01-03-2015, 03:58 PM
If you have, few, or simple physical constraints then excellent design is quite possible. I recently went to a presentation on the design of a 16x zoom lens - 16 - 250mm. The physical length was constricted to < 250mm, max diameter was 70mm, and the distance from the rear element to the sensor was constricted to > 20mm. Designs were achieved ranging from f/3.8-f/6 to f/6 constant to f/5-f/6.3 and nine of them were diffraction limited. In an optical sense, diffraction limited isn't so impressive at f/6, but for a 16x zoom lens it is excellent. The aberration that drove design was also astigmatism - chromatic aberrations were no trouble at all.
And this is with just 12-15 elements, no aspherical elements, and no use of CAF2 (fluorite).
When you start constraining the design physically you run into heaps of trouble achiving a very good result. Often it is best to start from scratch at first order, but some may stubbornly insist on trying to make a starting point at an old design fit the task. This can force you into a bad local maximum in terms of performance, or result in a waste of lenses. Often, adding more elements doesn't help and you could have achieved the same performance with the original element count.
And this is with just 12-15 elements, no aspherical elements, and no use of CAF2 (fluorite).
When you start constraining the design physically you run into heaps of trouble achiving a very good result. Often it is best to start from scratch at first order, but some may stubbornly insist on trying to make a starting point at an old design fit the task. This can force you into a bad local maximum in terms of performance, or result in a waste of lenses. Often, adding more elements doesn't help and you could have achieved the same performance with the original element count.