Lab experiments: Basic Lab color space enhancement in Photoshop CS3

I’ve been thinking back on my early days of photography, Tri-X black and white film developed in a closet converted to darkroom near my bedroom as a 7th grader. Fortunately for my parents, both were isolated in the basement of our house so the aromas were fairly contained. The process required to transform photons to prints was staggering in comparison to today’s digital workflows and my passion for it had to be fueled by youth.. and no alternative. Given the capabilities today, I can’t imagine spending hours in a safelight lit room, although when I get a whiff of vinegar, I’m still transported back in time to that little closet.

Recently, I’ve been experimenting in Lab, the color space, not the kind with test tubes and/or monkeys. I had heard a few buddies talking about Lab before but had never dipped a toe. Then someone was brave enough to show me his before pictures and an example of the adjustment process. The combination of the ease with which the adjustments were made and the dramatic results floored me and I could resist no longer. My first experiment with Lab was a huge success and since then I’ve been playing with old photos in my spare time (actually little mental health breaks from coding). I’ve been meaning to blog about some of my experiences and haven’t had the time, but I’m facing down a particularly ornery bug and I needed a break.

I’ll give you this warning now: once you start playing with color adjustments in Lab, you’ll feel really sad that you’ve shown people some of your photographs, even those you’re most proud of.

These are the steps I used to convert this picture from a recent hike that David and I took near Durham. The original photo warranted three stars in my Aperture library. It’s not a great picture, but I thought the colors were attractive and the composition somewhat interesting. The image looks a little flat and dull. Here is a cropped section of the original image:

Before anything

I opened the image in Photoshop CS3 and converted the image to Lab with Image -> Mode -> Lab Color. I created a new Curves adjustment layer (you can do this through the menus with Layer -> New Adjustment Layer -> Curves… or with a shortcut by clicking on the “Create new fill or adjustment layer” button at the bottom of the Layers panel (it looks like a half white/half black circle) as shown below and selecting Curves… from the pop-up menu.

Given that the original image seemed to have an overall blue cast to it so I started with the b channel adjustments by selecting ‘b’ in the channel pulldown menu in the Curves adjustment window. Lacking any known source of white or gray in the image, I used my best judgment. With a white balanced image, the normal a and b channel adjustments pull in the left and right corner points the same amount such that the line pass through the origin of the curves graph. As you play with the b channel, you’ll see that shifting the curve to the left (as I have my Curves setup, to the left if you have yours setup in the CMKY mentality) reduces the blue cast of the image.

b channel adjustment

Here’s the result of this adjustment:

Leaves after b channel adjustment

At this point the colors are warmer and the blue cast is reduced. Some may say I’ve gone too far, other may say not enough. On this day, with my laptop screen and nice overcast light coming in through the windows behind me, it looks better than it did and better than more or less as I played with it. Now on to the a channel.

a channel adjustment

The a channel curve is a more traditional Lab color enhancement curve in that it passes through the origin. The purpose of this change is to create a steepening of the curve as it passes through the values in use, essentially the same as what we do when adding contrast to the lightness channel. The corner points can be pulled in different amounts depending on the amount of adjustment desired and your mood that day, so it’s good to play with them. The amount I show here is a moderate, average adjustment in my experience. Remember that you can always tweak the adjustment with the layer opacity later, so even if it looks a little too strong, you can tame it down later. Here’s the result of the a channel adjustment:

Leaves after a channel adjustment

As I’ve been working my way backwards through the channels in Lab, the Lightness channel is the third and final that I adjusted. This channel contains no color information, only, well, lightness, and if you look at this channel on it’s own (command 1 on a Mac.. any Windows users willing to let us know what it would be on a PC?) it looks like a black and white version of the image (command ` will get you back to normal).

Lightness channel adjustment

In the Lightness channel, I only did a minor contrast enhancement S-curve. This alters the image in a similar way that a normal RGB-channel curve like this would. Here’s the final result of the color enhancement:

Leaves after Lightness channel adjustment

I flattened the image (Layer -> Flatten Image) and converted the image back to RGB (Image -> Mode -> RGB) and applied a light sharpening to the image using Filter -> Sharpen -> Unsharp Mask… using these settings:

unsharp mask

As always when sharpening in RGB, it’s important to apply the sharpening to the Luminosity channel, not the color channels. To do this, immediately after using unsharp mask, select Edit -> Fade Unsharp Mask… and select Luminosity from the pulldown menu:

unsharp mask luminocity

Here are the pre- and post-enhancement images for reference. Click through each to Flickr to see larger versions.

leaves before

leaves after

The adjustments definitely have an effect on the final image. The sickly blue haze has been lifted and the colors are more vibrant. This is certainly not the most dramatic effect I’ve experienced when working with Lab color enhancements. In fact, this and other Lab techniques can resurrect images that RGB approaches can’t help and would otherwise be fitting only for the delete key. In this example, I believe that an okay photo was improved, and that’s what matters.

The Lab color space is fundamentally different than RGB or CMYK which is why some of the magical color enhancements that take seconds to perform in Lab are impossible or extremely difficult in RGB/CMYK. If you’ve never played with Lab, it’s well worth the time. And if you remember the smells of darkroom chemicals, it’s interesting to consider how unimaginable things like Lab adjustments were back in the day. If there is interest, I’ll continue posting examples of image enhancements that I do using Lab over time. I’d also like to explore other software that is capable of working in Lab since not everyone is willing to splurge for Photoshop. If you’ve used other Lab-worthy software, please drop a note in the comments and let us know what it is.

  • davak

    Very nice changes.

    I am scared to see you playing in any kind of lab again. ;)

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  • Matthew Goff

    Thanks for the gentle but authoritative prod into Lab! I have been using Photoshop since version 1 and have never played around in it yet.

    Also, I will remember to sharpen in the luminosity channel next time. It looks like I learned TWO new things today!


  • vijayakumar

    I tried lab.I use CS 3.When I convert the image to Lab mode, image is completely filled with black. I am unable to use lab mode. what is the solution?

  • wert

    on the PC combination would be CTRL+1 for lightness channel and CTRL + ~ (tilde) to get back to composite.


  • Chris

    Really well explained – so we’ll all use lab from now on
    Thanks for taking the time to share with us.

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