A popular thing in home theater today is to use a 2.35:1 aspect ratio screen instead of a 1.78:1 which is what HDTV is. The idea is to present scope movies wider than HDTV without black bars on the top and bottom to provide a more immersive experience for these films.
Two common ways to do this are to use either a lens or the projector zoom. When the projector is placed properly the lens zoom can be used to resize the image to fit a 2:35 screen to fill the screen to it’s height limit. The other option is to use an anamorphic lens along with special scaling. This system stretches the image to eliminate the black bars vertically and then optically stretches the image to fill the 2.35 screen with a 2.35 image.
Each system has advantages and disadvantages. Zooming is going to be less expensive, sharper, allows the image to be pixel mapped, will spill light around the screen from the black border and have higher ANSI contrast. An anamorphic lens will have smaller pixels, less wear on the projector lens mechanism, lower ANSI contrast, less sharpness, and additional lens distortion. The reduction in contrast and image distortion from the lens will depend on the lens and projector used. Anamorphic lenses require longer throw ratios as well that will typically lower the projector light output and likely offset any gains from being able to use the entire imager for 2:35 movies. Dark walls are a requirement for using a zoom lens to avoid seeing the light spill.
I recently aided a test with a Prismasonic H-FE1500R and a JVC DLA-RS35. The projector was mounted at a 2.0 throw ratio. Using lens zoom to fill a 10 ft wide 2.35 screen the ANSI contrast was measured at 250:1. With the anamorphic lens the ANSI contrast fell to 112:1. The light level from the screen only dropped 9 percent using lens zoom versus the lens with the same projector location. That difference in light output would be less if the projector would have been mounted at the minimum throw ratio. The light spill caused by zooming the 2.35 image was not visible on a dark brown wall. The image in this case was clearly better zoomed than not.