Q: "I would like to paint fine details, like faces or pets within a painting. So I would like to know what brushes and brushstrokes are best for painting eyes, highlights, for example. I know the brush would be small, but which types and sizes and stiffness etc would be best for laying down tiny marks to portray an eye."
To answer this question, let's take a close look at two masterpieces by two of the world's most famous artists: John Singer Sargent and Sir Joshua Reynolds. The first piece we'll examine is a painting of a girl and her dog, Miss Beatrice Townsend, by John Singer Sargent.
If we zoom into the face of the dog, you can see a very bravura rendering.
But look closely. You can see what appears to be his initial layers. The area circled in yellow shows thin coats of paint - likely a base layer. These appear to be blended strokes, much like you see on the wall next to the dog.
In areas of the painting (such as the spot circled below), there appear to be areas where the colored ground was left exposed - used as lights.
I can't say how he developed some of his other base coats, but you can tell that he painted the darks of the nose and eyes with blended strokes and sweeps of feathered strokes. After his initial layers he seems to work dark to light (fairly typical with oil).
Below you can distinctly see where he painted bold, bravura strokes with a medium tone to create the dog's fur and define the face. These are confident, single strokes of paint.
This bravura work includes many of the lightest tones, which clearly sit as one of the very top layers. Notice how it sweeps over all prior layers. He let the marks of the strokes define aspects of the face.
Let's look at another painting - this time a painting of a girl and her dog by Sir Joshua Reynolds. This piece is called Miss Beatrix Lister (1765). I can't say with 100% certainty what his process was, but from examining the image closely and using my own painting knowledge, following would be my stab at how he painted the face.
Let's zoom into the girl's eye.
Notice what looks like a base tone - a neutral, fairly opaque-looking color that appears to have been painted with blended strokes all over the skin area; No seam lines and pretty solid looking. You can see above the eye what looks like a little glaze of red on top of the base.
You can also see glazes of a pale muted blue/green (first image below). Over the eye, it's blended out into the base tone. Under the eye, it looks a little more like a gradient as it fades to the left (second image below), thinning into the base tone from a darker area near the inner eye. These colors seem to be thin - transparent or translucent - allowing some of the color underneath to show through.
It's hard to tell exactly what he painted next, but I would assume he would have wanted to gently define the eye itself at this point. Notice the lighter tone at the base of the eye (arrow below). That looks directly painted yet gently blended into the area beneath.
Then you have the directly painted line that defines the crease in the eye. Notice that it's not blended in, but is gentle in tone. It looks to me like there is a similar tone (like the crease color) used to initially define the top of the eye as you can see a hint of it to the right of the rust color at the inner eye.
Once he felt good about the placement and definition of the eye boundaries, I would assume he painted the dark of the eye itself (the pupil and surrounding base tone for the iris). This would be directly painted.
Then he would likley paint the iris color and blend it a little into the black. If you could zoom in you could see that it seems to overlap, plus that would be logical.
Next, I think he probably painted that pale rust color, defining the top of the inner eye - perhaps to imply eye lashes? This, too, is directly painted. It doesn't really blend into the surrounding paint. Notice that it overlaps the black and iris color, so we know it was painted after those two.
And finally, the white highlight. The highlight would be a pure dab of white - a slight touch carefully placed and the final touch - not blended. So in summary, after the eye area was shadowed a bit and the lines defined, he seems to have worked dark to light.
As for her other features, it looks like the nostril on the left was worked light to dark. The tones softly blended until you get to the final two darkest tones that define the nostril itself. Those have pretty hard edges, so it was apparently directly painted. On the shadowed side, the shadow color and tone were painted with blended strokes.
For the lips, I'm guessing that he may have defined them with the shadow lip color - small blended strokes to define the shape on the inside. But notice the hard outer edge. He may have then directly painted the paler lip tone to distinguish the upper lip and light part of the lower lip - blending it into the shadow lip color.
If you can zoom in further to the lip area, the area where the arrow points is a spot that (to me) indicates that he painted the lip shadow color first followed by the lights.
Notice a blended highlight above the upper lip (arrow below). The highlight blends into the area between the lip and nose, but is left as a distict line where it meets the upper lip. That give the area dimension and value contrast - drama.
Also notice the gradient work on the left side of the face/cheek where the shadow from the hair very gradually leads into the cheek area. That would have been painted before defining any of the features, I would say.
Again, this is what I think based on a close analysis of the photograph. I don't claim to know for absolute certain.
If you'd like to zoom into this piece yourself, look up Google Arts & Culture and type in the painting title and/or Reynolds in the Search. Then click on the image and use the tools to zoom in. It's a wonderful website and tool.
In my own experience with oil landscapes, if I want an area to be very smooth, I would use a sable brush and typcially thin the paint a bit (sometimes a lot). If I want more of a textural effect or if I want to scrub the paint on, I use stiffer bristle brushes. If want to place a distinct highlight on my painting (like a iris), I us a stiff bristle like a very small hog hair round. Brush selection is a very individual thing, but I do think stiffer bristles give the artist more control when using thicker paint.
Here's a pretty interesting website on Reynolds if you're interested in learning more about him: