We compared sky factor, which is used in the field of architecture, and openness grade with measured relative illuminance. Sky factor and openness grade were measured on hemispherical photographs taken with an equidistant formula fisheye lens. The measurements were conducted under uniformly overcast skies at 20 points in a hinoki (Chamaecyparis obtusa) stand. Sky factor corresponded well with relative illuminance and the errors were ranged from -0.96 to 0.98 (%). By contrast, openness grade was lower than relative illuminance, and the regression coefficient between them was 0.62. The difference between sky factor and openness grade was due to the effect of the projection formula of the fisheye lens. In conclusion, sky factor was a better estimator of relative illuminance than openness grade. Furthermore, it was shown that it is important to state clearly the kind of fisheye lens used when measuring sky factor or openness grade.