Chasing Highlight Tones

Here at LensWork, we are so involved in the world of offset printing that I forget sometimes how foreign this world is to most photographers. Here is an example.

At the very highlight end of the scale, photographers often don't mind having a perfectly pure white tone in their prints. We call these 8-bit RGB 255, or 0% tone, or 100% white — the terms can be confusing. In the world of commercial printing, it's all about ink coverage, so I tend to think of pure white as 0% ink coverage.

Now, 0% tones are a problem in printing. It's theoretically the whitest white the paper can produce, but is problematic in two primary ways:

Any area that graduates from light grey to pure white will cross a threshold where, somewhat suddenly, there is no ink at all. Where this transition takes place leaves a squiggly line of demarcation that can be seen, somewhat similar to banding.

The second area where 0% ink coverage is a problem is when that tone is on the edge of the image, often in a sky. The tonalities in the print simply float off into the paper without defining the edge of the image.

In either case, the existence of pure, toneless white can be very distracting and one of the sure signs that the prepress folks didn't do their job.

One of our preflight checks is to examine every image before printing to be sure there are no pure whites. Photographers often send us images that have a pure white. If we find a pure white, we throttle back the tonal range of the print to be sure there is a very slight tone printed when the image is reproduced in ink. You don't want to pull back too far or the highlights start to look gray; not far enough and the press may not be able to physically print the dots. You'll need to find out what your printer is capable of producing and use that as your throttle point.

In our case, because the presses at Hemlock are so good, we dial back the highlights to 2%. That means that any spot of pure white tone in the image will actually print with 2% ink coverage. This is so slight you don't really notice it, but it's there — and most importantly prevents both of the problems that would be created with 0% tones if we didn't modify the clipped highlights.

Fortunately, this is easy to do with Lightroom. It's a good idea to be aware of this with any image you are sending in for reproduction just in case your printer doesn't check for this. Personally, I would never send off a file with pure RGB 255 whites just because I never know how sophisticated the printer is or whether or not they will check for this subtle detail.

Here's how we fix this in Lightroom:

We examine every image destined for ink (this does not pertain to on-screen images) to see if there are any areas of absolute white. If you don't know this trick, it's a good one. In the Develop module, look at the upper right corner of the histogram where you will find a small, upward pointing triangle. This is the highlight clipping toggle. You can either hover over it for a temporary display of highlight clipping or click on it to turn it on.

With the highlight clipping invoked, any pure white areas of the image will appear with a red mask like this.

These areas have no tone at all (well, technically they are 100% white, or 0% ink coverage, which actually is a tone, albeit one with no information at all).

If you see this when you are working with a RAW file, you might be able to pull out some tones that are available in the RAW data by using the "recovery" control in Lightroom — a preferred approach, but not the point of this discussion. Let's assume for now that this option is not available to you.

How do you create a 2% tone to pull back the highlights from clipping? We do it with a simple preset that will create this . . .

from this . . .

Even on the computer screen, you can now see the top edge of the image (barely) helping define the rectangular shape of the image because of that ever-so-slight tone in the sky.

Why not 1%? Actually, the real question is: Why not 5%? For ages in the printing world, 5% dots were considered the target for pure whites because most presses can't print a dot small enough to accurately reproduce anything less than 5%. Even if your file had 5% tone in the highlights like this . . .

The tones in the final image would be lighter because of the loss of subtle dots as they got smaller and smaller. So the above visual on-screen image may look more like this when printed because the highlight dots are just lost in the process of applying those tiny dots to the paper.

It was always a guessing game as to how much highlight detail would be lost. Since printers wanted to give themselves as much wiggle room as possible, 5% became the standard highlight dot target.

Like all things, technology has improved in recent years and Hemlock's presses are consistently capable of producing a 2% dot. This is a mere wisp of ink that is barely visible with a good loupe. But even the experts at Hemlock cannot reliably print 1% dots mainly due to variations in the surface texture of paper. That they can consistently do 2% is an amazing feat. Since we are motivated to squeeze every bit of quality out of our printing as we can, we use 2% and dance on the edge of disaster. Wheeeee!

The super-advanced class in all of this occurs when you have an image printed on the back side of the sheet. If the back side image has very dark tones and if the paper is not 100% opaque (no paper is), then the dark tone on the back side can have an affect on the light tones in the image on the front side. This is precisely why so many book designs have no image on the back side of the very high key images. The so-called "bleed through" of the dark tones can make the subtle highlights on the front-side image appear gray.

I guess this is yet another reason why I love the folio concept of individual prints where one does not have to worry about these kinds of issues that we sweat bullets over once we engage the offset press to make a traditional book.

Here is our 2% preset. Download 2% Highlights

This preset is a simple tone curve that remaps tones from one level to another. Notice the very slight downward curve at the very highlight end of the tone curve graph.

Here is the 5% curve preset that is even more visible . . .

The reason we use a preset rather than simply modify this curve is that the end points are not moveable by dragging. The only way to modify either end point is with a preset.

If you want to modify this to some other level, open the preset in a text editor. It will look like this:

s = {

id = "340E9E26-24BA-4558-9AE9-A0C9F48D3DDB",

internalName = "2% Highlights",

title = "2% Highlights",

type = "Develop",

value = {

settings = {

ToneCurve = {

0,

0,

32,

32,

64,

64,

128,

128,

192,

192,

255,

250,

},

},

uuid = "674990D8-49DF-11DB-94CF-000A9599D634",

},

version = 0,

}

If you want something other than a 2% change in the end point, then simply modify the last pair of numbers in the sequence. In the example above, the last two numbers in the sequence are 255 and 250 which simply remaps the level 255 RGB tones down to level 250 — roughly 2%. Simply change the 250 to whatever you want and save the preset with a new name and copy/move it to your Lightroom presets folder. A 1% tone is a remap to 252; a 3% tone is a remap to 247; a 4% tone is a remap to 245; and a 5% tone is a remap to 242.

Comments

[wtlloyd]

Thank you for this discussion, very useful.

You may not have been aware as of yet that Lightroom 3 will introduce "point" curves in the Tone control panel, allowing direct movement of the curve and end points....

[Brooks Jensen]

Point curves are all well and good, but will it allow the two end points to be adjusted, too?

[Neil Enns]

Very useful post Brooks, thanks for sharing!

[richardplondon]

Moving the ends of the point curves: Yes, it seems to work in the second Beta. No need to use ACR to tweak and save custom curves, you can do it in Lightroom directly. Single channel (luminance) not multichannel (RGB), obviously. Like working in just the L of Lab.

[Godfrey DiGiorgi]

Excellent tip for Lightroom 2 use, thanks very much Brooks!

I've been doing this for some time ... it's essential to make a good print (to my eye anyway!) that *some* small amount of ink be laid down even in the highlight areas. This makes it much easier to do without distorting the rest of the tonal range.