Here's a basic tutorial on how I do my eyes. This works for blue and brown eyes. I use Photoshop CC and I se a MAC with a mouse.
1. Prep your horse as you usually would and open a new, transparent layer. Some people like to duplicate the layer, it doesn't really matter which you use. Using the flexible lasso tool, set the feather to 2px. This will give the eyes a nice, soft edge and make them easier to blend into the socket. Select your eye, making sure to circle the whole inner eye.
2. Select a medium sized, round soft brush. You'll need to keep changing brush sizes throughout the editing but I always keep mine at 0% hardness unless I am drawing iris lines. In which case I use 50% hardness. The opacity will vary throughout the editing.
3. Select a mid-tone shade in your chosen colour. This is your base colour. We will be adding highlights and lowlights to the eye so make sure the mid-tone you select as a base is a happy middle between your other shades. I filled my eye in using two layers at 40% opacity.
4. Next, it's time to edge the eye with a low-light. Select a shade a few stops darker than your base and if needed, change the size of your brush to a smaller size. Along the border of your selection, add the darker shade. Like most mammal eyes, the colours o the iris are dark at the edges and brighten towards the pupil.
5. The blocking-in of highlights is next. Take a lighter shade than your base and around the middle of the eye, leaving a thin line of your base between the dark tone and the light tone, add in some high-light. Keep in mind eyes aren't perfect so lines won't be straight and nothing will be perfectly round within the eye. It's okay to be messy!
6. next is adding colour and 'mess'. Bouncing between your three shades, and, if need be, adding more, draw small patches and thin wiggly lines throughout all the colour. here is where reference photos and practice come in handy. To blend, use the smudge tool on a small, soft brush and 10% opacity and mix them.
7. The pupil is next. horses, like goats, have rectangular style pupils. I usually change the shape of my brush to a sort of fat sausage. Pay careful attention to here your horse eye is looking. Don't forget eyes are 3D so if your horse is looking off in the distance the pupil will appear shorter. I set my brush to pure black at a 50% opacity and block in my pupil.
8. Next is to make the eye really pop. Keeping it selected, go to > Image > Adjustments > Brightness & Contrast. Move the sliders around. You may need to click 'Done' and do it multiple times to get the correct look you want. I like un-realistically bright, colourful eyes. You may want a more natural look.
And we're done!
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