Colorimetric Correction

The tonal and chromatic rendering of images coated on transparent photographic films does not linearly reflect the brightness of the scene to which they were exposed. The sensitometric curves of silver emulsions have an S-shaped profile. Strong distortions appear systematically in extreme exposure ranges (the regions of high and low light). On the other hand, the "linear" region of these densitometric transfer functions has a rather variable slope (gamma) from one film manufacturer to another as well as depending on the conditions of the chemical treatment. If these non-linearities can be relatively simple to characterize and compensate in the case of invertibles, it is not the same with the negatives and particularly with the negative colours. Not only does the spectral response of the dyes used differs from one manufacturer to another but, more generally, the exposure latitudes also diverge between the dyes of the same emulsion.

Finally, real difficulties arise to balance the sensitometric curves of a raw scan. This is the difficulty encountered by the algorithms supposed to automatically correct the colorimetric profile of silver emulsions. Depending on the content of the image, automatic tone and colour correction can sometimes be successful or otherwise produce a rendering denatured or even disastrous.

Services included in our On-Line offer
Human intervention in the selection and adjustment of colour and tone enhancement tools is essential. This is precisely what we offer in the optional Post-Processing service of our On-Line offer.

For each image concerned, this post-processing is performed on the basis of the linear and neutral "raw-scan" image and produces a final rendering file in TIFF 48-bits format.

These operations and their settings can be partly performed with the Hasselblad FlexColor software editing tools and mainly under other more efficient tools like Photoshop.

Services not included
Restoration work aimed at correcting defects that may result from improper chemical treatment, deterioration resulting from improper preservation or handling of films (folds, scratches, stains, glued dust, fingerprints, etc.) are not integrated into the standard On-Line service offer. If necessary, we can respond to such requests based on Off-line business proposals.


Note

The only way to fine-tune the effects of the development and preservation of a photographic film would be to have on each film, information that would allow to recalibrate its real colour profile and to compensate for the distortions of its tonal curves. In other words, each film to be digitized should have a test image resulting from the shooting of an ideally illuminated calibration chart. To return to reality, it is therefore illusory to claim that one can correct accurately and fully automatically the tonality of an silver film, it is the same for its chromatic balance.

Unless satisfied with marketing arguments, the "perfect" does not exist in this area of activity.

Manual editing interventions are inevitable, and like any manual intervention, without reference to the original scene, they bring a slight degree of subjectivity about the settings made by the operator. But we are far from the gaps that can introduce the algorithms of "automatic correction".