Optical coating process

I agree. Since then, I have tried to build a model (Zemax) of the objective. It seems that microscope objectives have a principal sphere instead of a principal plane. This addresses the first order ray tracing. I'm not really sure how far this model can be used.

For a lens obeying the Abbe sine condition, e.g., any well corrected microscope objective, $\frac{D}{f} = 2\times\sin\theta$. The explanation for this is in the Wikipedia article you refer to. It is probably the cause of the discrepancy in your results for high NA objectives (where $\sin\theta\not\approx\tan\theta$).

High reflective coating

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Reflective coating paint

In LOOP, the LASEROPTIK ONLINE PORTAL, you can search for coated optics available in our stock or configure your own ideas.

Highest reflection, optimized for a wide angle of incidence (e.g. 0-45°), also available for a broadband wavelength range

Thus the 18.2 mm calculated here could correspond to a plane inside the objective. In this case 18.2mm is a plausible lenght since most of the objective I know have an external diameter superior to 20mm.

erratum : my bad F# = f/D thanks Kyle. But my main poin remain valid : D is the pupil diameter therefore while being a proper physical dimension, it may not be related to any physical object (i.e lenses), it's a product of the overall optical assembly.

Anti reflection coating

Numerical aperture (NA) $= sin(\theta)$ where $\theta$ is the half-angle (see: http://en.wikipedia.org/wiki/Numerical_aperture)

Ar coating

The lens diameter appears too large. There are no microscope objectives this large. Although I can accept that the optical prescription can increase (decrease) the marginal ray, the exiting beam from a point source at the object plane is collimated. Under what conditions can this beam be smaller than the "lens diameter"?

An other explanation could be that the F# is D/f where D is the diameter of the entrance pupil of the optical system (cf http://en.wikipedia.org/wiki/F-number), here an microscope objective.

By the way, I've modeled some microscope objectives in Zemax using US Patents. Some obey these equations above, some do not.