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Zoom Lenses
Third Quantum of Lens Design
Zoom lenses have a three-dimensional performance behavior. The third added dimension - the focal length, is a serious challenge for lens designers. The common way to change the focal length is by moving groups of lenses inside the lens.
Understanding the intrinsic function of every moving group is key
In
In almost all cases there are multiple solutions for the zoom problem. Selecting a solution is not trivial and selecting the best is the challenge.
Here are some solutions for a 28-80 mm zoom, full-field at F2.8.
The selected solution is a negative lead system with positive Variator and negative Compensator. Important criteria for selection are individual group power, ray height on different groups, variation of aberrations while zooming, sensitivities and other.
Power distribution is key in zoom lens design
During the transition from first-order design to real lens elements, a third movement was introduced to stabilize aberration variation during zooming. Therefore the negative compensator group was split into two groups so that each of the two subgroups has independent movements.
The final design has a relatively stable variation of aberrations and an excellent color correction for all the range of focal length.
The performance of zoom lenses is always a compromise. To find the optimum, you need decades of experience.
Long time design experience is essential for success.
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