  The use of low-dimensional objects in the field of cavity optomechanics is
limited by their low scattering cross section compared to the size of the
optical cavity mode. Fiber-based Fabry-P\'{e}rot microcavities can feature tiny
mode cross sections and still maintain a high finesse, boosting the
light-matter interaction and thus enabling the sensitive detection of the
displacement of minute objects. Here we present such an ultrasensitive
microcavity setup with the highest finesse reported so far in loaded fiber
cavities, $\mathcal{F} = 195\,000$. We are able to position-tune the static
optomechanical coupling to a silicon nitride membrane stripe, reaching
frequency pull parameters of up to $\mathrm{\lvert
G/2\pi\rvert=1}\,\mathrm{GHz\, nm^{-1}}$. We also demonstrate radiation
pressure backaction in the regime of an ultrahigh finesse up to
$\mathcal{F}=165\,000$.
