Highly Variable Magnification Zoom Lens System Having a Diaphragm With Its Apertures Diameter Varied in Relation With Zooming

The present invention is related to a zoom lens with its magnification variation range being able to be consecutively enlarged, wherein chiefly the halo generated by the use of a means to enlarge the magnification variation range is prevented.

A zoom lens is composed by a front zoom part to determine the magnification variation (ratio) of focal distance and a rear fixed relay part to determine the magnification variation range of focal distance. And a zoom lens, in which said rear fixed relay part is replaced with another zoom lens for enlarging the magnification variation range, is known as a so-called double zoom lens.

An object of the present invention is to prevent the halo or coma (aberration) generated in a highly variable mangification zoom lens system which consists of a front zoom part and a rear part and is made as a zoom lens having a diaphragm at its front end by providing such plural number of lens systems as can be relatively moved from one direction to the other within said rear part. For achieving this object, a diaphragm for adjusting oblique luminous flux, varying the aperture diameter along with the movable lens is provided.

FIG. 1 is a drawing to show a specific example of the present invention.

FIG. 2A and FIG. 2B are drawings to explain the state in which halo or coma (aberration) is generated.

FIG. 3 is a drawing, for example, of a zoom lens to which the present invention is applied.

FIG. 4A through FIG. 4F are drawings for various aberrations of a lens system shown in FIG. 3, wherein FIG. 4A is a curve drawing for spherical aberration, FIG. 4B is a curve drawing for astigmatism, FIG. 4C is a curve drawing for distortion of image, and FIG. 4D, 4E and 4F are curves for lateral aberration.

In FIG. 1, I is a front zoom part, II is a rear zoom part which is coupled with the front zoom part. The front zoom part I has a focusing lens 1, a variator lens 2, and a compensator lens 3. And the rear zoom part II has a fixed lens 21, a movable lens 22 to correct the shifting of image, a movable lens 23 having a function of varying focal distance, and a fixed imaging lens 24. 11 is a diaphragm to determine a relative aperture corresponding to the brightness of an object.

On the other hand, 20 is a diaphragm for adjusting oblique luminous flux and is provided within the fixed imaging lens 24. This diaphragm 20 has the size of aperture varied by a "diaphragm opening and closing means" 26 which is moved by a means to shift the movable lenses 22 and 23. As a means to shift such lens, for example, a cam ring is used and the rotation of said cam ring moves a diaphragm opening and closing lever through a combination of gears or a link mechanism.

Firstly, functions of this optical system will be explained. It is suppposed that the zooming ratio of the front zoom part I is 10 times, and the zooming ratio of the rear zoom part II is two times, and the shortest focal distance of the total lens system which is made up with the zoom parts I and II is 20 mm.

Since the front zoom part I is operated in zooming the focal distance of the total lens system changes from 20 mm to 200 mm, then, as the rear zoom part II is operated in zooming, the focal distance of the total lens system is changed from 200 mm to 400 mm. This zoom lens system can thus be used as a zoom lens of 10 times magnification having any desired range of focal distance within a range of 20 mm to 400 mm by the rear zoom part II at any desired fixed zooming position and operating the front zoom part I for zooming.

As this rear zoom part II has a diaphragm 11 in front of the movable lenses 22 and 23, it becomes a zoom lens of front diaphragm type. Here, if the diaphragm 11 is positioned behind the movable lens 23, the distance from the front lens 1 of the front zoom part I to the diaphragm becomes long and the incident position of the oblique luminous flux which passes through the front lens 1 of the front zoom part I will be largely apart from the optical axis, therefore the lens diameter will become very large to cover the same and will be difficult to be used in photographing. And in a zoom lens of front diaphragm type, great change will take place in a circumferential light volume for a picture by the zooming operation. The reason therefor will be explained now. FIG. 2A shows a case of the shortest focal distance in such zoom lens of front diaphragm type as the rear zoom part II, while FIG. 2B shows the case of the longest focal distance in the same zoom lens. In the drawings, 11 is a diaphragm and 20 is a diaphragm for adjusting oblique luminous flux. 101 is a secondary principal plane of this zoom lens while 102 is a lens holding ring of a lens barrel to determine the effective diameter of the zoom lens.

In the case of the shortest focal distance (FIG. 2A), the luminous flux .omega..sub.o on the axis will have its size of the luminous flux determined by the diaphragm 11, while in the off-axis rays .omega..sub.1, the lower light beam L.sub.1 is restricted by the diaphragm 11 and the upper light beam L.sub.2 is restricted by the effective diameter which is determined by the member 102. If at that time the upper light beam L.sub.2 is made to be shielded by the diaphragm 11, the vignetting by the member 102 will not take place, but the aberration correction of the lens will become very difficult on the other hand. Therefore in a zoom lens of front diaphragm type occurrence of vignetting is tacitly permitted.

Next, as zooming operation is done, the secondary principal plane 101 moves to the direction of the diaphragm 11 as shown in FIG. 2B and the imaging plane position is kept constant. In the case of the longest focal distance both the axial luminous flux .omega..sub.o and the off-axial rays .omega..sub.1 will have their size of luminous flux determined by the diaphragm 11 and vignetting will not take place, but halo or comatic flare will take place deteriorating imaging characteristics. While the reason for this kind of characteristics deterioration is that the circumferential light volume varies, the number of lenses composing the zoom lens must be greatly increased to correct aberration for preventing said variation, thus it is inconvenient.

In the present invention, when zooming operation is done for the rear zoom part II, the oblique luminous flux adjusting diaphragm 20 which is provided behind the movable lens 23 is so adjusted through the means 25 and 26 that the aperture diameter is large in the case of the shortest focal distance and the aperture diameter is small in the case of the longest focal distance (FIG. 1). And as a result, the movable lenses 22 and 23 are shifted thereby changing the focal distance, and as the focal distance becomes long the aperture of the diaphragm 20 gradually shrinks shielding the upper light beam of off-axis rays so that the circumferential light volume is always kept constant.

Next, one example of figures in accordance with FIG. 3 wherein the present invention is applied to TV lens will be shown below. The numbers and marks in this drawing correspond to those in FIG. 1, wherein A is 21, B is 22, C is 23 and D is 24. Focal distance : 24 to 800 Zoom ratio: 1 : 33.3 Maximum relative aperture: 1 : 1.8 to 1 : 4.6 (shortest focal distance - longest focal distance)

In the above table, .gamma. is radius of curvature. d is a thickness of lens or lens interval. n is a refractive index of lens. .nu. is Abbe number.

Figures with asterisk mark * show the cases of the focal distance being the shortest, middle and the longest. The mark shows a color resolving prism system.

The diaphragm to determine relative aperture will become the maximum aperture 64.7 mm.theta. at 1.5 mm behind the .gamma.20 in FIG. 3. The oblique luminous flux adjusting diaphragm will be 47.8 mm .theta. to 28.5 mm .theta. (shortest focal distance to longest focal distance) at 0.1 mm behind .gamma.45.

FIG. 4A, FIG. 4B and FIG. 4C show the spherical aberration, astigmatism and distortion aberration, for f = 24 mm, f = 234.2 mm, f = 800 mm in the total lens system in the above mentioned example. FIG. 4D, 4E and 4F show lateral aberration of the same lens system, and the hatched parts within the drawings show such portions as being shielded by the oblique luminous flux adjusting diaphragm according to the present invention, and as such portions are cut out, the quality of picture is improved.

As has been explained above according to the present invention, without necessity for replacing the rear fixed relay part or installing magnification amplification optical system, the magnification variation range of focal distance can be consecutively varied, and an oblique luminous flux adjusting means is activated in connection with the use of a means to vary the magnification variation range at that time, thus the upper light beam of the off-axis rays is properly shielded.