= Solution
\b[The weak <phalloidin> signal could include sparse or very short residual <actin filaments>, together with nonspecific background; it is not a direct image of <G-actin>.] <phalloidin> preferentially binds <filamentous actin>, so genuinely specific residual signal would indicate some polymeric <actin>. Long exposure also amplifies <autofluorescence>, out-of-focus light and nonspecific staining. Vehicle controls, unstained <biological cells>, matched acquisition settings and background subtraction are needed before assigning a small residual signal to <actin filaments>.
For wavelength $\lambda$ and <numerical aperture> $\mathrm{NA}$, the <Rayleigh criterion> gives a conventional lateral resolution of roughly $d\approx0.61\lambda/\mathrm{NA}$, typically a few hundred nanometres under these conditions, far larger than the roughly 7-nm <actin filament> diameter. It cannot resolve every short <actin filament>, distinguish all closely spaced <actin filaments>, or infer <polymer> mass directly from the displayed intensity. The tenfold exposure change is not itself a calibrated tenfold decrease in <F-actin>. Fixation, dye accessibility and detector saturation introduce further uncertainty.
Super-resolution <fluorescence microscopy> could localize remaining labelled structures more precisely, while <electron microscopy> or <cryo-electron tomography> could test ultrastructure, subject to preservation and sampling limits. Biochemical separation of polymeric and soluble <actin>, with the mass-balance and aggregation controls already discussed, supplies an independent population measurement. A live <actin> reporter can test dynamics but must be checked for perturbation of assembly. Agreement between these approaches would be stronger than concluding complete <actin filament> absence from weak conventional staining alone.
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