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<strong>Imatest</strong> <strong>Documentation</strong><br />

except for its autofocus. Mine doesn't autofocus as reliably as Canon lenses, but it works beautifully on manual. The autofocus<br />

problem is plainly visible when working with the distortion chart.<br />

Main figure showing modest pincushion distortion<br />

The Sigma has modest amounts of pincushion distortion at 125 mm and barrel distortion at 18mm, its widest angle setting.<br />

Corrected vertical lines are deep magenta; horizontal lines are blue. The following results are displayed on the left below the<br />

image.<br />

TV Distortion from the SMIA specification, §5.20. Referring to the image on the right,<br />

SMIA TV Distortion = 100( A-B )/B ; A = ( A1+A2 )/2<br />

The box on the right is described in the SMIA spec as "nearly filling" the image. Since<br />

the test chart grid may not do this, Distortion uses a simulated box whose height is 98%<br />

that of the image. Note that the sign is opposite of k1 and p1. SMIA TV Distortion > 0 is<br />

pincushion; < 0 is barrel.<br />

SMIA vs. traditional TV distortion<br />

Thanks to Mark Butterfield for pointing out that SMIA's distortion definition differs from the<br />

traditional TV industry definition by a factor of two. The traditional definition is shown in the<br />

illustration on the right, which has been adapted from the publication "Optical Terms," published<br />

by Fujinon. The same definition appears in "Measurement and analysis of the performance of<br />

film and television camera lenses" published by the European Broadcasting Union (EBU). SMIA<br />

TV distortion is double the traditional TV distortion definition.<br />

SMIA TV Distortion<br />

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