When filming, you’d like to have a clear idea where the media will be viewed from, and this would likely provide direction on the aspect ratio to use to best fit the format for the media provider. Here’s a way to visualize aspect ratios.
2.39:1 Aspect Ratio of current anamorphic (wide-screen) theatrical showings.
1.85:1 Aspect Ratio used for most U.S. theatrical showings since the 1960s
16:9 Standard Aspect Ratio for high-definition video (also called 1.78:1)
4:3 Standard Aspect Ratio and standard-definition video for older TVs (also called 1.33)