Pixel 'Aspect Ratio'
The Pixel Aspect Ratio is simply a fancy way of saying what the length of one side of a pixel is in relation to the length of the other. If the ratio is 1:1 it means that each side of the pixel is the same length as the other, in other words the pixel is a square. If the ratio is 2:1 it means that one side is twice the length of the other. With still images we will always be required to supply them with square pixels, so why would we worry about what the ratio is?. The trouble is that some video formats do use pixels that are not square, with an 'Aspect Ratio' that is not 1:1. So if you are working with video further on in this digital training course, or if you are receiving images that were captured from video, then you need to be aware of what the image Pixel Aspect Ratio is.
NOTE: The 'Aspect Ratio' does not state what the actual length of the side of a pixel is, just it's "relative" size of one side to the other. The actual length will depend on how much you stretch or shrink an image when reproducing it. But if the Pixel Aspect Ratio is 1:1 then the pixels will always be square, no matter what size the image may end up being.
FIGURE 1 Images are made with square pixels, even though we don't see that they are square
Images with square pixels does not mean that the images themselves are square, as they can be horizontal or vertical with sides as long or short as you wish. What it does mean though is that the building blocks, the pixels, are square.
FIGURE 2 If the pixel 'Aspect Ratio' is set to other than square, then the image will appear as either stretched or squeezed
Some video formats use retangular pixel shapes which may look correct in a video, but can be a problem if still images are captured from them. If the 'Aspect Ratio' is set to other than "square" then Photoshop will display the image with a vertical or horizontal distortion, depending on which setting was used. The effect is a little like going to a fair ground and standing in front of one of those distorting mirrors they sometimes have. You won't have changed, even if you do look a little odd in the reflection.
NOTE: Having an image with a non square 'Aspect Ratio' does NOT damage the image, as the number of pixels, and the data within the pixels remains the same. The Pixel Aspect Ratio is simply an instruction to Photoshop on whether to squeeze or stretch the image view. The "view" may look a little odd, but underneath the image will still be in a healthy state.
FIGURE 3 The Photoshop warning shown if the pixels are not set to square (left). This can be easily fixed from the menu bar (right)
Nornally Photoshop will warn you when you open an image if the pixels are not set to "square" (left). Fixing this is done very simply by selecting Image > Pixel Aspect Ratio > Square on the menu bar (right).