Understanding aspect ratios in a world shoe-horned
within the bounds of a rectangle
Author’s Note: This is one of those geeky-math articles that is necessary before we go on…
It is the nature of computer graphics that they live within the bounds of a rectangle. Any raster graphic created for digital use is forced into this construct. Unlike the analog world where a picture can be cut into a literal circle, all pictures are rectangles in the digital world. You can cut them into a circle if you like but your software will always keep them within a rectangular bounding box.
Furthermore, these images live in perfectly regular, level rectangular boxes. Rotate an image 45° and the computer will work to ensure that it is a regular rectangle by either filling in space or removing parts of your image. It is the nature of the bitmap or raster image that it will always be some form of a rectangle.
The rectangle’s shape is often described in terms of an Aspect Ratio. An aspect ratio describes the relationship of one side of the rectangle to its adjacent side. Depending upon your industry-orientation and the definition you source, the aspect ratio may be reversed. The Holy Crop! blog will use the following definition as it refers to digital images:
A number derived from dividing the horizontal number of pixels by the vertical number of pixels. It can be expressed as a decimal or as a ratio. Examples: 3:2, 16:9
Thus, images that are taller than wide will have a decimal value of less than one and images that are wider than tall will have a decimal value of greater than one. Square images or 1:1 images will have a decimal value equal to exactly one.
A Few Common Aspect Ratios:
3:2 – Most DSRs, 4×6 Print
4:3 – Video Standard/Television/Some DSLRs and many Point-n-Shoots
5:4 or 4:5 – 8×10 Print
16:9 – HD TV
Aspect Ratios come into play when we crop our image to meet a specific container. Aside from scaling, which is a different concern, it is important that our final images have the same aspect ratio as the destination container so that, when scaled, they fit the container exactly with no dead space (letterboxing) or pixels lost (cropping).
The next few articles in the Holy Crop! blog will rely on this basic understanding of the concept of Aspect Ratio as we discuss the three basic families of crop:
Portrait • Square • Landscape
Rikk Flohr © 2010