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Dark-field microscopy excludes directly transmitted illumination from the microscope objective, while admitting light scattered by the specimen. A transparent particle can then appear bright against a dark background even when it has little optical absorption.

One implementation illuminates the specimen with a hollow cone whose direct rays pass outside the objective's acceptance cone. Scattering of light at particles or edges redirects some of this illumination into the objective. Dust and scratches also become bright, so a clean blank specimen is a useful control.^[ref]

Detection is not the same as optical resolution: a very small particle may be visible as an unresolved spot whose width is set by the point spread function.

Use the principle of dark-field microscopy: block the unscattered beam where it is concentrated, while transmitting most light scattering at nonzero angles. All direct light rays of the collimated illumination are parallel, so the thin lens focuses them to one point in its back focal plane. Put a small opaque circular stop centred on that point, one focal length behind the optical lens. In the real image geometry above this is $11.719\mathrm{mm}$ behind the optical lens, well before the real image at $187.5\mathrm{mm}$.

A light ray leaving object coordinate $y$ with a small initial direction $\theta$ reaches the optical lens at $y+u\theta$. The thin lens changes its direction to $\theta -(y+u\theta )/f$. At the back focal plane its coordinate is thereforeThus direct illumination with $\theta =0$ is concentrated at the centre, whereas scattered light at different directions is distributed away from it, independently of $y$. A stop of radius $r_s$ removes approximately $|\theta |\lesssim r_s/f$; choose it just large enough to cover the direct beam's finite diffraction spot, illumination divergence and alignment errors. Scattered light inside that small angular cone is also lost, so negligible loss requires most useful scattered light to lie outside it. A stop in the object's real image plane instead removes object detail and does not generally separate scattered and direct light. For the virtual image interpretation, the same physical back focal plane filtering requires a downstream observation arrangement that actually passes through this plane.

In 1997, Hiroyuki Noji and colleagues attached a fluorescent actin filament to the central $\gamma$ shaft of isolated $F_1$. With ATP supplied, the marker made sustained rotations. This exposed rotation directly, but used ATP hydrolysis: there was no membrane $F_0$ motor producing ATP from a proton-motive force in this assay.^[ref]

A subsequent primary experiment made the optical readout much faster. Purified Escherichia coli $F_1$ was immobilized on glass in a $30\mu \mathrm{m}$-deep flow cell, with a $60\mathrm{nm}$ gold bead attached to $\gamma$. With $2\mathrm{mM}$ ATP and magnesium supplied, laser illumination and dark-field microscopy recorded bead positions at $4000$ or $8000$ frames per second. The position trajectory resolved turns and intervening dwells. This apparatus follows a shaft-attached marker, rather than imaging the shaft's chemical atoms.^[ref]

Controls addressed perturbation and chemical activity: comparison of $40$–$200\mathrm{nm}$ probes found similar rates for the two smallest sizes, supporting low drag; a separate coupled-enzyme assay followed ATP hydrolysis through NADH absorbance. But the rotational analysis excluded pauses longer than $0.1\mathrm{s}$, so it characterizes continuously active intervals, not total output including long inhibited periods. It neither counts transported protons nor measures simultaneous single-molecule ATP synthesis.^[ref]

The experimental distinction is essential. Sustained directional marker motion tests a rotary mechanism. Demonstrating proton-driven synthesis additionally requires intact $F_0{F}_1$ in a membrane with a controlled proton-motive force and a chemical ATP-production readout; dependence on that driving force and inhibition or respiratory uncoupling controls then test the proposed energy coupling. A rotation movie alone cannot establish the complete material and free energy ledger.