Modulation Transfer Function

Modulation Transfer Function (MTF) charts are one of the key components that lens designers utilize when designing or updating a lens. It is a chart speaking to the translucency of a lens. Specifically the lenses contrast and sharpness. It can also tell you the quality of the Bokeh. Bokeh is the Japanese term for the out of focus bits.

The vertical axis represents the transmittance of light. The maximum value of the vertical axis is 100% light transmittance. Glass is not completely translucent, so it impossible for a lens to achieve 100%.

The horizontal axis represents the distance from the center of the image to the outer edge. Some manufacturers measure this slightly differently. Graphs should only be compared by lenses of the same manufacturer and of similar focal lengths. Telephoto lenses normally have better MTF graphs compared to wide angle lenses. If the lines are relatively flat you will have a lens that performs better than ones that sporadically decreases from left to right.

Canon EF 50mm f/1.8

There are eight lines on the graph and half of them are thick and the other half are thin. Half are solid and the other half are dashed lines. The solid lines are referred to as “Sagittal lines” and they are parallel to the diagonal of the image. The dashed lines are referred to as “Meridionial lines” and they run perpendicular to the Sagittal lines. The Meridional and Sagital lines are grouped together in a pattern and the sharpness and contrast of these lines give us our data points. The lines are also colored to represent the different apertures.

 

Putting it all together

The thickness of the lines refers to the spatial frequency or lines per mm. The thick lines that represent the higher spatial frequency (30 lines/mm) tells us how well a lenses resolving power is. Think of this as sharpness. The thin lines show lower spatial frequency of around 10 lines per mm and can attest to a lenses contrast. If the dashed and solid lines are similar in slope and distance to each other than the lens should have decent bokeh. 

Works Cited:

EF LENS WORK III The Eyes of EOS, Japan: Canon Inc. March 2008, Tenth edition, 229-232

Canon U.S.A. : Consumer and Home Office: EF Lens lineup 2012 Canon U.S.A., Inc. 9 Sept. 2012 <http://www.usa.canon.com>

Nikon USA What is a lens MTF chart 2012 Nikon Inc., 9 Sept. 2012
<http://www.nikonusa.com/en/Learn-And-Explore/Article/h3c4udzf/what-is-a-lens-mtf-chart-how-do-i-read-it.html>