JPS5820007B2 - Retrofocus Gatakouka Cleanse - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a retrofocus wide-angle lens for single-lens reflex cameras, and particularly relates to a retrofocus wide-angle lens with an aperture ratio of 1:2.8, an angle of view of 75 degrees or more, and a long back focus. be.

Conventional retrofocus wide-angle lenses often use an extremely thick lens after the front group diverging system.
This made it possible to design a lens system with good aberrations, which is one of the features.

However, arranging such a thick lens in the lens system results in narrowing the distance between the lenses, which tends to cause strain on the structure of the lens barrel.

Furthermore, when a thick lens is used, there is a risk that performance may deteriorate due to eccentricity.

In view of the above points, the present invention provides a retrofocus type wide-angle lens in which a thin lens is placed after the front group diverging system to provide a sufficient air gap, and aberrations such as astigmatism are well corrected. be.

The lens system of the present invention is a front group diverging system consisting of a positive meniscus lens having a convex surface facing the object side, and a second lens and a third lens having negative meniscus lenses having convex surfaces facing the object side. 7 elements in 7 groups: the 4th lens which is a biconvex lens placed after that, the 5th lens which is a biconcave lens, the 6th lens which is a positive meniscus lens with its concave surface facing the object side, and the 7th lens which is a biconvex lens. This is a lens system with a configuration.

and the image side surface of the second lens, the object side surface of the second lens,
The radius of curvature of both surfaces of the third lens and both surfaces of the fifth lens is r2, r3 tr5 tr6 yrg + rlQ + the distance between the third and fourth lenses is d6, and the thickness of the fourth lens is d7. When the refractive index of the fourth lens is n4, it is characterized by satisfying the following conditions.

Among these conditions, condition (4) relates to one of the features of the present invention, which is to make the fourth lens thinner, and also allows for good aberration correction while maintaining the back focus of the lens system for a long time. It is something.

In other words, if d6/d7 exceeds the lower limit of the condition, the above-mentioned feature of making the fourth lens thinner and narrowing the air gap cannot be obtained.

Moreover, if the upper limit is exceeded, the sine condition deteriorates.

Furthermore, even if the conditions regarding d6i are satisfied, if d7 becomes smaller than 0.28f, it becomes impossible to maintain a long back focus and improve aberrations.

Moreover, condition (5) defines the refractive index of this fourth lens, and when the fourth lens is made to satisfy the above-mentioned condition (4), if the upper limit of this condition (5) is exceeded, , comatic aberration worsens.

Moreover, exceeding the lower limit is not favorable for the Petzval sum.

Next, condition (1) is a condition that defines the shape of the air lens formed between the first lens and the second lens, and when a thin lens is used as the fourth lens as described above, this air lens It is preferable to make the difference in the radius of curvature of the two surfaces forming the surface relatively large.

In other words, when the lower limit of condition (1) is exceeded and the radii of curvature of both surfaces become close to each other, the meridian plane at the intermediate angle of view will deviate significantly to the minus side.

However, if the difference is too large and exceeds the upper limit of the conditions, the curvature of the spherical image plane becomes strong and the performance in the peripheral area deteriorates significantly.

Condition (2) also defines the shape of the third lens.

When this exceeds the upper limit, the negative power becomes stronger and it is advantageous to lengthen the back focus, but spherical aberration worsens, and when it exceeds the lower limit, the negative power becomes weaker and the back focus becomes longer (in order to maintain it). This is not preferable because the load on the second lens becomes too large.

Finally, condition (3) is a condition that defines the shape of the fifth lens, and among them, rg <o indicates that it is a biconcave lens. When this becomes positive, the refraction of off-axis rays becomes stronger, so the present invention The lens configuration makes it difficult to correct aberrations.

If 0/frgl exceeds the lower limit of the condition, the curvature of the spherical image becomes dog-like, and if it exceeds the upper limit, the meridional image plane shifts to the negative side and coma aberration worsens.

Examples of the lens system of the present invention described above are shown below.

Example 1 As in the above example, the lens system of the present invention has an aperture ratio of 1:2.
．． 8. Back focus is 1.37 when the angle of view is 75° or more
It is an extremely long lens system with f~1.43 f and over 1.37 f.

In addition, the lens system is compact and has sufficient lens spacing, and is a high-performance retro-Zukas semi-wide-angle lens, as shown in FIGS. 2 to 6, which show the aberrations of each example.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of the wide-angle lens of the present invention, and FIGS. 2 to 6 are aberration curve diagrams of each embodiment.

Claims (1)

[Claims] 1. A positive meniscus lens with a convex surface facing the object side, a second and third negative meniscus lens with both convex surfaces facing the object side, a biconvex fourth lens, and a biconcave lens. A compact retrofocus type, comprising a fifth lens, a positive meniscus sixth lens with a concave surface facing the object side, and a seventh lens a biconvex lens, and is characterized by satisfying the following conditions: Wide-angle lens, r21 r3, r5 Hr61 rg H is the image side surface of the third lens, the object side surface of the second lens,
Radius of curvature of both surfaces of the third lens and both surfaces of the fifth lens, d6
td7 is the air gap between the third lens and the fourth lens and the thickness of the fourth lens, respectively, and n4 is the refractive index of the fourth lens.