I agree painting eyes can be difficult and a little time consuming but if done well can really bring a model to life and make it 'pop'.
It is perhaps more suited to some of the more 'heroic scaled' 28mm figs, that have a decent amount of facial features sculpted in.
It does take practice and steady hand and a fine tip brush.
Here's a pic of what i do, its a bit crude but you get the idea:
1. block paint base flesh colour
2. give a wash of brown colour, this will define nose and eyes
3. paint a white line across eye, this is usually the easy bit. If you make it too wide, let if dry and brush over with a slightly darker tone to base flesh tone you used (to match the base flesh tone plus wash)
4. Now do a vertical black line through the middle of the eye, starting by resting brush tip on middle of eye brow, and drag downwards. I find this far easier than trying to stab a black dot in the centre of the white line with a brush tip that will deform as you do so. It also helps your aim and focus and fines out the brush tip as you drag downwards.
5. Now reduce the excess black line carefully brushing over with a slightly darker tone to base flesh tone you used.
6. Add highlights to face to finish.
I will agree, this can be hard depending on posture of figure, and other things getting in the way; bushy beards and eyebrows, hats and helmets, especially helmets with face guards... but its still possible...
Another trick I have heard about though not tried is dotting in the pupil using a cocktail stick
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