Duotone. I suppose the first thing to say about them is to take a look at a few, to get a feel for what a duotone is.  The Wikipedia link gives a good idea, with a pretty good example.

It’s pretty easy to make a duotone in Photoshop – I’ve been using PS since version 5.5 and I don’t remember it *not* being there (although I didn’t really use it until CS2 came out, so my memory may be faulty).  The effect can be quite nice, and it’s a favorite technique that I often use with infra-red photos.

So, how to convert your  image:

1: Open your image in Photoshop

For this article, I’m using an older color photo that came out… poorly.  The light was good, but a combination of environment and lens resulted in an unnaturally low-contrast image.

Note the blues from the shadows, and how (typical Alaska), the slanted sun didn’t really have a lot of the golden hues that you’d like. What was a great view in life rendered… poorly in the camera.  But no worries: it makes a decent black & white image.

2: Convert to B&W

Use your favorite technique here.  You could go old school: desaturate, use curves to enhance contrast, and go on.  Or use Photoshop’s B&W filter – it does a pretty good job.  In my case, I used Nik’s Silver Efex tool to get here:

Personally, I use the Nik tool for speed.  Nothing better, and I’ve always loved the end result.  Plus, it feels a bit like the old film days: this conversion was all of 20 seconds.  Red filter (darken out those blues), a slight increase in contrast, decrease in structure, increase in brightness, then local controls to darken the sky.

The photo itself now gives me a bit more feeling, something I like with the perpendicular (but uneven) lines.

3: Convert to full Grayscale

Even though this image image looks like a B&W image, it isn’t.  Depending on your technique, it could just be a layer hiding color info.  Whatever the technique, you need to start by getting back to a single layer.  I’m keyboard oriented, so my technique:

• Click on the highest layer.
• Engage “the claw”: press ctrl-alt-shift-”E” (mac: opt-alt-shift-”E”).  Your hand will look like a claw as you press it, and you’ll get a new top layer with all layers merged.  I.e., a full B&W layer.
• Press ctrl-E (mac: opt-E) until you are back to a single layer.

An alternate technique: just above the “Opacity” option in the layers panel is a button with three lines & a down-arrow.  Click it, and one of the options will be “flatten image.”  Most will find this faster, but for me it’s a distraction (hand off the keyboard to the mouse makes it slower for me).

Now, convert to full grayscale by navigating through the menu to “Image” -> “Mode” and selecting “Grayscale”:

You may get a warning pop up:

Click “Discard” if you do.

4: Start the conversion to Duotone

This is pretty much the same as what you just did:

Just as before, it’s under the “Image” menu – select “Mode” and click “Duotone.”

5: Set number of tones and colors

In my mind, the “Duotone” option is really a “multitone” option.  You can use anywhere from one to four inks, so it’s not really “duo.”  That being said, the default is a single ink – we want two, so select that from the drop-down:

The next step is up to you.  You can click on each ink and select a color.  You can also select a preset.  I like using presets to get close to where I want to be:

Notice how there’s a dark green overlay to the entire thing?  If you’re familiar with cyanotype, you’ll see that we’re getting close to that kind of effect with this preset.

Once you’ve made your final selection of colors, click OK and you’re done.

6: Bask in the glory

Or perhaps enjoy your work.  Or post to Flickr, which I will do with this pic sometime soon:

Note that I deliberately selected a different (non-cyan) color scheme.  I personally like the cooler schemes, but hey – why not mix it up?

Other things to do

If you like this effect, some thoughts for play:

• Play with it.  Try different colors, or different numbers of inks.  Two, three, four – each has its own unique effect.
• Use clashing inks: spin the color wheel & go for something obnoxious.  Like mouldy pea-soup green with neon pink.  Who knows, might be cool!
• Play with the curves – you’ll get different gradations in the overall effect.