Coupled measurements of liquid film thickness and heat transfer coefficient during condensation inside small diameter channels are rare in the literature due to the technical complexity of performing accurate measurements at such scale. The availability of these experimental data would help to understand the underlying physical mechanisms and foster the development of new reliable correlations for the design of condensers. To cover this gap, in the present paper liquid film thickness and heat transfer coefficients have been simultaneously measured during vertical downflow condensation inside a 3.38 mm inner diameter channel. Condensation tests have been performed with refrigerant R134a at mass flux from 50 to 110 kg m^-2 s^-1. The instantaneous liquid film thickness and interfacial waves are detected by combining two complementary optical methods: shadowgraph technique and chromatic confocal imaging. Liquid film thickness data have been processed through statistical and spectral analyses and then employed to explain the heat transfer coefficient trends. In the data analysis, R134a experimental results are discussed in comparison to R245fa data taken during a previous experimental campaign. These two fluids have been selected because of the different values of vapor density (and thus vapor velocity) at the considered test conditions. On average, R134a displays higher liquid film thickness compared to R245fa. Moreover, high-amplitude disturbance waves, which promote the thinning of the liquid film and enhance the condensation heat transfer, are detected at 100 kg m^-2 s^-1 for R134a and at 75 kg m^-2 s^-1 for R245fa.