200 f2 VRII 52mm NC filter question

Started Feb 22, 2013 | Discussions thread
Leonard Migliore
Leonard Migliore Forum Pro • Posts: 14,601
The glass is flat but the beam isn't collimated

mpe wrote:

These flat filters like UV/NC can't be a part of optical formula since its glass surface is flat. These filters do not refract light like other elements in the lens.

Of course they refract light. The glass in the filter has an index of refraction of about 1.5.

Not all the rays of light passing through the filter are at normal incidence; with an f/2 lens, you can have considerable inclination. These oblique rays are shifted more than paraxial rays and will affect the correction for spherical and chromatic aberration. This is the same reason that high NA microscope objectives have to be corrected for the thickness of the cover glass, which is flat. Nikon, who also knows a few things about microscope optics, has the following explanation:


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