Maximizing Colour during Post-Scan Editing

Version 1.1, Page 2, ©2002 by Dale Cotton, all rights reserved.

One of my luckiest shots, 42-24, has long been overdue for re-working. When I got the roll of Reala film back from the lab I was overjoyed to see that I had not totally botched this awesome cameo I had stumbled upon one spring morning. My initial interpretation of the negative was constrained by the pure white sky resulting from a normal exposure scan.

I've learned a lot about both Photoshop and scanning since then...

• 1. Scan once at 0 exposure compensation and once +2 exposure compensation.

Fig 2a. 42-24 at normal exp. and 2b. at +2 exp. Click to download larger versions.

(If you wish to work along with this tutorial, click each image to open a larger version. Windows users can right-click on the larger version to save to your hard drive, presumably Mac users also know their own drill. Then open both in Photoshop. Leave the AdobeRGB colour space in place and do not crop at this time.)

I don't know if this works with your scanner or not, but with my Canon FS4000 I can re-scan the same frame multiple times with differing settings without ejecting the film holder. This results in multiple scan files with exactly the same registration. If you can't do this, there are methods for re-aligning mis-registered scans, but they are not covered here.

Again the normal exposure scan has a nearly white sky. The +2 scan has excellent sky colour but is otherwise much too dark.

Fig 3. Curves to remove colour casts plus result

• 2. Open the normal exposure scan. Use Image->Adjust->Curves to apply a + green and - blue curve to remove the colour cast. Save the curve, then re-apply it to the +2 scan.

As with most raw scans from the Canon using its supplied software this one has a purple cast. Unfortunately, there is no neutral grey tone to use with the mid-tones eyedropper of the curves dialogue for automatic colour cast removal, so we'll have to eyeball it. A moderate gamma curve increase in the green channel compensates for the magenta portion of the purple haze. Decreasing the blue channel by a similar amount removes the blue cast due to the colour temperature of the predominant shadow lighting.

You can experiment with the black point and white point, but in this case I chose to leave them alone.

Let me back up on my soapbox for just a bit more. If you are working with neg film and a scanner you may despair when faced with the challenge of removing colour casts like these. Skin tones can be a nasty business if there is no neutral grey to home in on. But for nature shots like this one there's no need to struggle. The goal is not some mythical faithfulness to the "real" scene. Even Provia or any other slide film won't buy you that. Which real scene? The one your eyes saw; or the one your mind saw after your visual cortex got through with it; or the one with the ugly blue colour cast the film recorded? Instead, the goal is to find a colour adjustment that pleases your eyes right now. If that happens to come close to the original scene - great. If not - pride yourself on your artistic sensitivity. Instead you might have used a warming filter (and incidentally skewed the sky colour). Is that any more faithful to the original scene?

• 3. Convert both to 8-bit.

Like most fanatics, I always scan to 16-bit, do as many adjustments as possible, then dumb-down to 8-bit mode. I would also spot before converting in real life.