We investigate the diffuse absolute calibration of the Infrared Array Camera (IRAC) on the Spitzer Space Telescope (SST) at 8.0 mu m using a sample of 43 H II regions with a wide range of morphologies near l = 312 degrees. For each region we carefully measure sky-subtracted, point-source-subtracted, areally integrated IRAC 8.0-mu m fluxes and compare these with Midcourse Space eXperiment (MSX) 8.3-mu m images at two different spatial resolutions, and with radio continuum maps. We determine an accurate median ratio of IRAC 8.0-mu m/MSX 8.3-mu m fluxes, of 1.55 +/- 0.15. From robust spectral energy distributions of these regions we conclude that the present 8.0-mu m diffuse calibration of the SST is 36 per cent too high compared with the MSX validated calibration, perhaps due to scattered light inside the camera. This is an independent confirmation of the result derived for the diffuse calibration of IRAC by the Spitzer Science Centre (SSC). From regression analyses we find that 843-MHz radio fluxes of H II regions and mid-infrared (MIR) fluxes are linearly related for MSX at 8.3 mu m and Spitzer at 8.0 mu m, confirming the earlier MSX result by Cohen & Green. The median ratio of MIR/843-MHz diffuse continuum fluxes is 600x smaller in non-thermal than thermal regions, making it a sharp discriminant. The ratios are largely independent of morphology up to a size of similar to 24 arcmin. We provide homogeneous radio and MIR morphologies for all sources. MIR morphology is not uniquely related to radio structure. Compact regions may have MIR filaments and/or diffuse haloes, perhaps infrared counterparts to weakly ionized radio haloes found around compact H II regions. We offer two IRAC colour-colour plots as quantitative diagnostics of diffuse H II regions.