At some point in the future I'm considering entering the market for a second-hand IR-converted camera, i.e. one that has had its internal IR-cut filter replaced with a long-pass filter that cuts visible light instead. This question is about the choice of replacement filter for such a camera.
I tend to gravitate towards the deeper infra-red, such as 830nm. This is because I'm mostly interested in black and white images, and because I'm less interested in the classic "infra-red look" than in the ability of infra-red photos to produce strong contrast between shadows and sunlit areas.
However, I do tend to use colour information quite a bit in producing black and white photos, so I feel I might be losing a bit of flexibility in going for 830nm, even if I only use it for black and white. Because of this, I'd like to get a sense of how much difference the 830nm filter really makes, in comparison to 720. Of course, if information is available about other wavelengths that's useful too, but those seem to be the most common choices.
In particular, I'd really like to see a comparison between the same scene shot with 720 and 830nm internal filters. The only one I could find was a now-removed page from LifePixel.com archived at archive.org, but in that case the monochrome images at 720nm ("standard color IR") and 830nm ("deep BW IR") look indistinguishable. Presumably this isn't the case for every scene, so I'd really like to see a comparison example where it does make a difference.
Additionally I'd really like to hear from IR photographers what the difference is like. In particular, am I right in thinking it will be easier to get the sky to come out really black and the shadows to be really dark?
I know that to some extent these effects can be achieved/enhanced by post-processing, but there is some aspect of it that I think can't be reproduced, which comes I think from the absence of scattered light illuminating the scene from the sky.
One other very specific question: one of my lenses, the Pentax DA15mm limited, produces a hot spot under some conditions when using an external Hoya 720nm filter. This hot spot is distinctly blue/purple in colour after white balancing, which makes it straightforward to remove when converting to black and white. Would I be likely to see the same thing with an internal IR filter, and if I switched to 830nm would the hot spot be likely to get better or worse, or is it hard to know?