The Panasonic projector I have can throw out several different aspect ratios.

The only problem with that is that by not using all the projector's pixels to show the image (the letterbox black bars above and below the projector could be considered wasted lumens or wasted brightness), the picture might not be as bright as it could be.

To get around that some folk add an anamorphic lense in front of their projector. That was you intentionally have the projector set as "distorted" (non-letterboxed) with image being stretched tall and thin. But you're using every lumen that the projector can throw.

The tall thin image passes through the anamorphic lens and gets "stretched" wider, which results in a normal aspect image.

My Oppo DVD player can't alter the aspect ratio, but I wouldn't be surprised if some can.