Why does the Hubble Deep Field image have a "stair step" shape?
The Wide Field Planetary Camera 2 (WFPC2) is actually four cameras in one; each camera looks at adjacent pieces of the sky. The resulting four separate pictures are combined together, like tiles, to create a mosaic.
Three of the cameras (labeled A, B, C) are "wide field" but only compared to other cameras that can look very far away. They each look at a piece of sky that we, on the ground, would measure as one-tenth the diameter of the full moon.
A fourth camera, called the "planetary camera" (labeled PC), has an even narrower view. It looks at an area of the sky that is only one-fourth the area of the wide field cameras; however, it has twice the resolution.
When astronomers combine the image from the smaller camera with the three images taken by the wide cameras, they create the unique "stair step" appearance of full-field WFPC2 pictures.