Using special filters and lens shades
I use several types of UV-pass filters for UV photography. Without exceptions, these filters are available from specialist suppliers and are either unmounted (i.e., all you get is the optical glass without a mounting frame), or mounted in metal cells without threads or with threads different from those normally used to attach filters onto camera lenses. The best solution is to mount, or re-mount, these filters into frames (i.e. metal cells) equipped with standard threads, as discussed below.
UV photography requires the use of lenses that transmit sufficient amounts of UV radiation at the desired wavelengths, and that are capable of forming high-resolution images at these wavelengths. These two properties are not always associated together. In addition, for multispectral photography, it is highly desirable that a lens that possesses the two properties mentioned above also displays little or no "focus shift" (which is actually a form of axial chromatic aberration) between UV and other wavelengths of interest. Lenses for UV photography fall into two categories:
Virtually all these lenses have front lens elements of small sizes. This is not accidental: lens elements made from special glass types are expensive, and lenses with elements of smaller diameters are easier to design and require fewer elements to correct aberrations. The filter mounts at the front of these lenses, however, usually have much larger diameters than the front optical element. In most cases, and especially with lenses of medium and long focal lengths, this means that relatively small filters can be used, with suitable adapters, without causing vignetting or dark image corners. Stopping down the lens usually reduces any vignetting caused by a filter and/or lens shade. Experimentation must be used to verify whether a given combination of lens, lens aperture, filter, adapters and lens shade causes an unacceptable vignetting.
Small filters can be (and should be) protected from off-axis illumination by correspondingly small lens shades. An unnecessarily large lens shade may invite flare and loss of contrast, especially with thick special filters that consist of multiple layers.
This page describes the adapters I assembled and/or modified for the purpose of using small filters and small lens shades on lenses with larger (usually 52 mm) filter mounts. I chose to standardize on the 52 mm size because most of the lenses I use in UV and multispectral photography have this filter mount, or are easily converted to this size. Most Nikon lenses of medium focal lengths also traditionally use the 52 mm size. Other brands tend to concentrate on different sizes (e.g., Micro 4/3 on 46 mm and Canon on 55 mm).
Mounting or remounting filters
Cheap UV or "protector" filters are my favorite source of filter frames for holding unmounted filters. 1 1/4" astronomy filters, once removed from their original frames, usually fit perfectly within 30 mm frames. Unmounted 1" filters fit within 28 mm filter frames. 2" astronomy filters are slightly more difficult to remount within standard filter frames. The best solution I found is using the high-quality blackened brass frames of 52 mm B+W filters, which have a sufficiently wide internal "ledge" that allows the mounting of filters of slightly different diameters. Thick 1" or 25 mm filters mounted in unthreaded metal cells are best epoxied or cemented with black silicone into a 28 mm filter frame without removing them from the original metal cell.
Special filters consisting of a stack of cemented elements or with different coatings on either face should be used in a specific orientation, and not be reversed. In astronomy filters, the attaching thread of the filter sits at its rear (away from incoming radiation), just like in camera filters. Special filters in unthreaded metal cells usually have an arrow or a "V" symbol on their girdle that shows the required direction of transmitted radiation. Unmounted filters with a thick transparent cladding on one side should usually be oriented with this cladding at the rear (away from incoming radiation). Using a filter in an orientation that differs from its specifications may result in an increased risk of flare, loss of contrast and "double edges" in images that contain sharp borders between light and dark areas. The materials of some multi-layer filters may even fluoresce strongly if the filter is used in the wrong orientation.
Metal cells that hold the optical filter with a threaded ring can be disassembled with a spanner driver adjustable in size and lockable at the desired size. These tools are relatively cheap and indispensable for this task. It is best to get spanner drivers equipped with interchangeable or reversible bits ending in both a flat driver blade (like screwdrivers) and in a rounded point. Spanner drivers shaped like a pincer are less suitable and more likely to damage the optical material of the filter.
In most cases, the retaining ring can be identified by two slits on opposite sides of its circumference that must be engaged by the flattened tips of the driver. Sometimes, two small drilled round holes are present, instead of slits. These holes must be engaged by pointed bits. The retaining ring is usually mounted at the front of the filter frame, but in some cases (e.g., B+W filters) it is mounted at the rear.
Do not overtighten the filter frame when replacing the retaining ring. You should stop turning the driver at the first sign of contact between retaining ring and glass. In some cases, a rubber or soft plastic gasket can be placed between glass and retaining ring. This is a good idea that prevents damage to the glass and at the same time prevents the glass from shaking around in a loose mount. If you are concerned about a retaining ring coming loose, use thread sealant instead of attempting to further tighten it. Any mechanical stress on the glass must be avoided. Some of the UV-pass filters I use cost hundreds of $ and can be easily cracked by overtightening a retaining ring.
Mounting the lens shades and filters
As usual in this type of construction, I purchased the necessay adapters and components from People's Republic of China, Inc. via eBay and set about to modify them for my purpose.
The first step was to take a 52 mm to 37 mm step-down filter adapter and to modify it by adding at its front a female 52 m thread. I did this by grinding away the male thread from a step-up 46 mm to 52 mm filter adapter and attaching the modified step-up adapter at the front of the step-down adapter with a few 1.6 mm screws. The end result is an adapter with three threads: male 52 mm, female 52 mm and female 37 mm (Figure 1, left). I did the same modification on a 52 mm - 28 mm step-down adapter (Figure 1, right).
Most of my current filters for multispectral photography are mounted (or re-mounted, in the case of astronomy filters originally mounted in 1 1/4" frames) in 28 mm or 30 mm frames (Figure 2). I added to these filters suitable step-down adapters that end in a male 37 mm thread, which allows coupling the filters to the modified adapter shown in Figure 1, left. 28 mm filters can be mounted directly on the adapter shown in Figure 1, right.
The lens shades are mounted in suitable step-down filter adapters that end in a male 52 mm thread (Figure 3). This thread couples to the modified adapters shown in Figure 1. The assembled lens shades + adapters can also be mounted directly onto a lens with 52 mm filter attachment for photography without filters.
Figure 4 shows how to connect the various pieces. A lens cap mounts at the front of the lens shade. The end result is a simple way to combine together filters and lens shades of different sizes and mount them onto a lens with a 52 mm filter mount.
The filters can be mounted at the front or rear of a 37 mm step-down filter adapter, and the latter can likewise be mounted at the front or rear of a 52 mm adapter (Figure 5). This allows the distance between filter and front lens element to be varied by up to 12 mm in four steps. This capability can be put to good use in placing the filter as close as feasible to the front element of the lens (or more precisely, as close as possible to the entrance pupil of the lens), thus minimizing the risk of vignetting. Of course you must pay attention not to touch the front element of the lens with a filter mount, which likely would result in damage to the lens. Always allow for at least 2-3 mm of distance between the front lens element and any adapter or filter.
The 37 mm step-down adapters are also useful to increase the physical size of the filter assemblies and to reduce the risk of accidentally touching the optical surfaces of the filters while handling them.
If it should be necessary to make the front aperture of a lens shade narrower, in order to exclude more off-axis light, it is possible to do so by mounting a step-down adapter ring at the front of a lens shade. Most metal lens shades have a standard filter thread at their front, which can be used for this purpose. On cylindrical lens shades, the front (female) thread is usually two standard sizes wider than the male thread at the rear of the lens shade.
I built a total of eight adapter and lens shade stacks, in order to keep pre-assembled, complete stacks of frequently-used filters and lens shades ready for quick swapping while shooting. I always use a lens cap to close the front of the lens shade when in storage. I did not find any cheap rear 52 mm caps to protect the opposite end of the stacks (metal caps are available, but a bit expensive), and in the end I decided to use 52 mm UV-cut filters instead, which are much cheaper.