Most of my professional work requires that I use or match photography of a person. I'm usually provided the photographic reference, from that, if I am lucky, I can pull focal length and camera type from the image properties. Matching the correct focal length of the character I am sculpting in ZBrush has usually been a guessing game. At the time this document was written, a proper way of inputting a realistic focal length has not been included in ZBrush 4.0. The current version of ZBrush has instead a slider for Focal Angle (Draw > Focal Angle). By default this is set to 50. This focal angle is analogous to angular field of view or FOV.
FOV is the extent of the observable world that is seen at any given moment, the maximum front facing FOV being 180°. Thus, the scale slider for ZBrush has a range between 5-180° of forward facing perspective. The conversion of focal length to diagonal FOV can be accomplished via the following formula,
This example is calculating the FOV of a 50mm FL with a focal length multiplier of 1.
Thankfully you can also use some web convertors to get the proper conversion. Here are two good online conversion tools. The first link has several options for converting to popular cameras; the second link allows for the manual input of the focal length multiplier, which is the field of view cropping that occurs by the camera sensor. This option is represented by camera scale under the camera attributes in Maya. In both convertors the FOV degree we are looking for is the diagonal.
http://www.tawbaware.com/maxlyons/calc.htm The Angular Field of View Section.
Below is a conversion chart for Maya, focal length multiplier 1, and a standard DSLR, focal length multiplier 1.5, use these two charts as a base line in determining the proper focal angle to use in ZBrush. Remember that distance and rotation are variables, so manual alignment to your photographic reference is still key to getting good results.
Focal Length to FOV (Diagonal) (Degrees)
FOV for Standard DSLR, Focal Length Multiplier 1.5
FOV for Maya’s Camera, Focal Length Multiplier 1
Most facial reference is shot at a fairly mid to high focal length, 80-110 which means you should be using a ZBrush focal angle between 20-15 depending on the type of camera that was used. If the focal length is unknown, 20 is usually a good starting assumption, but may need to be adjust later based on the level of distortion.
Hopefully this all makes sense, and helps you to avoid some unnessary distortion. Email me if you have questions, a better method, or if you think I am flat out wrong, I'm here to learn just like everyone else. -K