When I used to take head & shoulder portraits with my Mamiya C220 I always found the 135mm Mamiya lens ideal. The standard 80mm lens is really too wide for this and the 105mm was sold as an alternative "standard" so probably wouldn't be a great deal different. You could use the 180mm but I think you would find you would have to move the camera back quite a bit from the sitter and this isn't always feasible.