What is Aspect Ratio and Pixel Aspect Ratio?

High Definition (HD) and Standard Definition (SD)

The most common standard found in televisions worldwide is Standard Definition (SD).

Depending on your location on Earth, you might have access to one of these two major SD standards:

  • PAL (720×576)
  • NTSC (720×480)

In order to enhance the viewer experience by providing greater clarity and visual detail, high definition systems were introduced, with greater resolutions. There are two broad flavors of high-definition (HD):

  • 1280×720 (sometimes called HD-ready for who knows what reason)
  • 1920×1080 (also called Full HD to distinguish it from 1280×720)

Is there anything better than HD? Of course there is. There’s 2K (with a horizontal resolution of about 2048), 4K (horizontal resolution of about 4096), and so on. Digital photo cameras can reach up to 10328 x 7760 (80 MP) in a single sensor, as of 2012.

The future might even bring 8K to homes, with a standard called UHDTV, or ultra high definition television. Stay tuned, but don’t hold your breath.

Pixel Aspect Ratio

Pixels

can be rectangular or square.

Pixel Aspect Ratio = Length of pixel ÷ Breadth of pixel

It is notated by the form a:b. E.g. 1.78:1, 1.33:1, etc. Sometimes, people drop the ‘:1’ for brevity. It can also be less than one, as in 0.9 for NTSC 4:3.

Why is the pixel aspect ratio important?

The pixel aspect ratio changes the size of an image, and this can be illustrated with an example.

If an image with the resolution 720×576 has a pixel aspect ratio of 1.0 (square pixels), then the size of my image is 720×576.

However, an image with the resolution 720×576 but with a pixel aspect ratio of 1.422:1 (length of the pixel is 1.422 times the breadth) will have a size of:

(720 x 1.422) x 576 = 1024×576

This means that even though the actual pixel count horizontally is only 720, by stretching the pixel a manufacturer can claim it is 1024!

In the above example, it is important to note that the resolution of the image is still 720×576, but the size of the image has changed to 1024×576 – there is no additional detail, just a stretched image.

Squashed or stretched footage has its genesis in the selection of incorrect pixel aspect ratios.

Image Size

Image Size = (Horizontal Resolution × Pixel Aspect Ratio) × Vertical Resolution

The size of an image is not the same as its resolution, but both are notated in the form HxV, so beware!

Aspect Ratio

Aspect Ratio = (Horizontal Resolution / Vertical Resolution) × Pixel Aspect Ratio

The horizontal size of the image divided by its vertical size is the aspect ratio of the image, notated in the form a:b. Aspect ratios are usually rounded off to two decimal places.

E.g., a 1920×1080 image has an aspect ratio of 1920/1080 = 1.78:1, if it has square pixels (pixel aspect ratio is 1:1).

What about a 720×576 image with a pixel aspect ratio of 1.422:1? What’s the aspect ratio of this image?

It’s NOT 720/576=1.25. The answer is calculated as follows:

Horizontal size = (720 x 1.422) = 1024
Vertical size = (576 x 1) = 576
Aspect Ratio = 1024/576 = 1.78:1