Using Threshold to Track Highlights

Using Threshold to track highlights

One issue I am always running into is how to track/measure highlight luminosity as I post-process files. Usage of curves or levels, not to mention dodging adjustments can iinadvertently push close-to-the-edge highlights into dreaded blow-out territory. Sharpening and using USM to enhance local contrast can also take a toll. Conversely, soft-proofing for print often begs that highlights be pushed just a tad so that the eye sees a print's sparkle in context of the intended substrate's brightness.

I used to pepper my images with color sampler points and kept an eye on the info panel to judge how the highlights were doing. Then, I learned about this simple technique from George Barr, ( georgebarr.blogspot.com/ ).

Specifically, set up a threshold adjustment layer, as the topmost layer. Set the slider to 250 (or whatever highlight value you want to find), so that any pixels at or above 250 will show in white. Reduce the opacity of the layer to whatever you like, so that you can see the white as well as the underlying image. In the example at the left, I've set opacity to 60%. By toggling the layer on or off, and keeping it as the topmost layer, you can see where your "dangerous" highlights are in an instant . (In this example, it is not topmost because the file includes the output sharpening layers.)

And there's more, as Ron Popeil is want to say . When prepping for print, you can toggle on the threshold layer and brush-dodge for the highlights you want pushed for the print in a separate, lower layer. Then just toggle off (obviously) before sending to print.

Record this as an action and it is a one-click effort. You can do the same for locating shadow values below a certain point by setting the threshold slider to whatever shadow value you want to find. I've got that as an action too but use it much more rarely.

As a final preflight for any file, I often toggle off all adjustment/work layers, leaving just the background layer (on the bottom) and the threshold layer (on the top) active. Then I turn on each of the intervening layers in sequence, watching the impact change in the threshold layer. If I find something happening that is unwanted, a simple mask addition or mask touch up on the offending layer is usually enough to cure the problem.

George's tip has saved me a lot of time and helps with print prepping.