Tuesday, 26 May 2015

Infrared image

Here is my first effort at an "infrared (IR)" image - taken earlier today.  

Infrared image 

This is made with a Hoya R72 infrared filter (received in this morning's post) fitted to my Canon 5D mk3. The key steps in the process for producing this image were as follows:

  1. Screw on the filter.
  2. Set the white balance. None of the "standard" white balance settings are suitable because the image will be looking at the infrared component of light. The custom white balance is set by reference to brightly lit green grass. In practice when the filter is attached the camera sensor sees only the IR light from the sun that is reflected off the green grass. The sensor does not see green.
  3. Take the shot. This is easier said than done because the filter takes out essentially all visible light; nothing is visible through either the viewfinder or the LCD screen. The easiest way to focus is to unscrew the filter, set the focus, switch to manual focus and then re-attached the filter, taking care not to move the focus setting. After some trial and error with exposures, the shot was taken at ISO3200, f/8 with a  15s exposure. The exposure time would have been even longer if I had used a lower ISO value. All IR photographs obtained the method  described here require a long exposure time - you can see that some leaves are blurred as a result of the breeze and long exposure time. (In practice I am sufficiently familiar the focus settings on the lens that I could set the focus near enough without use of viewfinder or LCD, and without unscrewing the filter. For some shots it may be important to note that IR focuses at a slightly different  point than visible light. Here the f/8 setting gave sufficient depth of focus that the different focal point for IR did not matter.)
  4. Process the raw image in Photoshop. All the visible colours in the image are "false" because the IR itself is not visible. Processing decisions are therefore based on aesthetics and what other people have discovered to produce a pleasing image. My main processing steps were as follows: 
  • Add a Channel mixer layer: Image > Adjustments > Channel mixer. For the red channel, set red to 0% and blue to 100%. For the blue channel set blue to 0% and red to 100%. This effectively swaps the red and blue channels.
  • Add a Levels layer: Image > Adjustments > Levels. Adjust the sliders for black point, white point and grey point to obtain a pleasing range of tones. I was originally advised to automate this by clicking the Auto RGB button, but a bit of trial and error showed that manual adjustment is preferable.
  • Add Hue / saturation layer: Image > Adjustments > Hue/saturation. Use the targeted adjustment control to bring the saturation of the foliage colours into an acceptable range. Similarly, make adjustments to the saturation of the sky. 

That completed 95% of the processing but here I also made some small local contrast adjustments in the long grass-like foliage at bottom-left of image, and in the tree at right.

If it's not clear from what has been said, The real colours were the colours of spring in Scotland - green grass and leaves, and a pale blue sky with white fluffy clouds. The sky was not that colour and none of the foliage was pink. All the colours in an IR image are false colours.

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