Aspect ratio describes the proportions of a still image or the frames in a movie, expressed as the ratio of width to height, regardless of the image's resolution.

Various aspect ratios

Aspect ratio is usually expressed either as a ratio of width to height (for example, 4:3) or as a real value relative to 1 (for example, 1.333). For example, pre-1950s movies and non-HD TV shows have an aspect ratio of 4:3.

The aspect ratio is used wherever a bitmap is created or used. For example:

• When preparing to render, you can specify an aspect ratio for your rendered images with the Image Aspect parameter on the Render Setup dialog.
• When setting up a viewport background you can specify whether to use the viewport, bitmap or rendering output aspect ratio to display the bitmap.
• When you make a spotlight a projector, you can use the Bitmap Fit option to match the spotlight's aspect ratio to the bitmap's, and thus prevent the bitmap from distorting.