What is a lens MTF chart and how do I read it?

What is a lens MTF chart and how do I read it?

What is an MTF Chart?

MTF is an abbreviation that stands for Modulation Transfer Function, this is a measurement that is used to evaluate the performance of a lens. An MTF chart plots the contrast of a lens from the centre to its edges against an ideal lens that would transit 100% of the light that passes through it.

The contrast of a lens is important as this works in correlation to lens resolution. In trying to evaluate the quality of a lens, contrast and resolution are important factors. Lens resolution tests tend to be more subjective and are dependant on other factors which is why MTF charts are the preferred method of studying optical performance for lens manufacturers as they use theoretical equations to plot a graph.

On the y-axis (vertical axis) there is a maximum value of 1, which would indicate 100% transmittance of the light through a lens, although 100% transmittance of light is not possible as glass is not 100% transparent. The x-axis (horizontal axis) shows the distance from the center of the image toward the edges. So '0' represents the centre of the lens and the different numerical values represent the distance out towards the edge of the lens in millimetres.

How to read an MTF chart.

Below is an example of an MTF chart. This has various different lines and numbers which indicate how well the lens performs with different measurements. When measuring the lenses performance for an MTF chart the test is carried out with the lens working at it's maximum aperture, the resulting figures are then put onto a chart to produce the MTF chart you can see below. The MTF chart consists of measurement for the Sagital and Meridonial lines at both 10 lines per millimetre and 30 lines per millimetre. This produces a chart with 4 separate lines.

'Sagital lines' (the solid lines on the chart) represent a pair of lines that run parallel to a central diagonal line that passes through the middle of the lens from the bottom left hand corner to the top right hand corner.
'Meridonial lines' (the dotted lines on the chart) represent pairs of lines that also run parallel to the centre of the lens and pass through the middle, but running from the top left hand corner to the bottom right hand corner (so at 90 degrees to the sagital lines).

The red 10 line/mm (10 lines per millimetre) indicates the lenses ability to reproduce low spatical frequency or low resolution. This line indicates the lenses contrast values and the higher and straighter this line is the better. The higher the line appears the greater the amount of contrast the lens can reproduce.
The blue 30 line/mm (30 lines per millimetre) indicates the lenses ability to to reproduce higher spatial frequency or higher resolution. This line relates to the resolving power of the lens and again the higher the line the better.

As the line starts on the left of the chart this represents the centre of the lens and as the line moves to the right it indicates the edge of the lens. So you can see how much the contrast and sharpness of the lens decreases from the centre to the edge of the image.

Using an MTF chart to determine the Bokeh effect of the lens

Another factor that can be read from the MTF graph is the 'bokeh'. Bokeh is a term used to describe the quality of the out of focus areas a lens produces. The bokeh effect varies between lenses and the effect is influenced by the quality of the lens elements used and also the number of aperture blades in the lens design (more blades produce a better circle and therefore a better 'bokeh' effect). The closer the solid line and the dotted line are together, the better the 'out of focus' effect will be on a particular lens.

Lens Performance and further information

Lens performance is not measured only with MTF charts, as softening or blurring of colour also governs measurement. Comparing MTF charts across different manufacturers is not a simple process and test conditions may vary with the set up of the camera. The amount of sharpening applied as standard to an image in-camera can be different between manufacturers, this effects the contrast of the image and so can effect the results of MTF charts.