One of the secrets of retouching is enhancing small details that draw a viewer’s attention. For faces, that means eyes! Adding a little brightness and shine to the subject’s eyes will make them seem more lifelike to the viewer. This technique should be used with restraint, as overdoing it will be obvious, or at least disturbing.
The choice to use Soft Light here is a nod to that restraint. When I first started using this technique, I relied on Screen, which does not become useful until the opacity is lowered (multiply is the same way). That meant at some point I was not actually brightening the colors so much as adding white and desaturating. Soft Light is not nearly as strong as Screen, but has the advantage of preserving the saturation of the lighter colors.
On a blank layer above the photo, set the blending mode to Soft Light. Use a soft, round brush set to about 20% opacity and 30% flow, a little smaller than the iris (the colored part of the eye). Choose white as your foreground color and work gently around the lighter areas of the eyes, paying attention to and preserving much of the natural shading. The soft brush will also brighten a little of the skin around the eye, and that’s fine so long as it’s not overdone. The goal is subtle enhancement, not a glowing horror film effect.
Once the overall shaping is complete, decrease the brush size to about half that of the iris, then lower the opacity to about 10%. Look for where the image light is coming from and how it highlights the iris. There should be a lighter and darker area, and you will paint very carefully in that lighter region, creating a new highlight (avoid the pupil at this stage). If the effect is too much, undo it and lower the Flow setting to 10%. You should look for just the barest change here at first.
For softly lit portraits, this should probably be all you need for the highlights. You want to both enhance the shape of the eyeball and bring out the color without diluting it. Highlights over the iris are meant to show clarity as well as draw attention.
Now it’s time to work in the darks just a bit. Switch the paint color to black and ensure the brush is small enough to deal with the edges of the iris. Gently brush over the dark edges to give them a little more presence. You can also use this over the pupils, but don’t fill them in completely—a tiny variation in the blacks implies depth here.
The rims of the eyelid can also be painted over, especially at the outer edges of the subject’s eyes. This same technique can be applied to eyelashes as well.
Bring a little of the color back in with a new blank layer set to Overlay blending mode. Sample a part of the eye color that is reasonably saturated but not too dark. Make a few small strokes outward from the pupil to the edge of the iris. This is a really subjective part of the technique, as it’s easy to get wrong. The look you’re going for is to disrupt the highlight just enough to maintain realism, without creating artifacts or looking like the eyes have been colored in. Overlay will have the effect of darkening these areas most typically, unless the model has extremely bright irises already (such as you might encounter with colored contacts or some animals). Often, just a few dabs or tiny strokes will be sufficient.
Overlay is a bit stronger than Soft Light, and helps bring back some of the saturation of the colors you sample from the eye without losing detail or causing artifacts. Here is a little demonstration of using the various blending modes for this kind of dodge and burn.
Notice that Overlay (bottom pair of lines) has a stronger effect in the midtones when painting with black or white, while Soft Light is more uniform across the range from black to white. When you want to enhance what’s there, choose one of these two blending modes.
In some cases, you can get a dramatic, vibrant change using a slightly different approach. Rather than gently painting in the highlights, this is kind of a brute force method of getting a glassy-eyed look.
On a blank layer set to Normal blending, use a hard, round brush the same size as the iris. Set the brush’s Flow and Opacity options to 100%. Pick a neutral gray, around 55% brightness, and stamp a single dot over each eye. Add some noise for texture by choosing Filter > Noise > Add Noise, and choose Gaussian, about 15%–25%. Deselect Monochrome so you get some color variation.
Now change the layer blending mode to Divide.
Egad! It’s horrible! Lower the Opacity—quick. Depending on the original eye, the opacity range could be anywhere from 10% up to 80%, so just try it out. Now add a layer mask and soften your round brush. Mask the edges and the pupil. Alternatively, switch the Brush blending mode in the options bar to Clear. This erases the paint on the canvas using the same brush, and you can avoid creating a mask. Also, if the speckled colors are too strong, use a Gaussian blur of about 1 pixel to soften the look.
Now we have a very bright, highlighted iris. Both methods can be made to give nice results. The painting method gives you control over position and shape of the highlights, while the selection method is very fast. There is no reason the two can’t be combined, either. In general, Soft Light blending will be more precise and fit a wider variety of eye colors with little variation. The Divide method can be hit or miss, but gives really dramatic results very quickly.