Norbert Vischer <[log in to unmask]> writes:

>
> This problem is discussed in:
>
> Taco D. Visser, J.L.Oud and G.J. Brakenhoff:
>
> Refractive Index and Distance Measurements in 3-D Microscopy,
> Optik, 90, pp 17-19 (1992)

Other publications point out that the formula for the focal shift given
in the above paper is most likely *not* the correct one. See, e.g.,
Carlsson (1991) J. Micros. 163:167-178 and Hell et al. (1993) J. Micros.
169:391-405. Both these papers point out that the focal shift is much
better described by

  NFP = n2/n1 * AFP (with NFP=nominal focus position and AFP=actual
                     focus position)

Though this formula is the low-aperture approximation of the one given in
Visser et. al. it gives better agreement with experimental data when
using high aperture objectives. Test measurements that I did on our confocal
agree with that formula as well (I used an NA 1.25 objective).

Regards,

   Christian Soeller

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Christian Soeller                         mailto: [log in to unmask]
St. Georges Hospital Medical School       Dept. of Pharmacology
Cranmer Terrace                           London SW17 0RE