Optical transparency is a critical but often overlooked property of MOFs considered for optical applications and luminescence sensing. Zr-1,4-NDC samples with various crystallite dimensions (35 nm to 100 μm) were prepared and their bulk optical transmittance measured. The nanocrystalline (35 nm) sample exhibited the highest optical transmittance, which boosts the luminescence signal for sensing applications by reducing scattering loss.