We study the effect of inclination on the apparent brightness of star-forming galaxies in spectral passbands that are commonly used as star-formation indicators. As diagnostics we use mass-to-light ratios in three passbands: the UV continuum at 280 nm, the H$alpha$ emission line, and the FIR 24$mu$-band. We include a study of inclination trends in the IR/UV ratio (IRX) and the IR/H$alpha$ ratio. Our sample comprises a few hundred galaxies from the region around the clusters Abell 901/902 with deep data and inclinations measured from outer disks in Hubble Space Telescope images. As a novelty, the H$alpha$- and separately the NII-emission are measured by tunable-filter imaging and encompass galaxies in their entirety. At galaxy stellar masses above log $M_*/M_odot > 10$ we find trends in the UV and H$alpha$ mass-to-light ratio that suggest an inclination-induced attenuation from face-on to edge-on of $sim 1$ mag and $sim 0.7$ mag in UV and H$alpha$, respectively, implying that star-formation rates of edge-on galaxies would be underestimated by $sim 2.5times$ in UV and $sim 2times$ in H$alpha$. We find the luminosities in UV and H$alpha$ to be well correlated, but the optical depth of diffuse dust that causes inclination dependence appears to be lower for stars emitting at 280 nm than for gas clouds emitting Balmer lines. For galaxies with log $M_*/M_odot < 9.7$, we find no measurable effect at $>0.1$ mag. The absence of an inclination dependence at 24$mu$ confirms that the average galaxy is optically thin in the FIR.
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