polarizers vs ND filters for near IR

[Shinnen]

Hi,
My dsc f717 shoots near IR. This camera was seriously crippled (by Sony) when shooting IR, because of concerns that IR will see though some clothing. They restricted the shutter speed to no more than 1/60th of a second, and the aperture stays wide open. I'm not sure how this prevents it from seeing through fabrics, but that's not what concerns me. What concerns me is that, as a result of these restrictions, it is impossible to shoot IR outside during daylight, without filters. So, in order to capture IR during the day I have been using a combination of two polarizing filters, aligned so as to restrict as much visible light as possible. This produces images which, if what I read is correct, bare the hallmarks of true near IR, namely .... black and white images with very dark skies, bright white vegetation, and dark water. The question I have is ..... what am I doing here?
Thanks,
..... john

[Barefoot]

I wonder if you might have considered a Red Filter for IR photography like the one in the link? Or, maybe one like this that isn't so dark that it prevents composing when the filter is mounted. Either one may be of absolutely no use on digital, but on b&w film they are the bomb.

[Shinnen]

Hi Barefoot,
Yes, I've considered them, and I think they might work; but right now I'm using a couple of polarizers that I had lying around, which seem to produce a decent result. The only problem is .... I'm not sure what they're doing, i.e. are they doing the same thing as the filters you mention.
Thanks,
.... john

[Marko]

we need to see the images..... Normally we use the filters barefoot recommended OR the camera is already converted so we don't need the filter.

Polarizers cut reflections.... it's a different beast altogether. ND filters have nothing to do with this.

[Shinnen]

Hi Marko,
Right. Well, this camera does have an IR mode, but the images are kind of lack lustre, with a green tinge. When I put the double polarizer on, the sky and water turn very dark. I must emphasize that I resorted to this method because of Sony's crippling the camera in IR mode, making it impossible to shoot outside in daylight. The two pictures I'm sending were just taken. (It's overcast here, so the sky isn't as black as on a sunny day, and the clouds aren't as white.) The first image is with the double polarizer and the next one (the control) is with an ND filter, only. (I had to use an ND filter, otherwise the picture would be just white.) It think the double polarizer has the added benefit of removing any remnants of colour from the picture.
..... john





P.S. If you want to see another example using the polarizers, look here http://www.photography.ca/Forums/att...sc04595mod.jpg The black pond is ice and the sky is blue.