After a significant amount of modellling on the portrait I can make some evaluations about the sculpting process in ZBrush. There is still some ZBrush love in the air but the romance is becoming slightly tarnished by the troublesome reality of trying to get things done with it.
Starting with the positives:
The first thing to mention is that relative to traditional clay sculpting the most useful feature of digital sculpting is the ability to edit at different levels of detail. The limitation of clay sculpture is that you are committed to the gestures and pose that you build into the armature. ZBrush and other programs like it allow you to alter the pose at any time and the detail is re projected through to the higher levels of detail. Fine gestural changes can also be made on the low resolution base mesh without effecting the tertiary and superficial details at the higher levels of detail. This all means greater freedom to try alterations to the character and expression as you go.
Secondly, an interesting effect of the ability to zoom right in and out, change the focal length of the camera and quickly manipulate the viewpoint, gives a new kind of understanding of the surface.
You can visualise the surface almost independently of scale. With manual clay sculpture your mind gets hold of the form through the experience of the forces transmitted through the tools to the fingertips, this is synaesthised in your mind with the visual cues to create an internalised 3d model. With Digital sculpting tools you have the ability to zoom right in and move the camera all around the fine contours of the surface. This gives a more directly visual understanding of the forms, which feels closer to a sense of mapping the surface in mind than mentally modelling the forms. I should note that this mapping appears to be additional to mental modelling rather than replacing.
Obviously another positive is the level of detail that can be achieved with the digital method of projecting images to create form. For a traditional sculptor this almost feels like cheating, because it is so easy to achieve very fine and accurate surface detail from photo reference.
Another benefit is the infinite malleability of the surface. Clay has physical limitations on how much remodelling it will tolerate before it become friable or brittle.
Now for some of the negatives:
The biggest drawback of working with ZBrush is that the lighting is poor. Light and shade is based on calculative surface shaders, which are designed to give an approximate representation of natural lighting but are essentially just colour values applied to the model mesh. The result is not representative of natural lighting and it pays no account to occlusion.
This creates a perceptual deficit when reading forms the lack of shadows caused by protrusions, confuses the eye in close up situations meaning that you have to step back from the object to re orient yourself and refresh your understanding of the shape. This impinges on the quality of formal decision making and is frustrating as well as time consuming.
These problems could be solved with real time ray tracing, which would simulate real shadows more closely to reality. Realistic shadows would allow artists to read to surface more accurately and make more positive judegment throughout the modelling process. This is not really technically viable at present, but as computer get faster it will be. Perhaps Mudbox will steal the march in this respect as it refers to GPU rendering more readily. We will see.
The lighting problem also contributes to the problem of judging the depth of the surface relative to the viewpoint. This single point perspective is the second drawback to the digital sculpting methods. It necessitates continuously moving the model in order to see the proper relationship of shapes and forms. The erroneous lighting and flattening effect of the single viewpoint regularly causes you to misread the surface position and surface flow.
The freedom that these tools offer also creates the potential for tinkering about indecisively for a long time. One danger is than when beginning a design, the knowledge that you can repose at will means that you may not necessarily resolve the designs before beginning.
The most obvious downside to digital sculpting is the lack of tactile feedback. For me and my style of modelling this is not as great an issue as it is for some. My way of seeing form tends depend on visual profiles. The lack of tactile feedback does not stop the understanding of form, but it does slow down the process of internalising the forms as you make them. which overall just makes for less certain modelling. and more trial and error.
Some artists try to resolve some of these issues with force feedback haptic devices. I tried one and found that the issues remain. The disjunction of feeling force feedback in the fingers, but the context is removed. so the acquistion of knowledge of the object has an element of guesswork involved.
Generally, at this stage I would say that in comparison to traditional clay sculpting, digital sculpting gives you slightly more than you lose. If the lighting systems could be improved then the benefits would be much greater.