Today’s Question: Sometimes Photoshop offers me a choice of square pixels or other kinds of pixels. I thought pixel was just a pixel. Can you explain?
Tim’s Quick Answer: For photographic images, pixels are indeed generally square. The option for non-square (i.e., rectangular) pixels relates primarily to the creation of images for certain video formats.
More Detail: For the vast majority of imaging software used for still photos and illustrations, square pixels are the standard. Even in high definition (HD) video, square pixels are the norm.
However, there have been a variety of video formats (primarily standard definition or SD video) where rectangular pixels were used. This was mostly an issue related to increasing overall resolution without changing the number of “scan lines” in the video itself. The number of scan lines represents the number of physical rows of pixels on older analog devices, such as CRT (cathode ray tube) displays.
For the most part, you can ignore the fact that it is possible to create images using non-square pixels in Photoshop. This option is really there to provide compatibility with the maximum range of video formats, since Photoshop provides a variety of (quite powerful) features for working with video clips.