How to read MTF charts

Modular Transfer Function charts show the sharpness of a lens on the X axis, from its centre (on the left) to the extreme corner (on the right). A test chart with a series of very fine black and white lines spaced at 10 per millimetre and 30 per millimetre is shot and the amount of blur recorded. Two sets of figures are given; ‘sagittal’ indicates lines that are parallel to the centre of the lens while ‘meridonal’ are perpendicular lines. The Y axis is measured from 0 to 1 – and the closer to the top of the chart, the sharper the lens is.