BNRLoupe provides a loupe view for iOS that displays, pixel for pixel, the contents of an image--potentially at a zoom level of less than 1--that is directly under your finger.
The motivation for this project is to provide a way for users to do detailed manipulation of an image. The finger obscures the portion of the image they're manipulating, making fine adjustments hard to achieve. Using this loupe class, you can provide feedback to the user of what's under their finger.
This class relies on a UIImage to contain the image, so it is not appropriate for use with very large images.