Section 2 - Under the Stars!If we take two, perfectly manufactured APO lenses under the stars (an air spaced one and another with oil spaces), they have to face several effects. Some of them are:
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In the case of oil spaced lenses, mechanical de-centering of an element has very little effect on performance. The result is only some lateral color, but to make it actually visible, the lens members must be de-centered by an irrealistic value (maybe except for some extra fast focal ratios). On the contrary, de-centering the member of an air spaced lens might cause a serious coma, but it depends on the actual mechanical configuration (e.g. are there three small separate spacers between the members or spacer rings). De-centering can realistically cause problems with air spaced lenses, but it is not very common in the quality range we are inspecting, so we do not consider it to be a major problem.
Regarding mechanical forces that distort the lens members, air spaced lenses are the clear winners. If we bend a member of an air spaced lens, the two surfaces surrounding the member bend similarly, so the deformations null out each other optically. For this reason, it is enough if the cell supports the lens at three positions around the perimeter of the lens. On the contrary, in the case of oil spaced lenses, the deformation of an outer member does actually deform the wavefront (its opposite surface also bands similarly, but the internal surface can not correct the error caused by the outer surface, as the deformation of the internal surface is nulled out by the oil). So, oil spaced lenses should be supported by flexible O-rings to distribute the force evenly around the edge of the lens. This is not a problem and this effectively prevents deformations, so in this respect, the lens types are practically equal again. |
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| But if we start to examine the different lens tipes regarding thermal problems, there will be serious differences. | |
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