Over 25 years ago, Temkin et al (JAS, 1975) compared and contrasted the temperature sensitivity of shortwave optical depth (tau) at fixed cloud fraction (Ac), (dtau/dT)_Ac, with the temperature sensitivity of Ac at fixed tau, (dAc/dT)_tau, for a single-layer cloud at constant relative humidity. They found (dtau/dT)_Ac >0 and (dAc/dT)_tau >0 using an idealized non-stochastic model. I expand the work of Temkin et al and consider these response function within the context of a statistical cloud scheme. First, a unified model of low-cloud physical and optical variability is derived that incorporates a new subgrid distribution of cloud-top height fluctuations. Using this statistical scheme and assuming a symmetric distribution of unresolved variability, I find (dAc/dT)_tau <0 in contradistinction with Temkin et al. Implications for subgrid-scale low-cloud radiative feedback, in general, and Tselioudis et al (Nature, 1993)'s observation of a negative optical depth sensitivity, in particular, are discussed. The impact of cloud-base height skewness on these two response functions is also examined.