articles/Infrared/digitalinfrared-page3

Digital Infrared - part 3 of 1 2 3 4 5 6

by Mike McNamee Published 01/04/2012

RIGHT: The histograms show various stages in the workflow. At the top the as-shot histogram is centred. The levels adjustment is then made 27 on shadows and 205 on highlights. The third image in 8-bit shows some spikes which are absent in the 16-bit image.

LEFT: The Exposure and Blacks sliders are adjusted in ACR to just clip the highlights (marked red) and the shadows (marked blue).

LEFT BELOW: The difference between an ACR-levelled image and a Photoshop-levelled image. They are not quite the same.

'Exposing to the right' is the accepted mantra for full spectrum imaging, but all advice on the subject for IR suggests avoiding over-exposure and a more centred histogram. In the image shown, with a centred histogram there are two stops 'going begging' before any sign of clipping in the highlights. This two stops represents 75% of the available bit-depth from a 12-bit capture system, 2,048 from the first stop and another 1,024 from the second stop, leaving just 1,024 for the remaining tones of the picture.

Sharpening

For the image shown, when processed all the way to inkjet print, including 'normal' sharpening levels, there were significant artefacts visible as square blocks (similar to JPEG artefacts but without JPEG compression being applied!), especially in the sky. This was processed in Photoshop for both curve adjustments and sharpening.

For many, IR sharpening will be slightly new (or at least revisited) territory with many older cameras being converted to IR as a way of extending their useful life. They are thus likely to have smaller pixel counts and as such require a slightly lighter touch during sharpening. Then there is the compressed bit-depth we have spoken of in the previous section. Smeared on top of all this is the optimisation required between ACR pre-sharpening and Photoshop sharpening prior to printing (usually for inkjet). We are, therefore into new territory and some experimentation will be required. Working in 16-bit is an obvious first step to alleviate some of the bit-depth issues.

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• Digital Infrared page 6
  

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