Tuesday, August 12, 2014

False Colour Digital IR Processing

In this post I want to talk about digital IR processing, and in particular how to ‘recover’ some of the colour in the scene.
Before discussing the process, however, it is worth saying that not everyone likes the IR false colour effect. If you’re one of those, then stick with the monochrome approach; hint use Silver Efex Pro II.

To recover colour you first need to ensure your IR conversion, or if you are simply using an external IR filter, still has some colour information being recorded. My 50D conversion from http://www.lifepixel.com/ is a standard IR one, ie with only a hint of colour, but sufficient to get some colour recovery.

The first port of call is the camera. If you do nothing, other than leave the camera on its ‘normal’, RAW, ie don’t capture in JPEGs, settings, you will get a very red looking image. As you are shooting in RAW, this is not critical, it just makes in-camera reviewing a little more difficult.

To address this there are two things to consider. 

First set up a custom WB by filling the frame with green (living!) grass in sunlight; and use this image to set a custom WB.  

Despite the in-camera custom WB, the camera LCD review will still look weird, but at least you wont have a strong red cast! These two images show a normal sunshine WB compared to the (grass-based) custom WB. These also illustrate what the RAW looks like before post processing.

Remember these WB settings are only impacting the review JPEG. Such in-camera WBing has no permanent impact on the RAW file. In other words, it is questionable whether it is worth doing, especially if using ML’s Auto-ETTR (see below).

Sunshine WB
Grass-based Custom WB
Second, if the red-cast worries you, consider switching the image preview to monochrome. In fact, as an aside, try setting the image review to monochrome for normal colour photography as well: it will help you not be diverted in your composition by colour.

As for capture, be wary of two things: focus and exposure. Your auto focus mechanism will be thrown off by the IR spectrum; so use Live View or calibrate your focus mechanism to the IR-biased world. Note that the focusing Apps at www.georgedouvos.com allow you to factor in the IR dimension when calculating hyperfocal distances.

For me exposure is a non issue as I use Magic Lantern’s Auto-ETTR to nail the exposure: I find a 1-2% clipping works well.

Another trick is to set up a custom camera profile for initial use in Lightroom. The way to do that is to download the Adobe DNG Profile Editor and create a custom profile, which should be placed in the appropriate LR profiles folder for your system set up, eg Mac vs PC. Rather than go through it here, check out this link: http://dustyden.org/wordpress/?tag=adobe-dng-profile-editor
A final post processing trick, if you want some sensible looking colours in your IR image, is to carry out a Red/Blue channel swop in Photoshop, or some other image editor that allows channel swopping.  Hint set up an action to speed things up, ie I have allocated F12 for colour swopping and thus the process is a single click.

Having swopped the channels you can now post process further in Photoshop, Lightroom or your favorite Plug-in, knowing one of your image’s environmental anchours, the sky, is a sensible colour, ie blueish!

These final two images illustrate the ‘false colour’ IR effect: as I say, not to everyone’s taste!

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