Monday, 26 May 2014

False Colour Infra-red Processing (part 1)

This tutorial shows my workflow for taking an infra-red image from RAW to final, using the 'false colour' technique. The editing will be done in Adobe Camera RAW (ACR) and Photoshop CC. I chose this example image because it has a good mix of clear sky, clouds, foliage and rock. The strong direct sunlight and shadows provide nice contrast and dynamic range. One of the main features of this process is retrieving the blue sky, but this is a trick (is fake). Before I get ahead of myself though, here is some information about the image:
  • Settings: 1/125th, f/8, iso 100
  • Camera: Sony A7 (converted to full-spectrum)
  • Lens: Nikkor 35mm f/2.0 AF-D
  • Filter: Heliopan Red (#25)
  • Tripod: No
  • Area: Stone Circle - Keswick, Cumbria (Lake District), UK 
  • Time of Day: Evening, about 45 minutes before sunset

This is the unmodified RAW image. If you're taking infra-red photos I highly recommend shooting in this format if you can. RAW files offer greater flexibility with colours and levels, often resulting in lower noise and better tonal range. This is what a colour infra-red image looks like with the white balance (WB) incorrectly set in the camera. Fortunately editing the RAW file offers the ability to alter the WB values in post production with no loss of quality.

Here is an example of a better WB setting. The 'temperature' at it's minimum value of 2000 brings the red channel much closer into line with the green & blue. It's often still not possible to get it low enough changing this one setting, but don't panic yet.

Dragging the WB Tint value down will help align the green and blue to each other. This will help get a more balanced colour image, but it won't bring much more of the red channel's values down - still not time to panic.

By clamping the values of the green channel it vastly improves the colour balance and it also brings the red channel more into line. This is partially because the values you seen in the individual channels are an absolute, whereas the histogram you see at the top right is a result of the manipulation. To put it another way - we're clamping the 14bit values (16,384 tones) into 8bit (256 tones) in a better way and thus improving the dynamic range.

Clamping the blue channel values will bring the red channel further into line with the other two. With all of the channels potential maximised I'll take the image into Photoshop (CC), but before you open the image and go out of ACR you can do any other edits you wish at this point.

Small tweaks to exposure can be made and gradients and/or vignetting can be added but this will throw out the histogram values a bit. If you do this after the channel tweaks you'll have to come back and re-adjust the clamped values again. Contrast and Saturation adjustments will skew the individual channels however, so larger changes will be needed to bring the values back into alignment, so I recommend getting these the way you want them before doing the channel tweaks the first time.

Now for the 'Flase Colour' technique. This involves swapping the red and blue channels around and for this we will need to add a 'Channel Mixer' adjustment layer. Once added choose 'Red' from the 'Output Channel' drop-down menu and then manually swap the values of the red and blue sliders. Red should be changed to '0' and Blue to '100'.

Now select 'Blue' from the 'Output Channel' drop-down menu and manually swap the values of the red and blue sliders again. Red should now be changed to '100' and Blue to '0'.

To finish off my process I add a 'Hue/Saturation' adjustment layer. I moved the 'Hue' slider until the sky reached a realistic colour (although it can sometimes be fine where it is). I also boosted the Saturation of the entire image a bit, but this will be a very personal taste thing.

If the blues get out of control when you boost the colours you can select blue from the drop-down menu to reduce it's saturation independently.

Although I often make many little tweaks to the sliders in ACR, add gradients and vignetting etc. this will vary greatly depending on the image. Hopefully this covers everything for the basic principal, but if you think I've missed anything or you'd like me to add any other information please feel free to write a comment below.

PS. I will be back soon to talk more about camera hardware and will soon be starting the lens reviews specifically pertaining to infra-red performance. I should be able to test most lenses on the Sony A7, so if anyone has any requests please post a comment below.

1 comment:

  1. I just converted my Fuji EX2 to a 720nm camera. Can you share any processing workflow information, this is a new area of photography for me.
    Thanks in advance for the advise.