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| Recovering slides dynamic range
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Some of my Kodachrome slides are heavily contrasted, and scanning them is disappointing. To get better results, I have successfully adapted the technique described by Michael Reichman
(Digital Blending
on
The Luminous Landscape)
to my Nikon Coolscan IV ED scanner. Indeed, it is possible to adjust its analog gain (lighting LED's intensity). Before reading some more on the technique used, roll you mouse over the picture below to see the final effect (very noticeable on upper left foliage):
Technique revealed
The following image has been scanned "at best", adjusting the analog gain just before saturating the highlights. I noticed that I often need to push the analog gain with Kodachrome slides; this is not the case with Ektachrome. Could this be linked to the multi-layer specificity of Kodachrome ?
The darker areas have no detail, and adjusting curves will give relatively poor results as eye sensitivity is non linear. See this
Adobe document for more details.
A second over-exposed scan will nevertheless recover some details in the darker areas:
Because of over-exposure, it is necessary to adjust the levels "black slider" to recover contrast:
Applying one of the techniques described in Michael Reichman's
tutorial, I use a photoshop fusion mask to control transparency of the original image, revealing darker areas of the over-exposed image. This fusion mask contains:
- A B&W copy of the over-exposed image,
- To which a gaussian blur of 40 pixels is applied, recovering local contrast,
- Levels are finally applied, fine tuning transparency thresholds with the black and white sliders.
Un petit mot
