KR0148149B1 - Low magnification view finder lens - Google Patents

Abstract

The present invention relates to a lens for enlarging a small image, and more particularly to a low magnification viewfinder lens used in a camera having an enlargement factor of about 2 times. The apparatus includes a first lens having a negative refractive power disposed on the screen side, and a second lens having a positive refractive power disposed at a predetermined distance from the first lens, and the second lens moves along the optical axis. Diopter adjustment is possible according to the user's eye difference. This apparatus reduces the diopter adjustment amount by using two lenses having different refractive powers, and has an effect of improving image performance by improving distortion and chromatic aberration.

Description

Low magnification viewfinder lens

1 is an optical path diagram of a conventional viewfinder lens.

2 is an optical path diagram of a viewfinder lens according to the present invention.

3 (a) is a distortion aberration diagram at -5 diopters.

3 (b) is a distortion aberration diagram at +2 diopters.

* Explanation of symbols for main parts of the drawings

10: cathode ray tube L1, L2, L3: lens

r1-r4: radius of curvature d1-d4: thickness or spacing

P: dispersion

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lens for enlarging a small image, and more particularly to a low magnification viewfinder lens having a magnification of about 2 times, such as a viewfinder lens used for a broadcast camera.

In general, a viewfinder is used to view an image photographed by a camera such as a still camera or a movie camera. The viewfinder consists of a monitor on which an image captured by the camera appears and a lens that enlarges the image displayed on the monitor so that the user can easily see it. In addition, the broadcast camera has a superior performance than the general home movie camera. That is, the horizontal resolution is 800TVL or more, and accordingly, the viewfinder is required to have the same level. Therefore, most broadcast cameras use a high resolution Cathode-ray tube (hereinafter referred to as CRT) in the viewfinder. Smaller CRTs are mechanically advantageous, but the screen size should be approximately 1.5 inches to achieve horizontal resolutions above 800 TVL. The image formed on such a small monitor is magnified through the viewfinder lens and viewed by the user. At this time, the magnification of the lens is about 2 times, and thus the focal length should be about 120 mm.

In an optical device such as a camera, the lens may be an essential element. In particular, the performance of the lens that the user sees by magnifying the image, such as the viewfinder lens of the camera, is the performance of the device, and countless efforts are being made to develop a lens that performs better. On the other hand, the most important factor to improve above all to improve the performance of the lens can be said to improve the aberration. One conventional example of this is shown in FIG.

FIG. 1 shows the optical path diagram of a conventional viewfinder lens, and the two-dotted line shows the lens position at the time of diopter adjustment. The lens L1 shown here represents the lens position at -4 diopters, and the lens L1 'represents the lens position at +2 diopters. Reference numeral E denotes the position of the eye of the user. As shown, in order to enlarge and show the image of the CRT 10 to the user, one lens L1 is disposed between the CRT 10 and the user's vision. At this time, the lens L1 moves along the cam curve to adjust the diopter according to the user's eye difference to focus on the position E of the user's eyes.

However, such a conventional low magnification viewfinder lens has a problem in that the chromatic aberration caused by the distortion aberration representing the degree of distortion of the image and the refractive index difference according to the wavelength of the lens is bad when one lens is used. In addition, there was a limit in improving the resolution with one lens.

Accordingly, an object of the present invention is to provide a low magnification viewfinder lens capable of correcting according to the eye difference of the majority of users by improving the aberration characteristic of the lens to improve image performance and reducing the diopter adjustment amount.

The low magnification viewfinder lens according to the present invention for achieving the above object has a negative refractive power between the position of the CRT and the user's eye and is disposed at a fixed position and a predetermined distance from the first lens. And a second lens having a positive refractive power which is disposed at a predetermined position and is movable along the optical axis between the position of the first lens and the user's eye.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is an optical path diagram of a low magnification viewfinder lens according to the present invention. The low magnification viewfinder lens of FIG. 2 includes two first and second lenses L2 and L3. Here, the dashed-dotted line shows the position of the lens according to the diopter adjustment and represents the diopter adjustment amount according to the eye difference of each user. As illustrated, the first lens L2 having negative refractive power is disposed on the CRT 10 side, and the position of the first lens L2 and the user's eye at a predetermined interval from the first lens L2 ( A second lens L3 having a positive refractive power is disposed between E '). In this way, the two magnifications are configured so that the total magnification is about twice. At this time, the CRT 10, the first lens L2, and the eye position E ′ are fixed, and the second lens L3 is movable.

Light rays emitted from the CRT 10 pass through the first lens L2 having negative refractive power and enter the second lens L3. The incident light rays pass through the second lens L3 to have a positive refractive power and focus on the user's eyes. That is, the focus is at the eye position E '. At this time, the second lens L3 is moved along the cam curve in the optical axis direction according to the user's eye difference, and the focal length is adjusted. At this time, the diopter adjustment amount D is determined by the focal length f and satisfies the following equation.

^{D = f 2/1000 (㎜} )

On the other hand, in the conventional viewfinder, the focal length is required to be about 120 mm to enlarge the image by about 2 times. In addition, in order to fit most users' eyes, ± 4 diopter amount is required, and to obtain this diopter amount, a lens adjustment amount of 96.8 mm is required. However, mechanically, this adjustment amount is difficult to obtain, and only ± 2 diopters are secured. In order to solve this problem, the device uses two lenses to reduce the amount of lens adjustment according to the diopter amount. Aberrations for this are shown in FIGS. 3 (a) and 3 (b). FIG. 3 (a) shows the aberration diagram at −5 diopters and FIG. 3 (b) shows the aberration diagram at +2 diopters.

In the optical system, the distortion aberration is minimal when the image distance and the object distance are similar. However, in the general optical system, the image distance and the object distance are different, so that the distortion aberration is corrected by adjusting the arrangement of the shape and the refractive power of the lens. That is, the amount of distortion is reduced by canceling the amount of positive distortion caused by the first lens L2 having negative refractive power and the amount of negative distortion caused by the second lens L3 having positive refractive power.

In addition, the chromatic aberration correction of the lens is corrected by combining various types of optical sheaths. In the present apparatus, a chromatic aberration is ensured by using a first refractive index material having a high refractive index and low dispersion and a second refractive index material having a low refractive index and large dispersion. In addition, by optimizing the shape of the lens to improve the image performance of the peripheral as well as the center. Lens data of such a viewfinder lens is shown in Table 1.

Here, the distance d3 between the first lens L2 and the second lens L3 includes the moving distance of the second lens L3.

The first and second lenses L2 and L3 according to the present invention satisfy the following equation.

| F2 | | f | | f1 |

Here, f1 represents a focal length of the first lens L2, f2 represents a focal length of the second lens L3, and f represents an entire focal length.

As described above, the low magnification viewfinder lens according to the present invention has the effect of reducing the diopter adjustment amount by reducing the conventional one diopter adjustment amount of 9.94 mm to 4.55 mm by using two lenses having different refractive indices. have. In addition, although the conventional distortion aberration is within ± 1.5%, the apparatus has an advantage of reducing the distortion aberration to within ± 0.6%. In addition, it has an effect of improving image performance by improving aberration, such as distortion and chromatic aberration.

Claims (4)

A viewfinder lens for enlarging an image of a screen, comprising: a first lens disposed at a fixed position with a negative refractive power between a position of a CRT and a user's eye, and a position spaced a predetermined distance from the first lens And a second lens having a positive refractive power capable of moving along the optical axis between the position of the first lens and the user's eye. The low magnification viewfinder lens of claim 1, wherein the second lens is movable along an optical axis to enable diopter adjustment according to a user's eye difference. The relationship between the focal length of the first lens, the focal length of the second lens, and the total focal length f, which is the sum of the focal length of the first lens and the focal length of the second lens, satisfies the following equation. Low magnification viewfinder lens, characterized in that. | F2 | | f | | f1 | Here, f1 is the focal length of the first lens, f2 is the focal length of the second lens. The low magnification viewfinder lens of claim 1, wherein the first lens is made of a base material having high refractive index and low dispersion, and the second lens is made of a base material having low refractive index and high dispersion.