Lens MTF Area Summary and Figure of Merit

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Lens MTF Area Summary and Figure of Merit

MTF Area Summary

The MTF area summary presents the average and RMS (root mean square) of the Sagittal and meridonial responses at each of 10 and 30 lp/mm as a weighted average of the area of the frame.

Figure of Merit

To try and simplify the data somewhat the summary data indicating how far from the lens axis the MTF remains above the satisfactory and superior limit is extended to derive a figure of merit (FoM). The methodology is as follows:

i) Calculate how large a percentage of the frame area is covered above each limit.

ii) Weight this percentage to give more emphasis to the edge and corners of the frame (as this is what differentiates an excellent from a mediocre lens).

iii) Produce a weighted sum of the two limit ( Superior and Satisfactory) weighted areas for contrast and detail (10 and 30 lp/mm)

iv) Take the average of the contrast and detail figure of merit,

v) Do i to iv for 1.6X and 1X crop factor of 35mm

To clarify this in an equation:

Where:

Asc    =       Area of superior contrast

Asd    =       Area of superior detail

Agc    =       Area of satisfactory contrast

Agd    =       Area of satisfactory detail

Af       =       Area of format frame

Lsc    =       Limit line for superior contrast (0.8)

Lgc    =       Limit line for satisfactory contrast (0.6)

Lsd    =       Limit line for superior detail (0.7)

Lgd    =       Limit line for satisfactory detail (0.5)

W       =       Area weighting exponent (2)

The weighting exponent was chosen so that a lens needed to achieve the quality limit line over around 75% of its area to achieve a 50% weighted area. This relates to about a 9mm radius on APS-C and 15mm radius on 35mm.

To achieve a 100% FoM a lens need to provide superior contrast and detail over 100% of the frame.

A lens that achieves satisfactory contrast and detail performance across 100% of the format frame but no where manages superior contrast and detail will achieve a FoM score of 42%. This scoring scheme is intended to separate adequate lenses from excellent ones, the separation being 58%.

Last Updated 05/06/2008

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