Over recent years, the pixel size of uncooled thermal detectors has kept shrinking, going from 50 μm in the last decade to 17 μm today. The latest generation of detectors, with 12 μm pixel pitch and smaller, come with a new set of challenges. In this paper, we investigate the link between pixel size and optics cost and performance by relying on a well-defined framework and concrete examples. First, we briefly clarify the relationship between the reduction in pixel size and requirements on the optics, from both the radiometric and the resolution point of view. Within this framework, we study the effect of decenter corresponding to current state-of-the art manufacturing on performance and price of lenses. Finally we demonstrate that reducing the pixel size indirectly leads to much more demanding lenses. As the new generation of 12 μm pixel pitch arrays are emerging in the long wave infrared, lenses will become more complex and harder to manufacture. Consequently, optics with equivalent levels of performance can become more expensive.