Simple Experimental Methods for Determining the Apparent Focal Shift in a Microscope System.

Area of Science:

• Optical microscopy
• Biophysics
• Imaging science

Background:

• Refractive index mismatch between sample and objective lens complicates 3D optical microscopy.
• This mismatch induces focal shift, degrading the accuracy of 3D reconstructions.

Purpose of the Study:

• To present simple, broadly applicable methods for measuring focal shift in 3D optical microscopy.
• To quantify focal shift in aqueous environments and validate theoretical models.

Main Methods:

• Utilized fluorescent beads of varying sizes and ring-stained fluorescent beads to measure focal shift.
• Applied methods to total internal reflection objectives and samples near the interface.
• Expanded sub-critical angle theory using polynomial extrapolation and tested its validity.

Main Results:

• Measured a relative focal shift of 0.57 ± 0.02 for a 1.49-NA objective in aqueous environments (0-1.5 μm), significantly lower than the n2/n1 approximation (0.88).
• Validated the polynomial extrapolation method across refractive indices from 1.33 to 1.45 and numerical apertures from 1.25 to 1.49.

Conclusions:

• Developed practical methods for accurate focal shift measurement in 3D optical microscopy.
• Demonstrated that focal shift is often less severe than predicted by simple approximations.
• Validated an extended theoretical model for predicting focal shift in diverse microscopy setups.