JP3250190B2 - Ring-shaped ND filter - 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, which is the cause of the focus movement between the open state and the minimum stop. I hypothesized that it might be. According to this hypothesis, the space surrounding the concentric laminated circular ND filter, which is microscopically stepped, is filled with a transparent adhesive having a refractive index similar to that of the ND filter film so that no difference in optical path length occurs. The focus movement was improved. That is, in the conventional annular ND filter, the difference in the optical path length has a kind of lens effect, which is considered to be a cause of the focus movement, whereas the transparent adhesive is applied to the space surrounding the concentric laminated circular ND filter. When an agent is provided, this lens action can be prevented.

That is, according to the present invention, a circular ND filter film whose diameter gradually decreases on a transparent substrate is formed concentrically and
In a ring-shaped ND filter formed by laminating microscopically a stepped cross section, the minimum diameter N of the surface of the transparent substrate and the central portion is reduced.
The refractive index of the circular ND filter film is substantially the same as the refractive index of the circular ND filter film in the space surrounding the concentric laminated ND filter, which is microscopically stepped and defined by the extended surface of the surface of the D filter film.
A transparent adhesive made of a material having a refractive index is provided.

[0007] The concentric laminated circular ND filter and the transparent adhesive can be provided between a pair of transparent substrates, or the transparent adhesive can be applied to the outer periphery of the concentric laminated circular ND filter by spin coating. Can also be formed. The refractive index of the ND filter film is usually 1.3 to 2.
Considering that it is 6, the transparent adhesive is also 1.3 to
It is preferable to use one having a refractive index of 2.6.

[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 reference numerals are given to the same components as those in the conventional example of FIG. 4, the ND filter films (light absorbing films) 12 to 14 are sandwiched between the transparent substrate 11 and another transparent substrate 15. , This transparent substrate 1
A transparent adhesive 16 is filled in a microscopically stair-shaped space other than the ND filter films (light absorbing films) 12 to 14 sandwiched between the substrate 1 and the transparent substrate 15.

FIG. 3 shows another embodiment of the present invention, in which a microscopic step-like structure around the ND filter films (light absorbing films) 12 to 14 is used without using another transparent substrate 15. A transparent adhesive 16 is provided in a space defined by the following formula.
The transparent adhesive 16 can be formed by, for example, a spin coating method. That is, the transparent adhesive 16 is dropped on the center of the ND filter film 14 having the smallest diameter while rotating the transparent substrate 11, and the transparent adhesive 16 is applied to the outer periphery of the ND filter films (light absorbing films) 12 to 14 by centrifugal force. After being guided to a microscopic stepwise space, it is dried and hardened.

The refractive index of the transparent adhesive 16 is ideally N
It is preferable that the refractive index is the same as that of the D filter films (light absorbing films) 12 to 14, but the stepped space is filled with a substance having a similar refractive index (the transparent adhesive 16), thereby opening the mechanical aperture. The difference between the optical path lengths in the state and the minimum stop state is not large. For this reason, the problematic lens action is reduced, and the focus movement is at a negligible level.

In the above description, the difference in the optical path length between the ND filter films (light absorbing films) 12 to 14 causes the focus movement.
The surface accuracy and the size error (thickness error) of the step portion during the vapor deposition of Nos. To 14 may also be a part of the cause of the focus movement.
Even if the cause of the focus shift is different from the hypothesis, the effect of reducing the focus shift by the annular ND filter of the present invention has been confirmed, and does not affect the right scope of the present invention.

[0013]

According to the ring-shaped ND filter of the present invention, it is possible to reduce the focus movement in the open state of the mechanical diaphragm and the minimum diaphragm state.

[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 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 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 Ring-shaped ND filter 11 15 Transparent substrate 12 13 14 ND filter film 16 Transparent adhesive

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-73201 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 5/00 G03B 9/00-9 / 07 G03B 11/00-11/06 H04N 5/222-5/257

Claims (5)

(57) [Claims] 1. A circular ND filter film whose diameter gradually decreases on a transparent substrate is concentrically and microscopically cross-sectionally stepped.
A ring-shaped ND filter formed by laminating in a shape, a microscopic stepwise concentric shape defined by the surface of the transparent substrate and an extended surface of the surface of the ND filter film having the smallest diameter at the center. in the space of the outer circumference of the laminated ND filter, the circular
The refractive index is almost the same as the refractive index of the ND filter film
An annular ND filter provided with a transparent adhesive made of a material . 2. The ND filter according to claim 1, wherein the concentric laminated circular ND filter and the transparent adhesive are:
An annular ND filter sandwiched between the transparent substrate and a transparent substrate different from the transparent substrate. 3. The ND filter according to claim 1, wherein the transparent adhesive is formed in an annular ND shape formed by a spin coating method around a concentric laminated circular ND filter.
D filter. 4. The ND filter according to claim 1, wherein the refractive index of the ND filter film and the refractive index of the transparent adhesive are both 1.3 to 2.6. . 5. The ND filter according to claim 1, wherein the ND filter is used in combination with a mechanical diaphragm mechanism for changing a mechanical aperture diameter from an open state to a minimum diaphragm state.