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 buy to go along with it is one which will allow it to work like an unmodified camera again. I am only aware of one filter of this type currently available in a standard threaded package: the LDP CC1 (look for "Color Correction Filter" about 3/4 of the way down the page).

LDP is a good source for filters of obscure types at reasonable prices. My experience is that for less-obscure filters their prices are sometimes higher than for name-brand manufacturers, so shop around.

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. LDP produces equivalents of all of the big names (and some variations that can't be had elsewhere), but again, do some price-checking first, e.g. at B&H Photo/Video and/or Adorama.

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 weakness unique to their spectral band - 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, and when using it, I stack an LDP CC1 on top. There are other options for the NIR-blocking filter, including LDP's BP1 or "hot mirror" filters from a variety of manufacturers. This does not completely block near infrared light, but for most purposes it is sufficient.

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.

LDP offers a 330nm-cutoff filter (which must be paired with their bandpass filter) that I believe is more or less equivalent to the Hoya U-360. I have not tried this myself, but it or the B&W 403 would be worth investigating for photographers who can't work around the small diameter of the U-Filter, or at least want an option to use with their wider lenses.

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.

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 LDP CC1 and a B&W 403, although I would like to experiment with other options as time and money permit.

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.

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