= Solution
Start with the biological quantity to measure: compartment location, molecular colocalization, fast cell dynamics, deep-tissue activity or nanometre-scale organization. Specify the required field, depth, lateral and axial resolution, temporal sampling and acceptable perturbation before choosing the instrument.
For a thin fixed specimen or a shallow live preparation, <confocal microscopy> provides flexible multicolour imaging and effective <optical sectioning> with widely available labels. Adjust the pinhole and dwell time against <signal-to-noise ratio>, bleaching and speed. Repeatedly scanning a large volume can become too slow, and out-of-focus excitation can damage regions that contribute little useful signal.
For a living scattering specimen, <two-photon excitation microscopy> often offers better access to depth and confinement of excitation. Longer excitation <wavelengths> scatter less strongly in many tissues, and broad collection can recover scattered emission without a detection pinhole. These benefits do not eliminate depth-dependent <optical aberrations>, absorption, heating or focal <phototoxicity>. Pulsed excitation can also make instrumentation and suitable multicolour excitation more demanding.
If the priority is rapid, gentle volumetric observation of a suitable specimen, consider <light-sheet fluorescence microscopy> rather than forcing either point-scanning method to cover every use case. If subdiffraction molecular organization is required, consider a suitable super-resolution method and assess its labeling, speed and dose costs.
Finally, calibrate the <point spread function> in a sample-relevant medium, sample finely enough in all three dimensions, and check registration between colour channels. Match photon collection and exposure to the question, with controls for label-induced changes and <photobleaching>. \b[The best method meets the biological resolution and timing requirements with sufficient signal and acceptable perturbation; the smallest nominal resolution number alone does not decide.]
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