Filters for Full Spectrum Photography, Part I

2009-09-17

Some time ago, I wrote an article about Getting Started with Digital IR Photography. Now I’d like to continue with a discussion of the different filters that can be used with a full spectrum digital camera to achieve various effects. I am going to split this into 2 posts covering: (1) taking normal photographs with a modified camera and some general form-factor considerations, and (2) filters for infrared effects. This is the first post of the pair.

As I mentioned in my previous post, taking visible light photographs with full spectrum-modified DSLR can get tricky. Here’s an example of a photo I took out of my window in San Francisco shortly after obtaining my newly modded camera.

UV-Vis-IR Camera, No Filters

UV-Vis-IR Camera, No Filters

The problem here is that the substantially greater sensitivity of the CMOS sensor to infrared light than to visible light causes the image to come out almost enitrely monochromatic. The reason why it comes out red in particular is that the red elements of the Bayer Filter (used to separate red, green, and blue into separate channels) pass significantly more infrared light than do the green or blue elements. As a result, a properly exposed photograph taken without any filters results in a predominantly red image.

To fix this, it is necessary to use a filter equivalent to the one removed from the sensor to block the infrared light and pass the right spectral bands at the right intensities for the camera to take a properly color-balanced picture. MaxMax.com sells a pair of filters that they call X-NiteCC1 and X-NiteCC2 which they say can be used for this purpose, with different cameras requiring one or the other filter. Unfortunately, they only offer these as screw-in filters and, for reasons I’ll explain shortly, I didn’t want to go that route. It also wasn’t clear to me in reading their website which of these filters (if either) would properly restore the color balance to my XSi.┬áLuckily, the people who worked there were kind enough to tell me that the closest generally available filter glass to that used in the XSi is Schott BG-38.

Now, why wouldn’t I want to use screw-in filters? The first reason is simply cost: the kinds of filters you need for this type of work are not cheap. MaxMax’s filters tend to run from $100-$200. And no matter what size you buy, you always run the risk of eventually getting a bigger, better lens that has a wider diameter than your filters.

The second reason is that my initial filter collection was simply going to contain a BG-38 filter for visible light photography and an RG-830 filter (see my next post) for black & white infrared photography. For doing color infrared, I wanted to be able to take one visible light and and one b&w infrared photo of a scene and combine them in photoshop. That’s a technique I’ll discuss in a future post, but for now it’s simply important to realize that doing so requires the two images to be “in register”–i.e., aligned perfectly. Any change in the camera or subject’s position or the focus or zoom of the lens would thwart such a goal. So I needed a way to swap filters with minimal movement of the lens.

The answer to both problems was the venerable Cokin filter system which uses square or rectangular filters. Up to three of these can be slipped into a filter holder and attached to the camera via an adapter for the particular lens diameter. The adapter rings are cheap and the Cokin P series filters can be used with any lens up to 82mm in diameter. (The “P” denotes the size of the fitlers and the holder. Cokin offers larger and smaller sizes.) The point here is that changing filters doesn’t require screwing anything onto the lens. Filters are slipped in and out of the holder vertically.

At this point, it was simply a matter of finding an optics manufacturer that could cut a Schott BG-38 filter to 84mm x 100mm x 2mm to fit the Cokin P series holder. For this, I turned to Optical Instruments Laboratory, Inc. which was able to manufacture a BG-38 and an RG-830 to these specifications for about $170 each. Here is basically the same picture as above, but taken through a BG-38 filter.

UV-Vis-IR Camera, BG-38 Filter

UV-Vis-IR Camera, BG-38 Filter

Leaving aside the unrelated issues of overexposure and lack of sharpness, this second image successfully achieves the correct color balance for the scene.

Before I sign off for the evening, I’d like to add a word of caution about the BG-38 filter. This filter is extremely scratch prone. My understanding is that this is because the filter is made from phosphate glass rather than the more common silica glass, but in any case the bottom line is that even very small amounts of grit will gouge the surface of the filter. If you intend to buy one of these uncoated (and scratch resistant coatings can more than double the cost), then also buy a few P size plain glass filters to place in front of the specialty filters. These are typically under $10 and protects your expensive glass the elements much the same as the standard UV filter protects your lens.

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