Devious histograms

Jake Wartenberg is working on a wonderful chromolithograph of the Montana state capitol that's nearly done. He sent me a copy for review with a couple of challenges he was facing. And he was also generous enough to let me blog about it. So today we'll discuss more about histograms.

Changing the levels is just about the last step in restoration. It can be very useful to preview a levels adjustment in order to detect subtle problems that need to be addressed, but to do the actual fixes it's important to step back to a pre-levels version. It's a good practice to always save a version from immediately before the levels adjustment, and to keep that pre-levels version under a separate filename. Jake's got good habits and he's making more changes to his pre-levels version right now.

This is very good work and it's nearly complete. The bottom border needs cropping and an information tag that someone pasted into the lower margin needs to be clone stamped out. The data on that tag is useful--it ought to be transcribed for the image hosting page when the work gets uploaded--but it isn't part of the original work. Since histograms are dumb, that tag translates into bum data as far as the histogram is concerned. In turn, that has an effect on what the software wants to do with brightness and color adjustments throughout the image.

Another element to remember with older paper prints is that paper often acquires uneven brightness as it ages. Edges tend to dry out and darken more than the center of the paper. Notice how in this example it's the corners that are darkest of all, while some of the margin in the middle (top and bottom) is closer to white. So Jake's also going over his pre-levels image to create feathered adjustment layers that will even out the brightness and color balance.

While we were discussing this he asked me why that matters. How much difference does it make, really? Here's a demonstration from one of last year's restorations where the difference stands out.

Here's the unrestored version of a Robert Fulton submarine design from 1806.

And here's the final featured version. It took quite a bit of work to get to this point. The hardest part to fix was Fulton's description, because he had rubbed out the original description and written a second one in smaller lettering in its place. In order to get an even tone through that section I actually went in at 700% resolution, working with tool settings 2 and 3 pixels wide along the outside edges of each pen stroke. It took a lot of work, but the end result is a natural paper tone.

The original rubbed-out lettering is easier to see on a preview of the unrestored version.

But obviously, trying to work from this version isn't going to yield the best end results.

Histogram previews can be a wonderful way to glimpse the potential of an unrestored image. A histogram preview gives a hint of how far the restoration might go, and reveals challenges that might be difficult to detect otherwise. But whenever possible, do the other restoration work before a final histogram fix. The end result comes out better that way.