When selecting the right objective, all measurement conditions should be taken in consideration. A good microscope objective may have 80-90% transmission at 500 nm wavelength, but only 40% or less at 900 nm. The working distance between sample and objective and the opportunity to measure under specific conditions such as low-temperatures or in liquids are also important factors in that decision. Furthermore the correction of imaging errors, e.g. chromatic aberrations, is an important quality characteristic.
More than the magnification, the numerical aperture (NA) plays a crucial role: The highest collection efficiency and the best spatial resolution can be obtained by using an objective with the highest NA applicable under the measurement conditions. The NA additionally defines the resolving power and the power density of the excitation spot. However, a lower NA can be beneficial if the sample’s topography requires a broader collection range in the z-direction.