So, shall I get the awful puns out of the way now? Yes, the eyes have it. I am keeping an eye on you etc etc
Seriously, watercolour is a lovely medium for capturing human or animal eyes. Its transparnecy really lends a sparkle and if there is an eye in your painting, you know it will be the focus of attention.
Getting them right is down to careful observation and a little thought about their shape and how they sit in the skull.
If you can see both eyes, consider making them subtly different, with one slightly more dominant so that we, as the viewer, knows where to look first. Consider placement of the dominant eye on the third line….
BUT – the temptation will be to make the eyes too important in a painting or too large. They may end up looking like caricatures or cartoons if you do so. Again careful observation is the key.
These are often very simple. Retain the highlight as the brightest part and continue the colour of the eye into the surrounding feathers to help bed them in. Look carefully at shape and position (if it is a predator they will be at the front of the skull for binocular vision and if a prey, at the side to spot the predator coming). Look carefully at eyelids and edges; there is often a bright edge to the eye which indicates the wetness of the sphere. Some eyes, such as owls are more complex and should be handled like a mammal’s eye (see below).
These are usually more complex with visible iris, pupil, white, lids and lashes. Again careful observation is the key to shape and colour.
The highlight is going to be the lightest part – but look again, even the highlight might have variation and can tell a whole story. Often it is blue over the pupil and white over the iris. Remember the pupil is a hole. Try and mix a dark from colours used elsewhere in the painting to make it harmonious and interesting. Build in stages: iris retaining the highlight, outside/lids, shadow, bleeding the eye colour into the surrounding fur/feathers, finally the pupil (which will stop it looking like a zombie!).
Cat’s eyes look like marbles, so be careful with transparency and white edges. Pupils can be oval, horizontal, round etc – look, look, look! The white, if it shows, is often not white at all – and is darker than the highlight. It will show the shadow of the brow etc. Younger animals tend to have bigger eyes in comparison to their overall size – it’s why we find them adorable!
Top is a tawny owl without a bright highlight, as their eyes are deep set and below is a cat….
I came across this post on human eyes. It is so comprehensive I certainly can’t better it:I http://www.wetcanvas.com/forums/showthread.php?t=504052