You don’t need to have your DSLR modified to produce “real” infrared images. But it helps.
I began infrared with Kodak HIE film (no longer made) and a Leica M3 rangefinder. I used that for a very long time, and I have to admit, it was my favorite in terms of results, but using it could be a real pain because of how sensitive that film was to light. Maybe it was just me, but I found that I needed to load and remove the film in a film changing bag. Then I read about how you could take a digital camera and have it modified to capture infrared images.
Then I had an early digital Rebel modified for me, which I liked a lot. The big thing with a modified infrared digital dslr is that you can really see what you’re doing and you get a nice bright image to work with. You don’t need any special filters.
But if you don’t have the money, or don’t want to get your DSLR modified, it is quite possible to do infrared with an unmodified digital camera. You’ll need to choose the infrared filter of your choice.
In a nutshell you can choose from the following infrared filters:
Hoya R72 Filter (Wratten 89b, B+W 092 equivalent)
The Hoya R72 was my personal favorite. It lets in some visible light and is probably going to need an adjustment of six or more stops depending on the subject and the lighting. If you were shooting foliage (which emits infrared light) and the normal exposure for a sunlit day was f8 and 1/1000th of a second shutter speed, you’d need to open up to about f2.0 and 1/60th (or some combination that gave you about 8 more stops of light). But with a digital camera it’s so easy (compared to film) because you’ll be able to immediately check the exposure on the back of the camera.
And then there’s the let’s cut out all visible light filters.
The Wratten 87 and 87c Filters
Assuming that you are using a DSLR, both filters are tricky to use because once they are on the lens, you really can’t see much since you are looking directly through the lens.