We investigate the origin of Lambda cold dark matter parameter constraints in weak lensing, with a focus on the Hubble constant. We explain why current cosmic shear data are sensitive to the parameter combination S8∝σ8Ω0.5m, improving upon previous studies through use of the halo model. Motivated by the ongoing discrepancy in measurements of the Hubble constant from high and low redshifts, we explain why cosmic shear provides almost no constraint on H0 by showing how the lensing angular power spectrum depends on physical length-scales in the dark matter distribution. We derive parameter constraints from galaxy lensing in KiDS and cosmic microwave background weak lensing from Planck and SPTpol, separately and jointly, showing how degeneracies between σ8 and Ωm can be broken. Using lensing and big bang nucleosynthesis to calibrate the sound horizon measured in projection by baryon acoustic oscillations gives H0=67.4±0.9kms−1Mpc−1, consistent with previous results from Planck and the Dark Energy Survey. We find that a toy Euclid-like lensing survey provides only weak constraints on the Hubble constant due to degeneracies with other parameters that affect the shape of the lensing correlation functions. If external priors on ns, the baryon density, and the amplitude of baryon feedback are available, then sub-per cent H0 constraints are achievable with forthcoming lensing surveys.