Why Image Quality of F1.5 aperture is worse than F2.4 in S9|S9+?

Galaxy S9/S9+ is equipped with dual(F1.5, F2.4) aperture.

In dark environments (approx. 100 lux or lower), the F1.5 aperture works which can minimize noise by letting in more light while in bright environments (approx. 100 lux or higher), the aperture will automatically switch to the F2.4 aperture. This allows you to take photos that are sharp even in the corners. 
Like this, F1.5 aperture should be get more light to take clear images in dark places. Therefore the image quality may be slightly blurred compared with taken photo by F2.4 aperture depending on how much light passes through the lens.

If you do not want to use F1.5 aperture in dark areas, please capture the photo in Pro Mode. In Pro Mode, regardless of the luminosity, you can choose between the F1.5 and F2.4 aperture.