  Optical tweezers allow the measurement of fluctuations at the nano-scale, in
particular fluctuations in the end-to-end distance in single molecules.
Fluctuation spectra can yield valuable information, but they can easily be
contaminated by instrumental effects. We identify axial fluctuations, i.e.
fluctuations of the trapped beads in the direction of light propagation, as one
of these instrumental effects. Remarkably, axial fluctuations occur on a
characteristic timescale similar to that of conformational (folding)
transitions, which may lead to misinterpretation of the experimental results.
We show that a precise measurement of the effect of force on both axial and
conformational fluctuations is crucial to disentangle them. Our results on
axial fluctuations are captured by a simple and general formula valid for all
optical tweezers setups and provide experimentalists with a general strategy to
distinguish axial fluctuations from conformational transitions.
