What fading actually is
A photographic print holds its picture in dye or in metallic silver, and both of them change over decades of light, heat and whatever was in the air. The picture does not blur and it does not move. What happens is narrower than that: the darkest part of the print stops being as dark as it was and the lightest part stops being as light, so the whole photograph occupies a smaller slice of the available range. A scan of it is a perfectly accurate record of a picture with the contrast squeezed out.
That is a recoverable condition, and it is recoverable arithmetically rather than by judgement, because the missing range is measurable. Read the histogram of each channel, find where its data really begins and ends, and map that back onto nought to two hundred and fifty-five. Nothing is invented; the levels that survived are simply spread back out over the space they used to occupy.
| Channel | Range still held | After recovery |
|---|---|---|
| Red | 18 to 249, so 231 levels | Barely moved — this channel was fine |
| Green | 34 to 236, so 202 levels | Stretched, mid-tones lift slightly |
| Blue | 61 to 198, so 137 levels | Stretched hardest — this is where the orange goes |
Why per channel, and not one curve for all three
One curve applied to all three channels keeps their relationship to each other exactly as it is, which means it keeps the cast. It makes a faded orange print into a contrastier orange print. Measuring separately is what lets the blue channel — which lost the most — be stretched the most, and the cast comes off as a side effect of treating each layer according to what it actually lost.
What it will not do
A channel with almost nothing left in it is not recoverable. If a channel holds fewer than eight levels of range, the correction leaves it alone rather than multiplying a handful of values by thirty and calling the noise a picture. You will see this on badly light-struck prints, where one corner has gone and the rest has not, and it is the correct outcome: the corner has no data, and a tool that pretended otherwise would be selling you a guess.
A scan worth correcting
Fade recovery is only as good as the range in the scan it is handed. A scanner that auto-corrects on the way in has already made these decisions, usually badly, and has clipped whatever it decided was black. Switch every automatic option off and scan flat. A flat scan looks worse on the scanner’s preview and gives this page more to work with, which is the trade worth making. The scanning piece goes through the rest of it.
