Where regular interchangeable lenses can be mass-produced optically, even if hand-assembled for best quality, big fast lenses use glass elements of a size and composition that can only be produced in batches with a limited run. Some, including the latest designs from Canon, use fluorite crystal instead of glass, despite its softness and fragility.
The design cycle for lenses in the 300mm f/2.8 and above class involves a first computer and mechanical design, based on target values for the glass. The glass is then produced, and its refraction and dispersion figures measured. These will be very close to the target values, but hardly ever a perfect match. The design will be adjusted to compensate, including small changes to the curvature, thickness and spacing of individual elements. Both the elements and the mechanical parts of the lens body may be adjusted, using shims and spacers internally.
