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Taking Multispectral Pictures
This is the first article in a series that will walk you through the complete process I use for shooting and processing multispectral photos. If you are a professional (or a very wealthy amateur), you may be able to avoid some of the steps. There may also be a better way of doing some or all of them, although I haven't found it yet.
Before beginning, a word of caution: multispectral photography today (especially using equipment that's priced reasonably for hobbyists) requires a generous amount of patience and dedication. If you are the sort of photographer who hates tripods, always uses autofocus, refuses to shoot in RAW format, and spends the absolute minimum amount of time using image editing software, you are probably not going to enjoy this branch of the art at least in its present form. That having been said, if you are willing to invest the time and effort you can create images that are impossible to obtain otherwise.
Whether you are in the field or a studio, you will need these pieces of equipment at a bare minimum to capture the images:
- A camera modified to capture near infrared and/or ultraviolet-A light (see Cameras).
- One or more lenses that provide at least passable performance outside the human-visible spectrum (see Lenses).
- A bandpass filter for human-visible light, and bandpass filters for near infrared and/or ultraviolet-A light (see Filters).
- A decent tripod. If you are on a tight budget, I recommend a solid camcorder tripod as opposed to a low-end consumer camera tripod.
The following are not strictly required, but strongly recommended:
- An umbrella, or some other way of shielding the camera from direct sunlight. I was tempted to include this in the "required" section, because your results will generally be much better with one than without. Casting a shadow over the camera can help prevent the infamous near infrared hotspot and the ultraviolet-A equivalent that would be just as infamous except that fewer photographers shoot UVA. It will also prevent lens flare, which can utterly overwhelm a multispectral photo.
- A remote shutter-release control for your camera. Wireless models are best for most situations using this technique. I use this for all of my tripod-based work to avoid jostling the camera, irrespective of the exposure time. If you must forego the remote, an alternative is to use the timed shutter-release mode (the one that's generally used for photos the photographer wants to appear in).
- A WhiBal, or other grey card.
- A stopwatch for timing long exposures using "bulb" mode.
- If you are shooting landscapes, a few neutral density filters can be very useful for lengthening human-visible-light and near infrared exposures to match ultraviolet-A exposure times. Again, see Filters.
Before you begin, make sure your camera is configured to save its photos in RAW format. This is not optional. You must be able to adjust the white balance and maximize the dynamic range of your infrared and/or ultraviolet shots later, in an image-editing program. Even for regular photography I cannot recommend enough that you shoot in RAW format, because it provides much more flexibility later on, and is the easiest way to get perfect colour using a grey card like the WhiBal.
This should go without saying, but keep your equipment's optics as clean as possible. Dust on a lens or filter can show up more distinctly in other bands of the spectrum. Dust and debris on your camera's sensor will mean long hours in the image-editing software later on, or reshooting the photos entirely. I generally use a Giotto squeeze-bulb blower, a Lens Pen, and a microfibre cleaning cloth (in that order) on my lenses and filters. If I've just obtained a secondhand lens or filter (or one of mine has gotten a significant amount of dust on it), I will preface my standard cleaning process by using the blower, using the brush end of the Lens Pen, using the blower again, applying liquid lens-cleaning solution using a piece of lens-cleaning tissue, then cleaning the solution off using a microfibre cloth. This is sort of obsessive-compulsive, but being careless here can easily result in scratching the optics. For the sensor on my D70, I have not had any luck with traditional sensor-cleaning products. The only approach I've found that works is to lock the mirror up, hold the camera body upside down, then use an air compressor (with blower attachment) at about 50 PSI to give the sensor and camera interior a thorough blowing-out. This may be a terrible idea, but so far it's the only method I've found to get it clean. Despite my best efforts, dust and debris collect frequently on the sensor of my D70, and so even if I had much deeper pockets it wouldn't be practical to have a professional clean it.
The Shooting Process
The actual shooting process is not technically challenging, just time-consuming. Don't try to rush through it, because it's easy to miss a step or make a mistake that you won't catch until much later. As with so many other things, getting everything right (or at least as good as possible) early on will save you a lot of time and headache later.
You will need to perform the steps in this section regardless of whether you are also shooting near infrared, ultraviolet-A, or both.
- Set up the tripod and compose your shot with no filters in front of the lens. I will often do a rough composition handheld first, because it's easier to move the camera around this way and then set up the tripod when I have a general idea of the lens and angle that I like best. Leaving the human-visible-light bandpass filter off is important because it may interfere with the composition due to its blue tint. When you are composing for anything other than infinity focus (IE almost everything other than landscape photography), make absolutely sure to leave a wide envelope of focus headroom on both sides of the scale, because unless you are using a very expensive multispectral lens you will need to adjust the focus for the near infrared and/or ultraviolet-A exposures. If this means using a different lens and/or moving the camera, so be it. For landscape photography and other infinity focus situations, see the notes in the next section.
- Lock the tripod in place. Make sure it is stable and not going anywhere. The alignment process used in the next article can correct for the sort of minor shifts that usually crop up even with a tripod, but it has its limits. Be sure that you have easy access to the front of the camera. You wouldn't want to position it right on the edge of a cliff and then end up having a filter fall to its doom when you switch between them.
- Attach the human-visible-light bandpass filter to the front of the lens. For this first shot, you can take the picture as you would any other tripod-based photo, although I strongly suggest using an umbrella to shield the camera from direct sunlight even for this initial photo. Most of the lenses I use don't support light-metering with my D70, so my process is to set the aperture I want, then make an educated guess about the exposure length, and use the histogram display to determine if I need to adjust the exposure time. Obviously it may take a couple of tries to get this right.
- If you are using a grey card, take a shot of it now.
- If you are shooting a scene that would benefit from a neutral density filter (a waterfall, river, etc.) attach that filter and take that version of the shot using the same process as for the initial shot. If you are using a grey card, remember to take a shot of it through the neutral density filter as well! The white balance will not be the same as without the NDF.
- Remove any filters from the lens.
Near Infrared and/or Ultraviolet-A Exposure
The process for shooting these exposures is essentially the same; differences will be noted below. If you are taking both, you will need to follow the steps in this section once for each of them. This section can likely be simplified considerably on a DSLR with "LiveView" (the ability to lock up the mirror and preview the current shot on the built-in LCD instead of using the traditional viewfinder). Mine does not have this feature, so I haven't verified it myself. The focusing steps are not necessary if using a very expensive multispectral-specific lens (see Lenses.
- Verify through the viewfinder that the shot is in focus.
- Attach the near infrared or ultraviolet-A bandpass filter to the front of the lens.
- Adjust the focus to the best of your knowledge based on any factory markings (such as a red "infrared dot") and previous experience with the lens. Note that especially for close-up work, factory infrared markings are often wildly inaccurate. There is a lot of trial-and-error required here, especially for a lens that you haven't used before. It can be very helpful to make a note of objects in the frame which are behind and/or in front of the subject matter so that you can more easily determine in which direction the focus needs to be adjusted after examining the initial results. There is a very counter-intuitive aspect of ultraviolet-A photography which is important to mention: although the focus position for near infrared is always counter-clockwise from the standard focus position (at least on Nikon lenses), the UVA focus position may be in either direction, sometimes even further counterclockwise than for near infrared (particularly for values close to the near limit of the lens). I believe this has to do with the parts of the optical design that are intended to limit or prevent chromatic aberration in standard photogrpahy (see Lenses). For landscape photography and other infinity focus situations, keep in mind that in most situations near infrared is "far-sighted" and ultraviolet-A is "near-sighted" with regards to human-visible-light. Unless you stop the aperture way down for all of the exposures (f/22, for example), an infinity focus shot will have a deeper out-of-focus foreground in the near infrared and will not be focused on infinity in the UVA exposure. Generally I've found that the only way to produce a usable ultraviolet-A landscape shot is at or about f/22, and of course this carries with it its own complications like very long exposure times.
- Optionally, stop down the aperture to provide a greater margin for error with regards to focus. For ultraviolet-A, I will typically avoid doing this until I'm reasonably happy with the focus, because of course it increases the already long exposure times.
- Adjust the exposure time to compensate for the subject matter, spectral band, filter, and sensitivity of your camera. When using a Hoya R72 filter, I will generally have to decrease the exposure time by one stop. With a B&W 093, exposure times are generally the same as for visible light. Using an LDP 1KB, I may have to increase the exposure time by as much as six stops. For ultraviolet-A filters such as the Baader U-Filter or a stack of a B&W 403 and an LDP CC1 I will often have to increase the exposure time by ten stops or more, especially if I'm using a wide aperture for the visible-light exposure and a narrow aperture for ultraviolet-A. If the subject matter is plant life, you will most likely need to decrease exposure time by a stop or two to compensate for high near infrared reflectivity. Ultraviolet-A exposures are where I most often use the "bulb" exposure mode and a stopwatch. This isn't necessary in broad daylight, but in other situations I've had to use exposure times as high as 8 minutes to get a reasonable amount of UVA light. In these situations I will generally get the focus right using a shorter exposure time and maximum ISO setting on my D70, then return to ISO 200 and the long exposure for the "real" exposure, because even my patience has its limits. Note that to my knowledge, no camera body will perform light-metering correctly for near infrared or ultraviolet-A light, so you are stuck using the histogram method whether or not you also use it for human-visible light.
- While using an umbrella (or other implement) to shield the camera from direct sunlight, take a picture. Examine the results both for focus (zooming in on the LCD to verify sharp details), and for exposure using the histogram. I tend to leave my D70 configured to display the histogram overlayed on all shots for this purpose. Keep in mind that unless you've specifically calibrated the white balance of your camera for the spectral band you're working in, the histogram will appear "wider" than it will after processing, because the three channels will be shifted relative to each other. This means that (especially for ultraviolet-A, which cannot be fully white-balanced into greyscale in my experience) some clipping off the left and/or right of the histogram may be acceptable. You'll need to get a feel for this yourself based on the specifics of the equipment you're working with.
- Make any necessary adjustments to the focus and/or exposure time, and repeat until you are happy with the results. If you see a "hotspot" (a circular bright area near the middle of the frame), you have three options: use a wider aperture (and if necessary adjust the focus accordingly for the more shallow depth-of-field), switch lenses (different lenses are more- or less-prone to hotspots depending on their design), or live with it and spend a lot of time in post-processing minimizing its impact on the final image. I highly encourage using the first two options if at all possible. An umbrella or other way of shielding the camera from direct sunlight will help prevent hotspots (especially UVA hotspots), but there don't seem to be any guaranteed methods of preventing them if a given lens is prone to the effect. See Reducing and Eliminating Hotspots for a lengthier discussion on this topic.
- If you are using a grey card, take a picture of it now.
- For near infrared, you may optionally shoot an exposure using a neutral density filter (using the same process as for human-visible light) to e.g. help match up moving water with the exposure length for ultraviolet-A. Again, if you are using a grey card remember to take a picture of it through the NDF as well as without the NDF, because they do alter the white balance enough to make a visible difference. Be aware that the NDF may not reduce the light level by as many stops outside of the human-visible spectrum.
If you are shooting both near infrared and ultraviolet-A, repeat the previous section using the other filter. For subject matter that is prone to changing over the course of this process (trees whose leaves are blowing in the wind, shadows cast by the sun), I will usually do a first pass of human-visible/near infrared/ultraviolet-A, then repeat the near infrared and human-visible exposures. This is because the UVA exposure generally takes me the longest to get right (due to the longer exposure times), and I can quickly follow it up with the "known-good" focus position, aperture, and exposure times that I already obtained for the other two.
Finally, if you are at all unsure about the success of any of the exposures, repeat it before you unlock the tripod. Remember, getting it right now will save you considerable time later on.
One you've obtained at least one set of exposures, you can proceed on to Aligning Multiple Exposures in Hugin.