Star Test Symmetry

Greetings all,

I have a question that I am sure has been discussed a bit, but would appreciate some input on.

Should a triplet apo have symmetrical star test? How about a doublet?

I have read Roland's article on the subject and it seems to say that they should not be symmetrical. So if a triplet has a symmetrical star test there is something wrong with it?

How about an ED doublet.

Any input would be greatly appreciated.

Take care,
john

>>Should a triplet apo have symmetrical star test? How about a doublet?>>

Star test will have nothing to do with number of elements. All closely spaced lenses have sphero-chromatism, so the star test in white light will not look perfectly symmetrical. Testing in narrow band green or yellow-green light should reveal the center wavelength that the lens was nulled at. That would be the only place where the lens will have symmetry.

Longer focal ratio lenses will look more symmetrical in white light. Lenses with higher grade ED or fluorite elements will look more symmetrical also.