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Filters for infra-red plus

Petrochemist

New member
I shoot with a full spectrum converted camera so can play with a wide range of filters to select wavelengths.
The convertions I've seen generally use high pass type filters that block all wavelengths below a cut off, but there is an alternative 'superblue' conversion. With a full spectrum conversion any of these should be possible along with many others. (Most normal photography filters transmit IR)

Exactly which wavelengths are available to the superblue conversion is not disclosed, it's simply described as something like 'blue & infrared'.

Getting the best from a filter often requires more than just putting it in front of the camera/sensor. The appropriate white balance, lighting & post processing can all make a significant difference. So I thought perhaps those of us who shoot with modified cameras could share some tips on the filters available.

The most common high pass filters are IMO R72 (& equivalents ~720nm), Red 'highcolor' and perhaps orange/yellow 'supercolor'.
R72 & the filters with longer wavelength cuts are usable on non-modified cameras, and generally give little distinction between the color channels - standard processing for these is CWB on grass & processing an monochrome for a high contrast finish.

With the red to yellow varieties grass is again the most common CWB, though I've heard of concrete & paper being used as alternatives. Post processing frequently involves a Red/blue channel swap to give blue skies - though I often like the look SOOC.

The superblue gives blue skies SOOC. I don't have exactly that filter but have a few that should be similar. These include Tiffen 47 & 47B, Schott BG3, & Hoya U330 all of which transmit UV, & Infra red often with some visible thrown in. None have so far quite replicated the shot's I've seen on line for superblue conversions, but some of the results from U330 have been quite interesting.

My most commonly used filter at the moment is a 'variable wavelength' one. Basically it seems to be a red filter with additional variable neutral density. The crossed polarisers adjusting the proportion of visible passing through while having minimal effect on the infra red. Effectively giving a filter that can be adjusted between highcolor & R72 styles at will.

A standard variable ND filter works in a similar way cutting the amount of visible light while transmitting infra red at relative constant amounts. Unfortunately I've not found anything to reduce IR controllably while transmitting visible. Hot mirror types are about the only option that block practically all IR.

I've had variable results with everyday film type filters. An old cokin A020 (blue) gave nice results one day & rubbish the next time I tried it. Obviously more work needed!
 
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