Top: mean unsigned longitudinal component ⟨|B^L|⟩ of the magnetic field in the weak field limit as a function of the Fried parameter R₀ at the wavelength λ = 10 827 Å. Bottom: same but for the mean transverse component ⟨B^T⟩ of the magnetic field. Solid, dashed, and dotted curves correspond to telescope diameters 4 m, 1.5 m, and 0.7 m, respectively. Open circles indicate the ⟨|B^L|⟩ and ⟨B^T⟩ values for the best resolution that the GREGOR Infrared Spectrograph can achieve at 10 832 Å (Joshi et al. 2016). The rms continuum contrast δI_rms, as well as the ⟨|B^L|⟩ and ⟨B^T⟩ values for the spatial resolution of the 3D model (no smearing and no noise effects) and for reduced resolutions due to the telescope diffraction are shown in the panels. We note that the mean vertical ⟨|B_Z|⟩ and transverse ⟨|B_XY|⟩ components of the model are equal to 25.7 G and =49.5 G, respectively. The top horizontal axis gives the spatial resolution, corresponding to the Fried parameter R₀ shown in the bottom horizontal axis.