That's A Stretch
An image is a matrix of numbers that represent the values of a measured physical quantity at different points in space. The images returned by MESSENGER's MDIS camera measure the amount of light reflected from the surface of Mercury. There is a myriad of ways in which the image values can be presented for display. The translation from the inherent physical values of the image to the "digital numbers" of a computer display is referred to as the "image stretch". Here we have the same image of an area of the terminator in two different stretches. The left-hand image nicely presents the better-illuminated portions of the scene, but some details in the darker areas cannot be seen. The right-hand image has a "harsher" stretch that permits some areas of low illumination to be perceived, at the expense of saturating areas that received more sunlight. North is to the top in each image.
Date acquired: March ??, 2011
Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington