In the first part of this series we discussed converting your Nikon camera to be “full spectrum.” I also explained why I'm calling it “wide spectrum” and provided details on how to make the camera take regular, visible light photographs. Last but not least, we discussed how to use bandpass filters and the post processing of IR images.
In this part, we’ll discuss using longpass and IR filters which people commonly use. We’ll also talk about why you may wish to choose a specific IR conversion instead of trying wide spectrum.
The images used in this part are almost all taken at the same location and of the same subject. The reason for this is to show how the different filters affect the same scene. In addition, the post-processing of the images is very minimal, generally consisting almost entirely of a simple white balance alteration. The coloration of any IR image is always false and the resulting colorization of your own photos is entirely up to you and should be done according to your own taste.
A longpass filter cuts off shorter wavelengths and passes longer wavelengths of light. Such a filter is generally associated with a Wratten number, a system developed by Kodak. Table 1 below shows the various types of longpass filters available, and their associated Wratten number. The nanometer values listed are light wavelength cutoffs.
Table 1 shows nanometer (nm) light cutoff wavelengths for different Wratten filters. Filters that are associated with an nm value are generally labeled at 50% blockage values. For example, the Hoya R72 is equivalent to the Wratten 89B, which blocks 50% at 720nm. Values in parenthesis are the 50% light blockage values for some typical IR filters. This information was taken from Wikipedia, but it is also available online from the original Kodak sources (they’re not easy to read).
Most of the filters in Table 1 have other uses that do not pertain to IR photography, but do pass IR light. They are available as gels and some as glass screw in types. These filters each produce different effects when used on a wide spectrum camera.
The range on these filters goes from the 2C, which allows all visible and IR light to pass through, but cuts off UV, up to wavelengths of 880nm, which cuts off all UV and visible light, as well as some IR. As the filters move through the spectrum, each has its own particular look. However, you really don’t need all these filters, just those that give the results you are looking for.
This section will use yellow and yellow-orange longpass filters from the middle of the table. The yellow filter used in these photos will be a Hoya HMC Y (K2) filter, and the yellow-orange will be the Hoya O (G). The graph for the Hoya Y and O filters (as well as green XO and X1, and red 25A filters) is shown in figure 1.
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