For two decades, extraordinary optical transmission has amplified exploration into subwavelength systems. Researchers have previously suggested exploiting the spectrally selective electromagnetic field confinement of subwavelength slits for multispectral detectors. Utilizing the finite-difference frequency-domain method, we examine electromagnetic field confinement in both two-dimensional and three-dimensional scenarios from 2.5 to 6  μm, i.e., midwave infrared. We explore the trade space of deep subwavelength slits and its impact on resonant enhancement of the electromagnetic field. This builds a fundamental understanding of the coupling mechanisms, allowing for prediction of resonant spectral behavior based on slit geometry and material properties.