Filters - Photography - Beneath the Waves

Filters

Although they aren't as flashy as the camera body and lenses, filters are a critical component of any set of multispectral photograhy equipment.

Human-Visible Light Bandpass Filters

If you have a modified DSLR, or are thinking of getting ahold of one, the first filter you should probably buy to go along with it is one which will allow it to work like an unmodified camera again. The most common of these filters use Schott BG- (cyan-coloured) or KG-series (clear or extremely pale blue/green) glass to filter out most or all of the infrared. Some of the more commonly-available variations:

Schott BG38 - available from Robert Cairns and other suppliers. I am 99% sure that the LDP CC1 that I currently use is an uncoated, 2.5mm-thick piece of Schott BG38, so if I were to need another, that's what I would order from Robert Cairns or another supplier.
Schott BG39 - also available from Robert Cairns, and from B+W in the form of their 039 filter (which can be a bit difficult to actually find for sale, although as of this writing it can be custom-ordered from B & H Photo/Video in the US)^[4]. In my opinion, BG39 cuts out too much visible red light for use as a general-purpose filter on my modified D70, but it is no doubt extremely useful for more-specialized applications.
Schott BG40 - also available from Robert Cairns. I haven't used BG40 myself, but it has a cutoff curve in-between that of BG38 and BG39, so it could be very good.
Schott KG3 (and other KG-series glasses) - also available from Robert Cairns, as well as B+W in the form of their 489 filter (easily obtained as of this writing). Again, the B+W multicoating most likely affects the UV transmission compared to the uncoated glass. I believe that LDP's CC2 is either KG2 or KG3. These filters allow too much infrared through for me to use them as general-purpose filters on my modified D70, but are supposedly ideal on other models of camera.

There are several other types of filter which supposedly can be used for the same purpose. A number of photographers I respect use Baader's UV/IR Cut filter, which I have not tried myself. Several brands of interference-based filter are available, such as Tiffen's "Standard Hot Mirror", PECA's 918 (which I believe is the same glass as the Tiffen filter), and B+W's 486. I have had very poor experiences with interference-based filters of this type, so I don't recommend using them except for special applications. As always, your mileage may vary.

Near Infrared Bandpass Filters

Near infrared photography has been a somewhat-popular niche for many decades, so there are myriad choices for filters of this type. When I began experimenting with multispectral photography I went a little overboard and got ahold of three NIR-bandpass filters: a Hoya R72 (cutoff frequency: 720nm), a B&W 093 (cutoff frequency: 880nm), and an LDP 1KB (cutoff frequency: 1000nm). Although there is a measurable difference between the frequencies they allow through (see A Detailed Introduction), I have not found a functional difference between them in terms of field/studio use. That is, if something is bright or dark in one part of the near infrared, it seems to appear more or less the same in other parts of the near infrared. There is a difference in that my D70 at least has lower sensitivity to longer wavelengths, so using the LDP 1KB filter is effectively adding a neutral-density filter as well (see below). It is certainly possible that for certain applications this would be a more important factor. However, especially for hobbyists, I would recommend sticking to one or two instead of splurging.

Below 1000nm, numerous manufacturers produce more-or-less-equivalent filters. The Hoya R72, B&W 092, and Kodak Wratten 89B are all basically the same, for example.

Near infrared filters can also be improvised using a piece of processed (unexposed) Kodak slide film. You may get a funny look if you buy a package of such film at a drug store, then immediately take it to the photo department for processing, but it will work. Of course, the filter size is much smaller than a threaded filter, but many people have converted webcams and other small imaging devices using this method.

Ultraviolet Bandpass Filters

Ultraviolet photography has always had limited appeal compared to near infrared, and ultraviolet-bandpass filters are correspondingly less common and more expensive. Many of the historical standards (e.g. the Kodak Wratten 18A and Hoya U-360) are no longer available, or at best are special-order custom purchases of high cost.

Ultraviolet-bandpass filters are also vulnerable to a common weakness - all of them allow some amount of near infrared light to pass through in addition to ultraviolet light. Traditional filters (the aforementioned Wratten 18A, Hoya U-360, and B&W 403) were originally designed for use with ultraviolet-sensitive film (instead of digital cameras), and I believe (but do not know for a fact) that this acted as the second "filter". That is, the film was not sensitive to ultraviolet light, so it was only important to prevent contamination by human-visible light (which these filters do admirably).

In the digital world, these older filters must be paired ("stacked") with a second filter which blocks the near infrared light, because otherwise the near infrared will completely overwhelm the image (particularly when using standard camera lenses) and the photographer will not have taken an "ultraviolet" photo at all.

The first ultraviolet-bandpass filter I obtained was a B&W 403 (which is made from Schott UG1 glass), and when using it, I stack a piece of BG38 on top. This does not completely block near infrared light, but for most purposes it is sufficient. If you are interested in going the "stack" route, be careful about the infrared-blocking filter that you choose. Many of them (e.g. Tiffen's "Hot Mirror") use interference effects, and will cause extreme vignetting on wide-angle lenses, and many of them don't block near infrared light effectively enough to work for these purposes.As of today, the best ultraviolet-bandpass option for most photographers is the Baader U-Filter. Baader may not be a familiar name unless you are into astronomy, because their products are targeted at that market, not traditional photographers. Nonetheless, the U-Filter is a staggeringly good ultraviolet-bandpass filter. I've found it offers 1-3 stops of higher light-sensitivity compared to the B&W 403/LDP CC1 stack, and the contrast of the resulting images is higher.

The U-Filter has two downsides: first, it is only available in a 48mm package. Astronomers measure their filters differently than photographers, and call it a "2-inch filter". This is a smaller diameter than Nikon's typical 52mm threading, so a step-down adapter ring is definitely required. It can also lead to mechanical vignetting depending on the focal length of the lens and the crop factor of the camera's sensor. On my D70 (with its crop factor of 1.5), I start to get vignetting somewhere between 18mm and 20mm. This implies to me that on a camera with a full-frame sensor, the same effect would occur with lenses around 28mm-35mm or wider, although I have not tested this myself and it may be even more of a problem.

The U-Filter is also expensive compared to other individual filters - especially filters of about the same small diameter. However, it is worth saving up for if necessary, because there really is no comparison. It is far and away the best ultraviolet-bandpass filter available on the consumer market.

I got my U-Filter from Astro-Physics Incorporated, and have never regretted it after seeing the results it produces.

Another UG-series option is Schott's UG11, which has better transmission into shorter-wave UV than UG1, but doesn't transmit longer-wave UV as well. Like UG1, UG11 must be paired with an infrared-blocking filter for use on a converted camera.

Astrodon recently launched a competitor to Baader's U-Filter, which they call the "U-Venus". Unfortunately, it appears to leak significant amounts of visible and/or infrared light. In addition, the 48mm version comes as a piece of unmounted glass, so anyone who wants to use it for photography is stuck mounting it in a ring themselves.

In the summer of 2011, UVR Defense Tech, Ltd. released a new filter named the "Andrea-U" (in honour of the moderator for the UV/IR forum at NikonGear. This filter provides a very interesting alternative to other options. Its transmission curve is similar to what you'd get with a BG38 + UG1 stack, but has more-effective blocking of IR. It can also allow shorter exposure times than a U-Filter depending on lighting conditions and the sensitivity of the camera being used. As a result, I carry both around in my case.

Other Filters

The main additional filters I use are a pair of B&W neutral-density filters (the 106 and 110). Neutral-density filters are essentially "sunglasses for your camera" - they reduce the amount of light entering the lens, allowing for a longer exposure at a given aperture setting. This is how nature photographers produce the iconic effect seen in famous (and many not-so-famous) photos of waterfalls and rivers.

Neutral Density Filter Example

R-G-B (Long Exposure)

R-G-B (Short Exposure)

A comparison between the short and long (using a neutral-density filter) versions of one of my photos from Yellowstone National Park.

This type of very long exposure results in an image that approximates (roughly) the "wavefunction" of the water - a map of all the routes that it can take as it flows through its course. A related use is to create the "river of light" effect of traffic through a city at night.

Beyond the traditional nature photography use, I have been using these filters to balance the exposure time of the human-visible and near-infrared shots of such scenes with the ultraviolet-A exposure. I can only reduce the UVA exposure time so much, so I must increase the exposure time of the other two to match, in order for the water to appear consistent between them in a false colour composite.

Neutral-density filters can also be handy when taking pictures of high-traffic tourist attractions, because the people will be reduced to a blur or disappear entirely due to their milling about.

Various types of coloured filter can also be used to obtain interesting effects when used on a converted digital camera. See False Colour From Filters (and Simulated Filters) for more information on this topic.

Which Filters Do I Use?

My "standard set" is an LDP CC1 for human-visible light, a B&W 093 for near-infrared^[2], and a Baader U-Filter for ultraviolet-A. Beginning in 2009, I added a pair of B&W neutral-density filters (106 and 110) to even out the exposure times between the three bands. For lenses which require larger-diameter filters than the U-Filter, I am currently stacking the CC1 and a piece of Schott UG1 (or UG11, depending on the subject matter), although I would like to experiment with other options as time and money permit.

Buying Filters

My main advice with regards to buying filters is "shop around". I tend to buy more commonly-available filters from either Adorama or B & H Photo/Video, depending on who has the better price. Buying a Baader U-Filter requires ordering from an astronomy/telescope supplier.

If you keep an eye on eBay, you may be able to obtain a set of PECA forensic filters at a bargain price. These are 67mm filters which are clones of the Wratten 18A, and several different near infrared bandpass filters. If you already own other manufacturers' examples of these, you won't gain anything with the PECA set, but it does seem like a good opportunity for newcomers to obtain a variety of filters for less than the cost of 2-3 from any other supplier.

UVIR Optics

UVIR Optics is a US-based operation currently using eBay as their store. They are offering extremely high-quality filters at very fair prices. Get them while they last! :)

Omega Filters

Omega Filters operate off of eBay, and offer a five-star experience for a one-star price. I can't say enough good things about them. The one downside is that the reason for the low price is that they're apparently selling surplus merchandise. They may or may not have what you want in stock, and even if they have a particular type of glass, they may not have it in the size and/or mount you're looking for.

Robert Cairns Company Optics

Robert Cairns is an excellent source for filters in the US, and will ship to at least some other countries as well. Their website makes it appear as though they only sell in bulk, but they are happy to build one-off filters as well. Most of the time, it will be a custom build, so expect to wait a few weeks between ordering and shipping, but it is well worth the wait - their customer service, build quality, and packing are all top-notch. Their prices reflect the custom aspect, and are slightly higher than certain other suppliers, but a 10% premium is a small price to pay for a first-class experience. Their standard sizes are 52mm and 82mm diameter, so if you specify one or both of those when requesting a quote you may get a faster response than if you'd asked about another size.

LDP, LLC (AKA "MaxMax")

I used to recommend LDP without reservation (other than the caveat that it's a good idea to see if someone else has the same filter available for less). I have mixed feelings about them now. On the positive side, they stock several filters that are difficult to find elsewhere, and their president/owner seems to be a genuinely nice guy with an interest in this field.

However, awhile ago I placed an order with them for five filters. When I opened the box, I found that while three of them were in plastic cases, two of them were in Zip-Loc®-style bags(!), and all five had just been stuck into one big Zip-Loc®-style bag together. The larger bag had been stuck at the bottom of the box, and then the rest of the box filled in that terrible Amazon.com-style way that provides literally no shock protection for the contents^[3]. Unsurprisingly, the filters in the Zip-Loc®-style bags had both sustained damage, and one of the filters in a plastic case had been jarred so strongly that the metal filter ring had popped apart, and the loose glass had then been scratched by jostling around with the metal parts. I shipped them back for replacement (at my expense), and the "replacements" I received looked suspiciously like the ones I had sent back, after a hasty polishing with substandard cloth and the ring on the separated filter cemented back together. One filter in particular has many subtle scratches in the coating, and the other has a mark on it that I am sure was on the "original". I've also received filters with fingerprints on the glass (another indication of careless handling), and I've heard from other people who have e.g. received filters with the cement used to hold the ring together smeared over the filter glass(!).

For the prices LDP charges, this is simply unacceptable. They should either take the time to pack their products properly, or ship filters in metal "filter stacker" cap sets, so that even if jostled around, nothing will impact the filter glass. I've bought $5 glassware many times on eBay that was packed ten times as well as these $100+ filters. If you order from them, my advice is to order one filter at a time, so that there is no possibility of anything else ending up in the package which could cause damage. I've done this a couple of times since then, and while I've still had to clean the filters myself, they arrived with no permanent damage.

Finally, be aware that most of what LDP offer in terms of filters are simply re-labeled Schott glass. To my knowledge, this is the mapping from their proprietary names to the original glass, in case you'd like to order the same thing elsewhere:

LDP Filter Name	LDP Filter Thickness	Glass Type
CC1	2.5mm	Schott BG38
CC2	2mm	Schott KG2 or KG3 (?)
X-NiteNNNN (e.g. X-Nite 1000)	Various	Schott RGNNNN (e.g. RG1000)
X-Nite330	2mm	Schott UG11
X-NiteUVR	2.5mm	Schott UG5
XDP	2mm	Schott UG1

To my knowledge, LDP's BP-series filters do not have commonly-available equivalents.

Footnotes

1.	Back in early 2007 when I was putting together my equipment, this "2-inch" terminology confused me to no end, and it was Bjørn Rørslett himself who took the time to explain to me in email what was going on.
2.	For a period of about 6 months I favoured the LDP 1KB over the B&W 093 (and I still use it for certain applications), but it requires much longer exposure times.
3.	One of the reasons this practice bothers me so much is that companies like UPS publish right on their websites packing instructions that include text such as: Please be sure that you wrap each item separately. Fragile articles need separation from each other, and from the corners, sides, top, and bottom of the box. Each item should be surrounded by at least two inches (5.08 cm) of cushioning and be placed at least two inches (5.08 cm) away from the walls of the box. This prevents product-against-product damage and protects contents from shock and vibration, which can pass from the outside of the box to the contents. ... UPS does not provide special handling for packages with "Fragile", package orientation (e.g., "UP" arrows or "This End Up" markings), or any other similar such markings. Essentially, the only safe assumption to make when packing something for shipping is that the package will be treated like a football (of either American or rest-of-the-world variety), and/or a stepping stone.
4.	Note that B+W's 039 is multicoated, which most likely reduces ultraviolet transmission compared to uncoated BG39. For purposes of transmission calculation, I measured the glass in my 039 as being 2mm thick.