Quote Originally Posted by DWThomas View Post
The Rollei IR400 makes some statement that it "extends to 820 nm"
Inimitable marketing BS, a unique selling point for Maco stuff

And that applies to filters, too. If some chinese manufacturer puts "900" on a filter that only means there is a "900" on a filter unless you use a filter from a premium manufacturer, then it will have a pretty sharp cutoff at 900nm. The el-cheapos might be labeled "sort of IR", but they work fine. I used a Cokin IR filter - acrylic stuff - for two years until it was so scratched I replaced it with a good glass filter. It's better, but not by a huge amount.