f 4 = -0.152, f W / r 10 = -3.78 Example 3 f = 36.05~67.9, F / 3.64~F / 5.2 maximum image height 21.6, wide Edge telephoto ratio 1.20 r ₁ = 15.1724 d ₁ = 3.6000 n ₁ = 1.49216 ν ₁ = 57.50 r ₂ = 66.7752 d ₂ = 1.7000 r ₃ = -27.2601 d ₃ = 2.9431 n ₂ = 1.71736 ν ₂ = 29.51 r ₄ = 68.5097 d _{4 = 3.4436 r 5 = 27.8714 d} 5 = 3.4000 n 3 = 1.51742 ν 3 = 52.41 r 6 = -18.1090 ( _{aspherical) d 6 = 1.0000 r 7 =} ∽ ( stop) d ₇ = D ₁ (variable) r ₈ = −30.1386 d ₈ = 3.0000 n ₄ = 1.49216 ν ₄ = 57.50 r ₉ = −17.5805 (aspherical surface) d ₉ = D ₂ (variable) r ₁₀ = −9.2526 d ₁₀ = 2.0000 n ₅ = 1.49216 ν ₅ = 57.50 r ₁₁ = −189.5103 Aspheric coefficient (Sixth surface) P = −3.2018, A ₂ = 0 A ₄ = −0.14123 × 10 ⁻⁴ A ₆ = −0.18344 × 10 ⁻⁶ , A ₈ = 0.21423 × 10 ⁻⁸ (No. 9 Surface) P = 2.22519, A ₂ = 0, A ₄ = −0.67556 × 10 ⁻⁴ A ₆ = −0.67061 × 10 ⁻⁶ , A ₈ = −0.20258 × 10 ⁻⁷ f 36.05 50 67.9 D ₁ 10.871 5.372 2.200 D ₂ 4.803 4.457 3.900 (ΣΔx _I ) / f _W = 8.0 × 10 ^-4 (ΣΔx _II ) /h=1.5×10 ^-2 f ₅ / f ₄ = −0.250, f _W / r ₁₀ = −3.90 Example 4 f = 36.05-67.9, F / 3.64-F / 7.0 Maximum image height 21.6, Telephoto ratio at wide-angle end 1.23 r ₁ = 19.2603 d ₁ = 3.0000 n ₁ = 1.49216 ν ₁ = 50.50 r ₂ = 53.0583 d ₂ = D ₁ (variable ) R ₃ = -19.0163 d ₃ = 4.0043 n ₂ = 1.58362 ν ₂ = 30.37 r ₄ = 169.8082 d ₄ = D ₂ (variable) r ₅ = 22.1427 d ₅ = 3.4000 n ₃ = 1.49216 ν ₃ = 57.50 r ₆ = − 16.1589 (aspheric surface) d ₆ = 1.0000 r ₇ = ∽ (aperture) d ₇ = D ₃ (variable) r ₈ = -33.3918 (aspheric surface) d ₈ = 2.6000 n ₄ = 1.49216 ν ₄ = 57.50 r ₉ = -22.8566 d ₉ = D ₄ (variable) r ₁₀ = −9.7414 d ₁₀ = 2.0000 n ₅ = 1.49216 ν ₅ = 57.50 r ₁₁ = −80.5917 (aspherical surface) Aspherical surface coefficient (Sixth surface) P = 0.5168, A ₂ = 0 , A ₄ = −0.72823 × 10 ⁻⁴ A ₆ = −0.25576 × 10 ⁻⁶ , A ₈ = 0.40774 × 10 ⁻⁸ (8th surface) P = 0.762, A ₂ = 0, A ₄ = 0.71911 × 10 ⁻⁴ A ₆ = -0.96400 x 10 ^-6 , A ₈ = 0.83133 x 10 ^-8 (Surface 11) P = 11.2777, A ₂ = 0, A ₄ = -0.21558 x 10 ^-6 A ₆ = -0.35885 x 10 ^-7 , A ₈ = 0.18755 × 10 ^-9 f 36.05 50 67.9 D ₁ 2.096 1.700 1.851 D ₂ 1.906 2.916 2.629 D ₃ 12.472 6.209 2.200 D ₄ 4.900 4.811 4.946 (ΣΔx _I ) / f _W = 1.5 × 10 ^-3 (ΣΔx _II ) / h = 1.4 × 10 ^-2 f ₅ / f ₄ = −0.167, f _W / r ₁₀ = −3.70 In the above data, r ₁ , r ₂ ,... are the radii of curvature of each lens surface, d ₁ , d ₂ ,. Is the wall thickness and air space of each lens, n ₁ , n
_2, ... is the refractive index of each lens, [nu _1, [nu _2, ... is the Abbe number of each lens.

The first embodiment has the lens configuration shown in FIG. 1, and changes only the distance between the first and second units during zooming. The lenses are all plastic lenses, only the second lens is polycarbonate, and all others are acrylic. The aspheric surface is used for the image-side surface of the third lens and the object-side surface of the fourth lens.

The polycarbonate used for the second lens has a higher anomalous dispersion than ordinary glass, and the Hertzberger equation for calculating the refractive index of each wavelength from the d-line refractive index and Abbe number does not hold. For reference, the refractive indices of the four main wavelengths are as follows.

n _d = 1.58362 n _c = 1.57809 n _F = 1.59731 _ng = 1.60888 Next, the effects of temperature and humidity when using a plastic lens will be described.

The refractive index and shape of a plastic lens change due to changes in temperature and humidity, thereby causing defocus.
This defocus is greater due to a change in the refractive index than a change in the shape. However, the resulting defocus can be sufficiently corrected by an appropriate combination of a positive lens and a negative lens.

The refractive index of plastic is approximately -10
^-4 / ° C. For example, if the temperature changes by ± 30 ° C., the refractive index changes by about 0.003. In the lens system of Example 1, when the focus shift amount when the temperature changes by ± 30 ° C. and the refractive index changes by 0.003 is calculated, the focus shift amount is from ± 0.3 mm (wide angle end) to ± 0.8 mm.
mm (telephoto end). This value is a value that does not cause any practical problem even if the influence of humidity is included.

The aberrations of the first embodiment at the wide-angle end, the intermediate focal length, and the telephoto end with respect to an object at infinity are shown in FIGS. 5, 6, and 7, respectively.
As shown in the figure. FIGS. 8 and 9 show aberrations at the wide-angle end, the intermediate focal length, and the telephoto end, respectively, when the second unit is moved to focus on an object 2 m from the front surface of the lens.
As shown in FIG.

Example 2 is a lens system having a lens configuration shown in FIG.
At the time of zooming, the distance between the first lens unit and the second lens unit is changed, and the distance between the second lens and the third lens in one lens unit is also slightly changed. In this embodiment, only the first lens is a glass lens and all are plastic lenses. The aspheric surface is used for the image-side surface of the third lens and the object-side surface of the fourth lens.

When the effects of temperature and humidity in this embodiment are calculated in the same manner as in Embodiment 1, the focus shift amount is ± 0.2 mm (wide-angle end) to ± 0.4 mm.
(Telephoto end). This is an effect of using the glass lens as the first lens in about half of the first embodiment.

The aberrations at the wide-angle end, the intermediate focal length, and the telephoto end of an object at infinity in this embodiment are shown in FIGS. 11, 12, and 13, respectively.
As shown in the figure.

Example 3 is a lens system having a lens configuration shown in FIG.
At the time of zooming, the distance between the first lens unit and the second lens unit is changed, and the distance between the fourth lens and the fifth lens is slightly changed.
In this embodiment, the second lens and the third lens are glass lenses, and the remaining lenses are plastic lenses. The aspheric surface is used for the image side surface of the third lens and the image side surface of the fourth lens.

When the effects of temperature and humidity in this embodiment are calculated by the same method, the focus shift amount is ± 0.2 mm (wide-angle end) to ± 0.4 mm (telephoto end), which is almost the same as that of the second embodiment and the focus shift amount is small. . This is also due to the use of a glass lens.

In this embodiment, the total length of the lens system is very short,
When converted to a telephoto ratio at the wide-angle end of 43.26 mm, it is 1.20.

Wide-angle end for an object at infinity in this embodiment. The aberrations at the intermediate focal length and telephoto end are shown in Figs.
This is as shown in FIG.

Example 4 is a lens system having a lens configuration shown in FIG.
At the time of zooming, the distance between the first lens unit and the second lens unit is changed, and all other lenses are also changed by a small amount. In this embodiment, all lenses are plastic lenses. The aspherical surface is used for the image side surface of the third lens, the object side surface of the fourth lens, and the image side surface of the fifth lens.

The effects of temperature and humidity of this embodiment are almost the same as those of the first embodiment.

The aberrations at the wide-angle end, the intermediate focal length, and the telephoto end of an object at infinity in this embodiment are shown in FIGS. 17, 18, and 19, respectively.
As shown in the figure.

The shape of the aspherical surface used in each of the above embodiments is represented by the following equation when the x-axis is taken in the optical axis direction and the y-axis is taken in a direction perpendicular to the optical axis with the intersection point with the optical axis taken as the origin.

Here, C is the curvature of the reference spherical surface, and P and A _2i are coefficients.

[Effects of the Invention] The zoom lens of the present invention is a small, lightweight, low-cost lens system having a zoom ratio of about 2, the number of lenses is extremely small and five, and many plastic lenses are used, and the optical performance is also extremely good. It is.

[Brief description of the drawings]

1 to 4 are sectional views of Examples 1 to 4 of the present invention, FIGS. 5 to 10 are aberration curve diagrams of Example 1, and FIGS.
FIG. 13 to FIG. 13 are aberration curve diagrams of the second embodiment, and FIG. 14 to FIG.
FIG. 17 is an aberration curve diagram of the third embodiment, and FIGS. 17 to 19 are aberration curve diagrams of the fourth embodiment.

Claims (2)

(57) [Claims] 1. A positive refraction comprising a first lens of a positive lens having a convex surface facing the object side in order from the object side, a second lens of a negative lens having a concave surface facing the object side, and a third lens of a biconvex lens. A first group having a positive power, a fourth lens of a positive meniscus lens having a concave surface facing the object side, and a fifth lens of a negative lens having a concave surface facing the object side. In a lens system that performs zooming by changing the distance between the first and second groups, all of the first to fifth lenses are homogeneous lenses, and at least three of the lenses are uniform. Plastic, having a stop between the first group and the second group, and having at least one aspheric surface in each group, and particularly having an aspheric surface in the third lens. Zoom lens. 2. The zoom lens according to claim 1, wherein a surface of said first group closest to said stop has an aspheric surface.