I'm looking at getting into IR photography, and am trying to decide what cutoff frequency filter to buy. Has anyone done a comparison of the same scenes using different filters?
Well, I haven't personally, but the folks over at Life Pixel have. In summary:
Pictures of all of the above options can be viewed at the Life Pixel website. Full disclosure: I am a customer of Life Pixel, having gotten my Canon 20D converted by them a couple years ago (went with the 720nm, Hoya R72 equivalent and I completely love it!). I don't get anything for mentioning them... Just a happy customer. :-)
I would go for the 720 nm, any longer (800s and you're limited to pure black and white - i.e. you get no colour response. Any shorter (500-600) and you start to lose some of the IR look, that is to say the very dark skies and glowing vegetation.
A 720nm filter is a good choice for a first IR camera as you can get the look of any of the other filters (with a bit of extra post production) so you can decide which look you like best:
Here's a deep IR conversion, in monochrome.
Here I've gone for a super vibrant colours look (for demonstration purposes ;)
Finally here's the look I prefer, leaving a subtle hint of colour.
These were all shot with a Canon 450D with 720nm conversion done by Protech in the UK. I was 100% satisfied with the service but it's worth pointing out that another company, ACS (advanced camera services) also do conversions though their prices are a little higher.
I use a Sigma DSLR for IR work, as you can remove the internal IR filter to allow IR wavelengths to pass and then use a filter atop your lens and then put it back when you want to switch back to color photography. A suggestion I would make is to look first at using gels - they are very cheap flexible sheets that cut IR (I have a Wratten 89b gel), and you can use them in Cokin-P square filter holders which in turn are also very cheap (the gels may require some cutting down to fit in a holder).
You can also try shooting without any filters at all, it can be especially interesting around dusk to gain more light - here's an example which I believe used the camera with no IR filter whatsoever (no internal and no external filter):