JP3131828B2 - Ring-shaped ND filter with concave surface - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an annular ND filter having a light transmittance reduced toward the center.

[0002]

2. Description of the Related Art For example, in a CCTV lens, in order to reduce the amount of light beyond the limit of mechanical small aperture (large F value) and obtain an apparently large F value, an annular ND is used.
A filter is used. FIG. 4 is a conceptual diagram showing a circular N having a diameter gradually reduced on the transparent substrate 11.
D filter films (light absorbing films) 12, 13, and 14 are formed concentrically by vapor deposition. Transparent substrate 11
Is usually around 0.5 mm, and the thickness of each circular ND filter film 12 to 14 is on the order of several tenths of μm.
According to the annular ND filter 10, the smaller the mechanical aperture is, the smaller the amount of transmitted light is. Therefore, a large apparent F value can be obtained.

However, this annular ND filter 10
When the image performance is compared between when the mechanical aperture is opened and when the aperture is minimum, the focus shift occurs slightly,
It became clear that the image performance deteriorated.

[0004]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a ring-shaped ND filter which does not cause a focus shift when the mechanical aperture is opened and at the minimum aperture, and does not deteriorate the image performance.

[0005]

SUMMARY OF THE INVENTION The present inventors have concluded that a circular ND filter formed concentrically and laminated is extremely thin on the order of μm, but when viewed as an annular ND filter as a whole, the laminated thickness at the central portion and the peripheral portion is large. However, since the refractive index of the ND filter film is 1.3 to 2.6, the difference in the lamination thickness appears as the difference in the optical path length, and the difference in the optical path length has a kind of convex lens effect. As a result, it was hypothesized that focus movement might occur between the maximum aperture and the minimum aperture. According to this hypothesis, when a concave surface in the direction to cancel the convex lens action was provided on the transparent substrate on the side opposite to the ND filter film laminated surface, the focus movement was improved.

That is, the present invention relates to a ring-shaped ND filter formed by concentrically laminating a circular ND filter film whose diameter gradually decreases on a transparent substrate. On the opposite side of
It is characterized in that a concave surface is provided in a direction in which the convex lens function of the laminated circular ND filter film is canceled.

This concave surface can be formed directly on the transparent substrate or can be formed in a hybrid manner. In the case of direct formation, the concave surface is formed by polishing a glass material or integrally formed on a transparent substrate made of a molded product of a glass material or a resin material during the molding. In the hybrid type, a transparent resin film having a concave surface is formed by lamination on a transparent substrate.

[0008] The annular ND filter of the present invention is effective in combination with a mechanical aperture mechanism that changes the mechanical aperture diameter from the open state to the minimum aperture state.

[0009]

1 and 2 show a first embodiment of an annular ND filter according to the present invention. When the same components as those in the conventional example of FIG.
1 includes ND filter films (light absorbing films) 12 to 14
The concave surface 15 is formed on the surface opposite to the surface on which the laminate is formed. The curvature of the concave surface 15 is determined so as to cancel out the convex lens action by the ND filter films 12 to 14.

The magnitude of the convex lens action (magnitude of refraction action) by the ND filter films 12 to 14 is calculated by performing ray tracing based on the refractive index, diameter and thickness of the ND filter films 12 to 14. be able to. The curvature of the concave surface 15 is determined so as to cancel the convex lens effect of the ND filter films 12 to 14 by this calculation. Concave 15
Can be formed by polishing the transparent substrate 11 made of a glass material, or can be formed integrally with the transparent substrate 11 made of a molded product of a glass material or a resin material at the time of molding.

FIG. 3 shows another embodiment of the present invention. A transparent resin film 1 having a concave surface 15 on a transparent substrate 11 is shown.
6 is a hybrid type formed by lamination.

As described above, by providing the transparent substrate 11 with the concave surface 15 for canceling the convex lens function of the ND filter films 12 to 14, the problem of the lens function is reduced, and the focus movement becomes a negligible level.

[0013]

According to the annular ND filter having the concave surface of the present invention, the focus movement in the open state of the mechanical diaphragm and the minimum diaphragm state can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view taken along line II of FIG. 2 showing one embodiment of an annular ND filter having a concave surface of the present invention.

FIG. 2 is a front view of the same.

FIG. 3 is a cross-sectional view corresponding to FIG. 1, showing another embodiment of the annular ND filter having a concave surface of the present invention.

FIG. 4 is a schematic sectional view corresponding to FIGS. 1 and 3, showing a conventional annular ND filter.

[Explanation of symbols]

 Reference Signs List 10 annular ND filter 11 transparent substrate 12 13 14 circular ND filter film 15 concave surface 16 transparent resin film

────────────────────────────────────────────────── (5) References JP-A-2-73201 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 5/00 G03B 9/02 G03B 11 / 00

Claims (5)

(57) [Claims] An annular ND filter formed by concentrically laminating a circular ND filter film whose diameter gradually decreases on a transparent substrate, wherein the transparent substrate has a surface opposite to a surface on which the circular ND filter film is laminated. A ring-shaped ND filter having a concave surface, wherein a concave surface in a direction to cancel a convex lens effect of the laminated circular ND filter film is provided on a side surface. 2. The ND filter according to claim 1, wherein the transparent substrate is made of a glass material, and the concave surface has a concave surface formed by polishing. 3. The ND filter according to claim 1, wherein the transparent substrate is formed of a molded product of a glass material or a resin material, and has a concave surface formed integrally with the concave surface when the transparent substrate is molded. 4. The annular ND filter according to claim 1, wherein the transparent substrate has a concave surface on which the transparent resin film having the concave surface is laminated. 5. The ND filter according to claim 1, wherein the ND filter has a concave surface used in combination with a mechanical aperture mechanism for changing a mechanical aperture diameter from an open state to a minimum aperture state. filter.