Solar reactors can convert intermittent solar radiation into storable chemical energy in the form of fuels that are transportable. In order to use solar energy as a source of high temperature process heat in a solar reactor, incident radiation needs to be concentrated over a small surface area, the inlet of which is called the aperture. The image of the incoming solar radiation over the aperture can be approximated by a Gaussian distribution where the solar radiation inside the reactor varies by the peak value and aperture size. Due to the transient nature of solar energy, there is a critical need for proper control to maximize system efficiency under field conditions. This paper provides numerically proven advantages of having a camera-like variable aperture, one which is sensitive to natural variations in solar flux, and having the ability to shrink or enlarge accordingly in order to maintain quasi-constant radiation inside the reactor. Our numerical results from optical, thermodynamic, and flow dynamic simulations led us to develop a computational two dimensional heat transfer distribution model inside the reactor in order to validate our optical results. The simulation results show that a changing aperture diameter with respect to a changing incoming solar flux density facilitates keeping quasi-constant and homogenous temperature distributions inside the reactor. Since the temperature has a major impact on reactant to product conversion efficiency, by keeping the temperature constant, process efficiency is kept high. By maintaining the internal temperature despite variable operating conditions the system can maintain peak performance through a wider insolation range than fixed aperture systems.