The portrait is pretty much finished at this level of subdivision.
I am doing as much modelling as I possibly can at the lowest levels of subdivision. The reasons for doing this are related to how the model will be rendered later on. The way ZBrush works means that the alterations to the form are stored as brush strokes on an image map. This is called a displacement map. When you are sculpting you are effectivlely looking at a live displacement map applied to the base geometry (the lowest level of subdivision). When the model is rendered, the renderer takes the base model, and divides it. The modelling you have done is then reapplied from the displacement map, which the renderer uses to calculate how far to move the points on the divided mesh. This is a similar process to tesselation in hardware rendering.
That seems straightforward, but there are limits to how far you can push points with a displacement map. If you exceed the limits then you lose the fidelity to the sculpting. The best way to protect the quality is to have the base mesh as close to the final modelled surface as possible.
Another reason for taking an approach of working on the low resolution meshes as much as you can is to mitigate against the common habit of over committing to detail. Almost everyone is prone to doing this. e.g. You can work on a nostril, work it up to perfection and then you are reluctant to change it- even if it is wrong, because you have invested time and devotion to it.
It is good practice to approach the formal structure of a sculpture in a heirarchal way. By this I mean that there are primary, secondary, tertiary forms, and then surface detail.
- Primary forms are the large overall shapes like the skull, neck, shoulder, etc.
- Secondary forms are smaller but still have their own identity like noses, lips, ears, chin, finger, etc.
- Tertiary forms are dependent on secondary forms, e.g. nostrils, ridges in the ears, eyelids and major folds in the skin.
- Details are lines, pores, minor folds, etc.
The stage I am at now with this portrait is dealing with the secondary forms. When making a portrait, the likeness of a person is all contained in the relationships between the primary and secondary forms. The human eye looks for the proportions of the boniest parts of the face to establish recognition, and the fleshy parts to establish mood. So it pays to spend some time on this stage.
One of the best aspects of working with ZBrush is that unlike clay sculpture it offers the ability to come back and revisit these forms at a later point, to change the expression, and without losing the earlier work. This would be done by creating a layer and adjusting the fleshy components of the face. You can even ‘mix’ the expression between layer positions.
In conjunction with 3D printing any expression can be instanced to an .stl file and printed out as a sculpture. Furthermore the different expressions can be exported and used as blend shapes for facial animation.
You have to be impressed with ZBrush!