The accuracy of the Hubble constant measured with extragalactic Cepheids depends on robust photometry and background estimation in the presence of stellar crowding .
The conventional approach accounts for crowding by sampling backgrounds near Cepheids and assuming they match those at their positions .
We show a direct consequence of crowding by unresolved sources at Cepheid sites is a reduction in the fractional amplitudes of their light curves .
We use a simple analytical expression to infer crowding directly from the light curve amplitudes of > 200 Cepheids in 3 SNe Ia hosts and NGC 4258 as observed by HST – the first near-infrared amplitudes measured beyond the Magellanic Clouds .
Where local crowding is minimal , we find near-infrared amplitudes match Milky Way Cepheids at the same periods .
At greater stellar densities we find that the empirically measured amplitudes match the values predicted ( with no free parameters ) from crowding assessed in the conventional way from local regions , confirming their accuracy for estimating the background at the Cepheid locations .
Extragalactic Cepheid amplitudes would need to be \sim 20 % smaller than measured to indicate additional , unrecognized crowding as a primary source of the present discrepancy in H _ { 0 } .
Rather we find the amplitude data constrains a systematic mis-estimate of Cepheid backgrounds to be 0.029 \pm 0.037 mag , more than 5 \times smaller than the size of the present \sim 0.2 mag tension in H _ { 0 } .
We conclude that systematic errors in Cepheid backgrounds do not provide a plausible resolution to the Hubble tension .