Filter sizes, filter threads, and re-mounting astronomy filters

UV photographers use UV-pass filters to block VIS and NIR, in order to image only the NUV part of the spectrum with digital cameras. Typically, these cameras are modified commercial cameras where the NIR- and NUV-cut built-in filter has been removed, and usually replaced with a window that transmits at least from 300 to 1,000 nm). UV-pass filters for this application are specialty items, often available only in sizes and mounts different from standard filters designed for mounting at the front of camera lenses.

This page discusses how to adapt astronomy filters for imaging on digital camera lenses.

Sizes of camera filters

Virtually all filters designed for direct attachment at the front of camera lenses are mounted in a metal ring with a male attachment thread. This thread screws into a matching female thread at the front of the lens barrel. A female thread is usally present at the front of the filter ring, and allows the stacking of two or more filters, or the use of lens shades and lens caps in front of the filter.

The size of a camera filter with threaded mounts is the diameter of the male thread at the rear of the filter ring. These threads are specified with two measurements:

  • Diameter of the male thread, measured on the outside of the thread. This measurement is, in practice, the diameter of a drilled hole just wide enough to let in the threaded portion of the mount. For example, the male threaded portion of the metal ring of a 28 mm filter passes through a 28 mm drilled hole (but the rest of the filter ring has a wider external diameter and does not pass through the 28 mm hole).
  • Thread pitch, measured between adjacent threads one whorl apart from each other.

Standard threaded filter mounts on camera lenses are metric, and their diameter is measured in mm. The standard specifies a number of specific filter sizes, usually spaced apart at intervals of 2 to 5 mm. Some of these standard values are very common, e.g. 52 mm, 55 mm and 58 mm. They also include a few fractional mm sizes, especially for small filters, like 34.5 mm and 40.5 mm. Certain diameters, although standard, may be unusual among modern lenses, and filters of these sizes are difficult to find.

In addition, a few non-standard filter thread diameters have also been used in the camera industry, at one time or another. An example is the Series VII thread, which has a thread diameter of 54.346 mm (more on this below).

For the large majority of standard camera filters, the thread pitch is either 0.5 mm (for really small filters) or 0.75 mm (for larger filters up to approximately 100 mm of diameter). For some standard filter sizes, filters may be available in both 0.5 and 0.75 thread pitch. For non-standard filter diameters, the thread pitch can also be non-standard. The Series VII threaded mount mentioned above uses a 36 TPI pitch, i.e., non-metric, which is approximately equal to 0.70 mm.

The two mating threads of filter and filter mount cannot have identical diameters, since a little clearance (usually, about 0.1 mm) between male and female threads is necessary to allow an easy screwing and unscrewing. For practical purposes, however, male and female threads that are supposed to match use the same nominal threaded mount, e.g., a lens with a 52 mm x 0.75 filter attachment accepts 52 mm x 0.75 filters.

Metric screw diameters are often prefixed with MA (= Metric Aerospace standard). MA screws typically also have has a standard thread pitch chosen among those specified in the ISO standard, and a few other standard measurements. When you buy 5 mm dia. machine screws in Europe, for instance, you always get MA5 x 0.5 screws, unless you also specify a different thread pitch. MA 14 ISO screws, instead, come in 1.0, 1.25 and 1.5 pitch, so you need to be more careful.

Astronomy filter sizes

Two sizes of mounted astronomy filters are commonly used: 2" and 1¼". The most common question in this context is how to mount a 2" threaded astronomy filter on a camera lens that accepts a standard, metric filter size (1¼" filters are less of a concern, and are discussed last). There is a considerable confusion in the information available on the web on how to mount 2" filters on camera lenses. Dozens of sites and bulletin boards repeat the statement that a mounted 2" astronomy filter can be adapted for use on a camera lens via a 52 mm (or whatever other filter diameter the lens accepts) to Series VII adapter. This is simply wrong. The Series VII female thread is much too wide. There is no way to screw a mounted 2" filter into a Series VII adapter, even with generous amounts of thread tape or glue. In addition, although a female Series VII thread is wide enough to admit the non-threaded portion of a 2" filter ring, a typical Series VII female thread is too shallow to completely contain a 2" filter ring and a Series VII retaining ring to keep the filter in place.

The 2" size specification for astronomy filters means something completely different from thread diameter, or diameter of the glass blank contained in the filter ring. It means instead that the outer diameter of the metal filter ring is 2", or 50.8 mm. The actual diameter of the filter ring is actually slightly less than 2", since the filter must fit within the inner 2" diameter of the eyepiece tube (or in other fittings, like a filter wheel, where similar space constraints apply).

A Series VII female thread does accept a non-mounted optical filter with a 2" diameter, or 50.8 mm. You also need a Series VII retaining ring to keep the filter in place (unless you glue it in).

Although adapters are available to attach a Series VII accessory to a 52 or 55 mm camera lens, you need a second adapter, this one with a Series VII male thead and a 52 mm or 55 mm female thread, to mount a lens shade with standard thread at the front of the filter. With some luck, you may find a slightly out-of-specifications 55 mm lens shade that screws sufficiently deep into a Series VII thread, but this is a hack, rather than a reliable solution suitable for daily use.

2" astronomy filters have a male 48 mm x 0.6 thread. You are reading correctly. The pitch is 0.6 instead of the standard 0.75 used in 48 mm camera filters. This means that the male thread of an astronomy filter will usually seize after being screwed into a standard 48 mm x 0.75 socket for about one turn. Still, one turn is better than nothing, and with normal care the filter is unlikely to unscrew and fall off.

2" astronomy filters differ from standard camera filters in one more respect. Standard camera filters (with the exception of filters in very shallow mounts designed to avoid vignetting with wide-angle lenses) carry a female thread at their front, of the same size as the rear male thread. This allows the easy stacking of two or more filters, as well as attaching a lens cap or a lens shade at the front of the filter. In 2" astronomy filters, instead, any of the following can apply:

  • Some 2" filter rings have an MA48 x 0.6 female thread at their front. This is similar to what you should expect of a camera filter.
  • Some 2" filtes rings have a different (often smaller than MA48) front thread diameter.
  • Some 2" filter rings have no front female thread at all, and the retaining ring that keeps the glass in place is friction-mounted, or glued, or is a C-spring held in a shallow groove. Sometimes, there is a threaded retaining ring, but smaller than the nominal filter thread and screwed in from the rear, with the glass resting on a machined shoulder at the front of the ring.
  • Some 2" filter rings have a short front thread that is filled by the retaining ring. If there is an exposed portion of female thread, it is too shallow for safely attaching anything at the front of the filter. This is the case of the PrimaLuceLab U filter.
  • Although the mounting thread of a 2" filter is supposed to be always M48 x 0.6, the diameter of the glass is not standardized, and varies by several mm across different filter brands, series and types. It can be as much as 8 mm smaller than 48 mm.

The last of these characteristics poses an obvious problem if the filter must be re-mounted for use on 52 mm x 0.75 lens filter mounts. Adapters for mounting a 2" filter on a camera lens are about as easy to find as hens' teeth.

Warning: The rest of this paragraph is a diversion from the topic of this page. You may safely skip it. A research group recently manipulated gene expression to modify the skull of a chick embryo into something resembling a dinosaur skull (birds evolved from dinosaurs, and in fact all birds are highly specialized dinosaurs), but they were so far unable to give back to the chick its dinosaur ancestor's teeth. Until then, my statement about hens' teeth and 2" adapters still holds, in spite of the fact that machining a filter step-down ring should be easier than altering the morphogenetic programming of an embryo to make it regress in evolution 120-odd million years.

Even a 52 mm to 2" adapter does not solve the problem of mounting a lens shade onto the 2" adapted filter, so re-mounting a 2" filter into a standard camera filter ring solves the problem of both rear and front threads, and is often a better option.

Filter adapters

Filters can be mounted, via adapters, on a lens that accepts a different filter size. Two types of adapters are available:

  • Step-up adapters, to use a filter larger than the lens filter thread.
  • Step-down adapters, to use a filter smaller than the lens filter thread.

In addition to the two thread sizes (for example, 28 mm and 37 mm), you need to specify whether the adapter you are ordering is a step-up or step-down type, or you may get a useless adapter. Most eBay ads use the step-up/step-down nomenclature, but some retailers use other ways of indicating the adapter type.

Step-up adapters rarely cause vignetting, and typically only on extreme wide-angle lenses. The risk of vignetting is higher with step-down adapters, but many lenses do not vignette even with filters much smaller than the lens filter mount. In fact, there are often good reasons to use a filter much smaller than the filter thread of the lens.

Re-mounting the Baader U

The diameter of the glass of the 2" Baader U is a bit too small to sit safely inside the metal ring of an ordinary 52 mm camera filter. It sits well, instead, in the filter ring of a 52 mm B+W Pro filter, which has an unusually wide inner shoulder and is machined from blackened brass. This type of filter ring keeps the glass in place with a retaining ring screwed in from the rear of the filter.

The Baader U filter, in its original 2" mount, does not have its identification data engraved on the filter ring. Instead, rather disappointinly for such an expensive filter, the data is printed on an adhesive sticker fastened to the ring. Nonetheless, after re-mounting this filter, it is possible to carefully peel the slicker off and reattach it to the new ring.

Re-mounting the PrimaLuceLab U

The glass of the 2" PrimaLuceLab U is even smaller, and falls right through the metal ring of a 52 mm B+W Pro filter. It does fit, instead, in a 48 mm B+W Pro filter ring, but the glass of the PrimaLuceLab U is much thicker than typical B+W filters, and this does not allow a retaining ring to be used.

In this case, I had to be creative. My solution was removing the retaining ring of the PrimaLuceLab U are replacing it with a 48 mm to 52 mm step-up adapter ring. I found one with a short rear 48 mm thread that screws completely into the PrimaLuceLab filter ring without forcing its thread and without putting pressure on the glass.

I screwed the rear thread of the PrimaLuceLab filter into a 52 mm to 48 mm step-down adapter, which also fits well.

This results in a relatively thick, but still manageable, stack of filter and two adapters, starting and ending in the desired 52 mm threads (above figure). The filter data engraved on the outer perimeter of the filter mount is still visible.

Sources of filter rings

Second-hand filters, or cheap UV and "protector" filters from China, are an obvious source of filter rings to use for re-mounting special filters. Make sure that you are purchasing a filter equipped with a threaded metal retaining ring. Some cheap filters rings have press-fit retaining rings or are crimped around the glass, and cannot be disassembled. Lately, cheap filters come with a threaded retaining ring made of plastic, best described as "one-time use only".

If a well-built and apparently suitable filter ring resists your attempts to unscrew its retaining ring with an appropriate spanner driver, smashing the glass with a hole punch against an anvil. Place a coin or similar metal object between filter and anvil, so that the glass (not the perimeter of the filter ring) touches the anvil. It may also be a good idea to put the filter in a thin plastic bag, to prevent glass shards from flying away. Wear safety goggles and keep your fingers away from the glass while punching it. After the glass shatters, remove the filter shards by gently prying them with a screwdriver tip (not your fingers or fingernails). You may need to punch two or three holes before the first glass shards can move. The rest are easier to remove. A large-diameter needle or sharp dental pick helps to remove any bits trapped between filter ring and retaining ring. Once the glass is completely removed, the retaining ring is unblocked and easily unscrewed.

For filter sizes not easily found on eBay, Edmund Optics sells empty filter rings of a variety of standard sizes, including several uncommon sizes.

Standardize on one or two sizes only

It helps to choose one or two filter mount sizes, and adapt all your filters to either chosen size. The first obvious choice is the filter size used by the lens you most often use for UV imaging, as long as it uses a standard filter size. I standardized on 52 mm and 37 mm filters, lens shades, lens caps and other accessories for multispectral photography. 52 mm filters attach to many macro and special lenses, including the CoastalOpt 60 mm APO, CoastalOpt 105 mm UV, Nikon UV Nikkor 105 mm and Tochigi Nikon UV Nikkor 105 mm. 37 mm step-down adapters are versatile enough to carry all small filters of diameters up to 37 mm. In practice, I use special-purpose filters all the way down to 20 mm by re-mounting them in standard filter rings, and then screwing the latter into 37 mm step-down adapters.

I discuss elsewhere in detail how to build stacks of filters, adapters and lens shades of standardized sizes.