I dedicated this article to infrared photography because I rarely see anyone experimenting with it, which is a shame as some stunning photographs can be made in this invisible world. If you need some motivation, scroll down and look at examples in this article. They're far from being representative for all of IR photography, but they may be enough to get you interested.
I'll talk about shooting infrared on digital SLRs, as I don't expect anyone already shooting IR film needing any help so all examples you will see were shot with Canon EOS 300D. You can apply most of this to compact digital cameras which can carry filters (or to all compacts if you're willing to hand-hold the filter in front of the lens).


IR filters - distinction

IR stands for infra-red. Just saying "infrared filter" is somewhat imprecise because today exist two kinds of filters that deal with this part of spectrum.
First kind are IR-block (a.k.a. low-pass) filters which block IR wavelengths. Virtually all digital cameras have these built-in, as sensors are sensitive to this part of spectrum which isn't visible to human eye. This would result in conversion from infrared light to visible pixels, which degrades image quality (ie. produces something that doesn't resemble the way you see it in the real world).

Second kind are IR-pass filters. They block all (or most) visible light and let through only long wavelengths corresponding to near-infrared and infrared parts of the spectrum. In some manufacturers, you can tell from the filter's name which wavelengths it will pass. For example, Hoya R72 filter lets through 720 nanometer and longer waves. RM90 blocks everything shorter than 900 nanometers and so on.


Hands-on

Take such filter in your hand, look through it and -- surprise, surprise -- you won't see anything. Pitch black. That's due to the fact you don't see infrared and the filter blocks everything else.
This also means you'll have hard time shooting, because you don't see anything through your dSLR viewfinder.
Now, in case of infrared film, things are somewhat simpler as this film can usually be used with deep red filters, so you can still compose and focus your shots with the filter screwed on the lens. (However, getting your hands on this kind of film is very hard, and exposing it properly is even harder.)

With digital, on the other hand, you won't have any luck without a tripod (unless you own certain Sony or Fuji models. But that's a different story).
That's due to several reasons:
1) As you can't see anything through the dSLR viewfinder, you need to compose your shot before putting on the filter. Only feasible way to do this is by using tripod.
2) Autofocus won't work, as it too won't 'see' anything. (certain pro SLRs are excluded) So you need to use tripod and pre-focus, but also take into consideration IR light focuses differently than visible light. (more on this later)
3) Due to built-in IR-block filter, only the smallest fraction of IR light will be actually admitted to the sensor. When you screw on IR-pass filter, it will block visible light and you'll remain with almost no light. Only practical way to battle this is using a long exposure. By long, I mean in the ballpark of 15 seconds on brightest sunny day (ISO 100, f/4).


Focusing, shooting...

So now that we've established some basic requirements for shooting IR with a dSLR, let's get a bit deeper.
I mentioned IR light focuses differently than visible light. I won't go into physics of that, but I will say that lens manufacturers are aware of this and many have solution in the form of a red dot on the lens. (This is only possible if you have a distance scale on your lens though, and even then it's not guaranteed you'll have the red dot.)
So if you have this red dot, you would focus normally on whatever you want to be in focus, then disengage autofocus and check the distance scale. You remember subject's distance and turn the focus ring until that distance is lined up with the red dot, instead of your usual focus marker.
Second thing you should now do is compose the shot as you want, and given that the camera is on tripod, secure it. Put the IR filter on the lens and cover your viewfinder. Failure to do this will produce hotspots, due to light coming through your viewfinder which by reflections inside the camera eventually reaches the sensor. It will accumulate during a long exposure (more than a few seconds) and produce a hotspot, ugly and hard to remove later.

Now you're ready to shoot. Simply guess your first exposure, because there are too many variables to estimate it. If it's sunny, try 10 to 15 seconds at f/4 and ISO 100, and go from there. Depending on your built-in IR cut filter (on your sensor), it might be enough.

Given that you don't have a special camera which can remove internal IR cut filter on command, or specially modified camera (mostly used in astrophotography), you will have to battle with chronic lack of light in infrared part of spectrum, so any kind of photography that isn't under direct sunlight will suffer from minutes long exposure, if not more. For example, this exposure at dusk took 7 minutes f/5.6 on ISO 200 (and also sent noise levels sky high). At exposures this long, it's almost impossible to effectively guess the exposure as after 10 or 20 minutes dusk is no longer dusk but night... so you get the picture.


First results, fake colors...

When the shot is taken, consult your histogram to evaluate exposure. Any kind of other visual assessment is likely to mislead you. Don't be surprised, colors on the image won't be like anything you've ever seen, it will be a red monochromatic image (or red & white). Some people call this fake colors, but depending on the filter you've used they may be more or less fake.
The thing is, many filters let through light of 720nm which is still in the visible area (eyes can see up to about 780nm, which is deep red), so part of the visible red spectrum will influence the shot. That part isn't fake colors, they really are red.
The longer wavelengths, however, will produce fake coloration because your sensor's red pixels will absorb these waves and portray them red, which is hardly accurate as humans who don't see these wavelengths can't really visualize them nor put a name to them.
Depending on your sensor and it's built-in filter, some cameras may see deeper into the IR part of spectrum than others, but generally speaking that whole area is called near infrared. For real infrared shots (including heat waves) you will need infrared film or specialized digital equipment. Preferably military. (;
However, near IR can be fun too.


What to shoot?

Lastly, a few words on the choice of subject. Infrared world looks differently than what we're used to. A black and white photo will look very differently from IR shot converted into B&W. (that's the whole point of IR photography, after all)
*EXAMPLE*
Notice how IR image packs a punch, cuts through haze, accentuates the clouds, increases shadow contrast...

I'll just mention a few things that, from my experience, look most effective on IR shots, and you do the rest of experimenting for homework.

Sky. Sky doesn't reflect any IR radiation from the sun, so it appears very dark to black. Very effective on shots.
*EXAMPLE* (10s f/3.5 ISO 200)
Water. Water absorbs almost all IR radiation that falls onto it's surface, so it also appears black.
*EXAMPLE* (6s f/3.5 ISO 200) (also see example at the bottom)
Green plants. Most plants contain chlorophyll, which reflects most of IR radiation that falls on the leaves, so they appear white. You must have seen white grasslands or forests on IR shots, and now you know why.
*EXAMPLE* (10s f/5.6 ISO 400)
Black clothing (and clothing in general) won't turn out the color you may expect, and you'll get wide variety of effects depending on the fabric in question, but mostly shades of gray. Sometimes even some skin or undergarments can show from underneath. (;
So if you plan on shooting a model, keep in mind his/her clothing might not turn out the way you wanted.

Finally, you may have noticed two kinds of images - black & white and monochromatic red. Camera produces latter ones, and it's up to you to convert them into B&W or leave them as they are. In theory, IR photography should be all black and white, but who cares about the rules?


Now here are a two more examples to top it off. Read EXIF for exposure info.




All examples in this article were taken with Canon EOS 300D and Hoya R72 filter, tripod and cable release. Lens used: EF-S 18-55mm f/3.5-5.6, EF 50mm f/1.8, EF 28mm f/2.8.


Hope you enjoyed... and good hunting in this invisible world!