I assume you are already familiar with the subject of infrared photography with a digital camera. If not, read my article on that.

So, what's so special with using an SLR for the purpose? Nothing much: the main problem remains the same. Roughly speaking, when you shoot infrared with an IR filter on a stock (unmodified) SLR, you face the CDI (Curse of Digital Infrared):

• The IR filter on the lens stops the visible light, letting only infrared through;
• The anti-IR filter in front of the image sensor stops the infrared, so nothing reaches the sensor to form the image. Goodbye, next customer, please.

Luckily, both filters are not 100% strict in doing their respective jobs. In particular, the anti-IR filter in front of the sensor lets a tiny fraction of infrared through, and that's what allows us to shoot in infrared using unmodified cameras. That fraction is, however, really tiny; the resulting exposures usually are a thousand times or so longer than in visible light. Stepping the lens down to F/8 usually requires exposures of 2-4 seconds or even longer.

All this means that just removing the anti-IR filter would greatly increase the camera's infrared sensitivity: the silicon in the sensor is, actually, very sensitive to infrared. Bingo, home free: we can walk around and take pictures as usual.

Well, yes and no. Nothing comes free, and an SLR camera is not just a device controlling the shutter. Some other features we take for granted may suffer. And this will be discussed in the present article.

If you search the Web, you will find a number of companies (ranging from specialized outfits to individuals working at their kitchen table) which offer a service of modifying your digital camera to become infrared-capable. While these modifications always involve removal of the anti-IR filter, they will turn your camera in either an infrared+visible or infrared-only device. These two kinds differ considerably, so they should be discussed separately.

Infrared+Visible conversion

This conversion boils down, in principle at least, to removal of the anti-IR filter. More exactly, it cannot be just removed, for a number of reasons.

The most important one is that the effective optical path length in a millimeter of typical glass is the same as in a 1.5 mm of air. Therefore removal of a filter which is, say, 1 mm thick reduces that effective path by 0.5 mm. Now, remember that the autofocus sensor and the SLR viewing screen in a well-adjusted SLR are at the same effective distance from the lens as the imager. "Moving" (not literally!) the latter half a millimeter closer will cause that what the AF (or viewfinder) sees as in focus, to be out of focus in the captured image; more exactly: a backfocus, with the sharpness zone moved away from the camera. Not good.

This is why the anti-IR filter is usually replaced with a plain glass plate of the same effective thickness (i.e., thickness times refractive index). There will be further complications, to which we will return later.

Such a V+IR camera will register both visible and infrared light, in approximately equal amounts. The images from it cannot be treated as "regular" color ones, because all three kinds of photosites (red, green, blue) will respond to infrared, throwing the colors hopelessly off-balance. On the other hand, we cannot treat the resulting image as a "classic" infrared one, as it contains also visible-light input, diluting the IR effect.

This is, however, easy to remedy: you just need two filters (used on the lens). With those, you may use the camera in two distinct ways:

• For visible-light photography use an anti-IR filter, similar to the one you've just removed from the sensor and transparent only to visible light. Your camera will become "normal" again, or at least close. The only problem you may experience is a slight shift in color balance, if the external filter is not a close match for the removed one.

  The autofocus and autoexposure should work just fine in this mode, dealing only with visible light for which they were designed.

• For infrared, use a regular IR filter (like the R72). This will stop visible light from messing up your IR images.

  Unfortunately, with the IR filter on the lens you will see nothing in the optical viewfinder; this is where cameras with Live View will have an edge.

  The AE and AF sensors also get only IR light this time; still this should be enough for them to work, if barely. The AE (light metering) will, with most cameras, need a compensation (if a fixed value); the AF will become slower and less accurate (less light!) and may become shifted in a particular direction.

  The latter is mostly due to the fact that the refraction index of glass depends on the wavelength. This is true also for various colors within the visible light spectrum (vide: chromatic aberration of lenses), but the differences increase when we go into infrared. Any filters (original or replacement) on top of the sensor will contribute to the effect.

Thus, the V+IR modification, with two external filters, will allow you to use the camera in two different capabilities. You will not have optical viewing when shooting in infrared, but at least the exposure times will be dramatically shorter (a factor of 1000× or so) than in the original camera.

This may look like the best of both worlds. This is, however, not necessarily true; keep on reading.

Off-the-shelf options

At present (March, 2008) there are two digital SLRs providing off-the-shelf IR capability. Both belong to the V+IR category described above.

The Fujifilm IS Pro it is not cheap at $2500 (body only). The Fuji S-5, which is basically the same camera for visible light only, sells for about $1000 less.

The Sigma SD14 can be easily modified by removing the anti-IR filter, which can be done by removing a few screws without disassembling the camera. That filter is positioned somewhat uniquely: in front of the mirror chamber, so that it affects equally the imager, AF sensor, and the viewing screen. All this means that the filter does not have to be replaced with a compensating plate discussed above.

This filter placement has an extra advantage: dust and dirt on its surface are too far from the sensor to cast a "shadow" on it. You may wonder why isn't everybody using that solution? The problem is that any particles freed of the moving shutter and mirror have unobstructed access to the sensor surface (or whatever extra filter may be sitting in front of it) — but this is a separate problem, outside of the scope of this article.

Infrared-only conversion

This modification is similar to the one above, except that the anti-IR (infrared-blocking) filter in front of the sensor is replaced with an IR (infrared-passing) one. This difference has some far-reaching implications:

• Shooting in visible light is no longer possible.
• There is no need to use an IR filter on the lens.
• Normal SLR viewing is possible, using visible light.
• The AE and AF sensors receive both visible and infrared light.

The last circumstance is a mixed blessing, and it deserves some explanations.

First, autoexposure works exactly like in an unmodified camera, designed to record visible-light images, with that visible light then cut-off just before reaching the sensor. In principle, the exposure as metered has nothing to do with the IR light reaching the sensor. In practice, however, both are quite well correlated, so that in a given kind of light (say, afternoon sunshine), a fixed value of exposure compensation may work just fine. (You may have to use a different compensation value on cloudy mornings, though.)

Second, autofocus also works as if nothing happened, being tuned for shooting in visible light. Because of different refraction coefficients, however, the focus plane for visible light is different than for infrared.

When shooting through an IR filter on the lens, AF does not suffer from this systematic shift, as it works with what is available: infrared only, even if the light intensity is affected by the filter.

Some IR-only modifications take this into account, adjusting the thickness of the IR filter at the sensor to compensate for the effect. The compensation is, however, rather crude, 0-th order only, as the visible-to-IR focus shift depends on the lens design, the current focal length (for zooms), and even on where the lens is focused. Take it or leave it: this is all what is possible without re-designing the AE and AF systems in the camera, a task beyond any shop's capabilities.

A manufacturer designing an IR-only SLR would have a seemingly easier task. Both AF and AE sensors are based on silicon, which is quite sensitive to IR. Therefore these sensors (most probably) have some anti-IR protection, so that their readings are not confused by those wavelengths in visible-light use. The "right" approach would remove that protection, replacing it with one against the visible light, so both systems would work with the same kind of light as the sensor.

Using the image sensor itself for both autofocus and exposure metering (a trend observed among recent SLRs with the Live View feature) would automatically address these issues. Ironically, this is where an IR-adapted non-SLR camera has an edge over an SLR. In practice, however, a skillful application of exposure compensation and closing down the lens to gain some extra depth of field should be enough to provide very satisfactory results from an IR-only converted SLR.

Doing an infrared-only conversion you have to decide what kind of IR filter do you want to have installed over the sensor. This is similar to the choice of an IR filter described in my other article, but various vendors may have various options, so this is worth checking out.

Which conversion is for you?

Don't ask me. With the basic facts listed above, you should now have enough information to reach a conclusion best for your needs and preferences. I can only say what's best for me.

Here I have no doubts: an IR-only camera suits my needs best. I am not sure about color quality of an IR+V camera using an external anti-IR filter, and I prefer to be able to use the optical viewfinder — after all, this is why I'm using (mostly) SLRs.

Another advantage of the IR-only solution is that this way I'm avoiding the cost and hassle of multiple IR (and anti-IR) filters. At present my lenses use four different filter threads; eight such filters (four of each kind) would cost $700 or more. Step-up rings are not really a solution; after all, why would I have to give up using lens hoods?

For these advantages I am willing to forgo the capability of using the same camera for visible-light imaging, and that of experimenting with various kinds of IR filters. Your mileage may vary.

How to do the conversion

Simply: leave it to someone who is qualified to do it. Disassembling your digital SLR is a daunting task; re-assembling it — even more so. You need proper tools, proper glass (kind and thickness) for the replacement filters, and industrial-class clean conditions. Most of all, you need to know exactly what you are doing.

If you are one of a dozen or so people in this country who are qualified (and have all else what's needed) to do the job — well, power to you; you should be writing this text, not I. For the rest of us mere mortals there is only one way out: finding someone whom we trust to perform the operation.

I've seen some outfits or individuals offering this service on the Internet (some are showing up and disappearing after a few months). I was also trying to find opinions of the people who had their cameras modified. Based on this, I came with a very short list of companies who may deserve your trust here:

• LDP of New Jersey (or MaxMax.com)

  These are people specializing in IR and UV applications, not just camera conversions. They also sell IR-block and IR-pass filters, IR lights, and other related items, including thermal and surveillance equipment. Their Web site is factual and informative. A typical SLR modification costs $450 for a wide variety of brands and models.

• Life Pixel Infrared Conversion Services of Washington (or LifePixel.com)

  They seem to specialize in conversions only, offering this service for Canon, Nikon, and Fuji cameras, with prices of $350-400 (somewhat more for high-end models). The Web site, while less informative than the other one, provides also do-it-yourself instructions for selected cameras, and offers the necessary parts for sale.

While I haven't (yet) dealt with either of these two companies, the LDP makes a good, solid impression, and they seem to have lots of positive feedback. Besides, they do convert some Olympus cameras (E-500, E-510), so I've decided to try them out with the former model. I'll be sending it for conversion in a week or two, and the whole story will be described elsewhere.