An apparatus for evaluating downhole fluids is disclosed. The apparatus includes: an optical block having an adjustable opening that receives electromagnetic energy emitted by an electromagnetic energy source; a controller operatively associated with the optical block for adjusting the opening size, wherein the opening size is adjusted at least in part based on one or more estimated downhole parameters; and a sensor that receives the electromagnetic energy emitted by the electromagnetic energy source after the emitted electromagnetic energy interacts with a downhole fluid. A method for evaluating downhole fluids is also disclosed.