You are deciding between two lenses. Both product pages show a graph with a few curved lines running left to right, some numbers along the bottom, and nothing on the page explaining what any of it means. That graph is an MTF chart, and it is one of the few pieces of genuinely objective optical data a manufacturer publishes. Learning to read one takes about ten minutes and it will change how you compare lenses. This article covers what MTF measures, how to read the chart, and — just as importantly — what it cannot tell you.
What is MTF?
MTF stands for Modulation Transfer Function. It is a measure of how faithfully a lens transfers the contrast of a subject into the image it projects.
Every lens loses some contrast. Point one at a pattern of crisp black and white lines and what emerges on the other side is never quite as crisp — the blacks lift slightly toward grey, the whites drop slightly, and the finer the lines, the more of that original contrast is lost. MTF puts a number on how much
