Modulation Transfer Function
A measurement of how faithfully a lens transfers the contrast of fine detail from subject to image, plotted against position in the frame.
Formula / rule
MTF = (I_max − I_min) / (I_max + I_min)
Measured in
% contrast
In depth
MTF charts usually show two spatial frequencies: a low one, around 10 line pairs per millimetre, describing overall contrast and 'pop', and a high one, around 30 lp/mm, describing fine resolution. Each is drawn twice, for sagittal and meridional detail, and the gap between those pairs reveals astigmatism. Higher and flatter curves are better.
Examples in practice
A lens whose 10 lp/mm line stays above 0.9 across the frame will look crisp and contrasty even where fine resolution is falling.