articles/Infrared/digitalinfrared-page2

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

by Mike McNamee Published 01/04/2012

Lenses

The matter of lenses and hot spots in the IR range is complex. It is to do, in part, with len's element coating and internal reflections, and has little to do with cost or len's marque. Some classic lenses such as the Nikkor 12-24 f4 are unsuitable (although the 14-24mm f2.8 Nikkor is OK). The flaw, which manifests itself as a large flare spot in the image is both aperture and focal length dependent (in a zoom). It can often be eliminated with judicious use of an adjustment layer and mask in Photoshop. However, check the lists before you commit or you may find it essential to buy a new lens to attach to your newly converted camera body. A list may be found at: http://dpanswers.com/content/irphoto_lenses.php which details lens hot spot issues (and a lack of them).

A full spectrum and an IR image in ACR. Note the difference in the histograms.

Exposure and Bit Depth

The IR filter in a camera rejects wavelengths below 720nm, only allowing wavelengths above that point to proceed to the sensor. This inherently results in a more restricted dynamic range, a situation that is even worse on a dull, overcast day. Despite this the overcast day is quite a good one for many IR images.

The question then arises, where do you centre your exposure and how should the ideal histogram be placed? The chances of a camera's TTL providing an accurate exposure are slight; manual setting is the order of the day! In the example shown, the histogram is reasonably centred, the 'dynamic range' extends from 31RGB points to 201RGB points. This was processed with 'zeroed' RAW settings, followed by a Levels adjustment in Photoshop, where the Alt key was used to reveal (and avoid) clipping at both ends of the scale. The process was carried out using both 8-bit and 16-bit images from the RAW file. Conversion to monochrome was made using Image>Adjust>Black&White. Alongside this the RAW file was processed in Adobe Camera RAW (ACR) using the Exposure slider and the Blacks slider along with the Alt key to reveal clipping points. Conversion to monochrome was then made using the ACR 'grayscale' conversion with default settings. The RAW-processed file was preferred visually, although the significantly higher image contrast might not suit every subject. The main issue with the workflow was the vulnerability to loss of bit depth and posterisation effects.

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