The rapid development of digital technology has enabled continual betterment of the functionality and performance of digital products, along with a decline in selling prices that has propelled such products into extensive use.  Digital still cameras, for example, have the advantageous features of real-time display and digital image storage, allowing users to browse and screen the images taken and process and share the images conveniently.  In fact, these advantages have made digital still cameras immensely popular among the general public and a rising star in the consumer electronics market; almost everybody has one nowadays.  The boom of the digital still camera market has also brought plenty of business opportunities to the market of camera peripherals. 
Generally speaking, one who photographs with a digital still camera would prepare various filters in order to add different effects to the images taken.  Some notable examples of photographic filters include soft-focus filters, cross screen filters (or better known as star filters), polarizing filters, neutral-density (ND) filters, and a variety of color filters.  Referring to FIG.  1, a filter F can be mounted to the front rim of a camera lens (depicted in the drawing as a camera lens assembly L by way of example) and then mounted to a digital still camera C along with the camera lens assembly L so that images taken with the digital still camera C will have a special effect corresponding to the filter F. While the advancement of image processing software has forced many of the aforementioned filters to give way to the powerful digital technology, polarizing filters and ND filters are still difficult to replace.  As its Chinese name (light-reducing lens) suggests, an ND filter is a filter configured for reducing light transmittance, and ND filters come in different specifications each featuring a distinct level of light transmittance.  A camera lens can be mounted with one or more ND filters of different specifications to meet practical photographic requirements, or more particularly to properly reduce the amount of light passing through the lens so that images can be taken either in the illumination of a relatively strong light source or using special photographic skills (e.g., to achieve uniform exposure by waving a black card in front of the lens when taking images in a high-contrast environment).  A polarizing filter, on the other hand, is a filter constructed on the principle of polarization of light and includes grate-like crystals arranged in a parallel manner.  By rotating a polarizing filter and thus adjusting the angle of its grate-like crystals, the amount of light that has a specific polarization angle and is allowed to pass through the lens can be adjusted.  A polarizing filter mounted on a lens can be properly rotated to effectively reduce the reflection in an image (e.g., the reflection from a water surface, a glass display cabinet, or the leaves of a plant), with a view to attaining the desired photographic effect. 
Conventionally, an ND filter of a certain specification corresponds to a certain level of transmittance.  A photographer, therefore, has to buy and carry ND filters of different specifications with them in order to deal with the light intensities of different shooting environments by mounting the appropriate ND filter(s) to a camera lens.  The foregoing approach, however, incurs a huge financial burden on photographers and enormous inconvenience of use, simply considering the trouble of carrying a plurality of ND filters around.  In addition, the need to mount and detach ND filters repeatedly compromises the ease of camera operation.  Although the conventional polarizing filters can lower light transmittance to some extent, they are not designed specifically for that purpose and hence cannot be directly used as substitutes for ND filters.  Furthermore, it is not uncommon that a polarizing filter may have to be used in combination with ND filters of different specifications to cope with the light intensities of different shooting environments. 
To overcome the drawbacks described above, the inventor of the present invention developed an “adjustable multiple-ring polarizing ND filter”.  This adjustable multiple-ring polarizing ND filter is essentially a “three-ring polarizing ND filter assembly” that includes a fixedly connecting ring, a first pivotally connecting ring, and a second pivotally connecting ring.  The fixedly connecting ring has one end configured to be fixed to the front rim of a camera lens.  The first pivotally connecting ring is fixedly provided therein with a first polarizing lens and has one end pivotally connected to the other end of the fixedly connecting ring.  The second pivotally connecting ring is fixedly provided therein with a second polarizing lens and has one end pivotally connected to the other end of the first pivotally connecting ring.  Each of the first pivotally connecting ring and the second pivotally connecting ring can be rotated independently with respect to the camera lens so that the first polarizing lens fixedly provided in the first pivotally connecting ring and the second polarizing lens fixedly provided in the second pivotally connecting ring are rotated with respect to each other.  That is to say, the polarization angle of each of the first and second polarizing lenses can be adjusted by rotating the corresponding first or second pivotally connecting ring, and the transmittance of incident light can therefore be regulated by changing the included angle between the polarization angles of the polarizing lenses.  The single lens set is thus capable of adjusting the amount of light reduction rapidly and effectively for the intended photographic effect.  Products adopting this technical solution grabbed the attention of photography enthusiasts immediately after they were launched, with great market responses soon afterward, but the inventor never felt complacent about the “three-ring polarizing ND filter assembly”, despite its significant improvement in convenience and utility over various conventional filters.  Instead, the inventor still hoped to design an even more convenient and more useful polarizing ND filter assembly to reciprocate the support of those who also take great interest in photography. 
The inventor has been not only using the “three-ring polarizing ND filter assembly” personally since its invention, but also sharing his understanding of and experience in photography with fellow-photographers actively.  During the process, the inventor has found that, although the “three-ring polarizing ND filter assembly” allows free adjustment of the transmittance of incident light and doubles as a polarizer for screening specific polarized light, most picture-taking environments do not require photographers to adjust the amount of light reduction but require the angle of passage of specific polarized light to be adjusted so that reflection from the surface of an object being photographed (e.g., a water surface, a glass display cabinet, or the leaves of a plant) can be effectively eliminated from the images taken, in order for the images to have the desired photographic effect.  When using the “three-ring polarizing ND filter assembly” to adjust the polarization angle, however, it is very likely that the second pivotally connecting ring (or the first pivotally connecting ring) will be inadvertently moved by the user rotating the first pivotally connecting ring (or the second pivotally connecting ring), or that the user will have problem rotating the first pivotally connecting ring and the second pivotally connecting ring stably in unison, the reason being that each of the two pivotally connecting rings is designed to be rotated independently with respect to the camera lens and that the two rings are in close proximity to each other.  Should either of the aforesaid scenarios occur, the included angle between the polarization angles of the polarizing lenses will not stay at a fixed value; in other words, the amount of light reduction will not remain at the expected level.  The quality and effect of the images taken will be impaired as a result. 
The issue to be addressed by the present invention, therefore, is to design an adjustable polarizing ND filter assembly that, in addition to preserving the convenience of the foregoing polarizing ND filter assembly, has two polarizing lenses configured to be rotated stably either in unison with respect to a camera lens so that the polarization angle of the assembly can be adjusted while the amount of light reduction remains constant, or separately with respect to the camera lens so that the amount of light reduction can be easily adjusted and fixed at the desired level, thus effectively enhancing the convenience of use, accuracy, stability, and usefulness of the assembly.