Theoretically the lower powered binoculars will be brighter due to a larger exit pupil.If you divide the lens size by the power you arrive at the exit puil.42 divided by 8 results in 5.25 compared to 4.2 for the ten power binocular.Unfortunately theory does not always hold true in the optic world.The quality and amount of lens coatings also has a big influence on brightness.In other words a high quality 10x42 binocular could be brighter than a 8x42 binocular with lesser quality coatings.