Here are typical spectra of M5iii in red (R Lyr) and A0v in blue (Vega) with a typical sensor QE curve (black) overlaid, first at equal V magnitude and then with the correct relative magnitudes assuming RR Lyr Vmag =4.0.
R Lyr is ~ as bright as Vega in the I band but should still look significantly fainter integrated over the response of an unfiltered image. (Unless perhaps close to the horizon where atmospheric extinction would reduce the difference)
Cheers
Robin
