Lens Calibration: Canon EF-S 15-85 f/3.5-5.6 IS USM

My image processing workflow is nearly complete. FotoPreProcessor is working as expected (small quirks aside), UFRaw proves to be a good RAW converter and The GIMP is adding finishing touches. But one building block is incomplete: lens correction, done by UFRaw via the LensFun library. All my lenses are present in LensFun’s library except for my current standard lens, the 15-85.

But this can be fixed, because the LensFun developer spent some time on a manual dealing with lens calibration: http://lensfun.berlios.de/lens-calibration/

Here’s my result, based on 64 test images and an evening of data processing:

<lensdatabase>
	<lens>
		<!-- author: Frank Abelbeck <frank.abelbeck@googlemail.com> -->
		<maker>Canon</maker>
		<model>Canon EF-S 15-85mm f/3.5-5.6 IS USM</model>
		<focal min="15.0" max="85.0" />
		<aperture min="3.5" max="5.6" />
		<mount>Canon EF-S</mount>
		<cropfactor>1.6</cropfactor>
		<type>rectilinear</type>
		<calibration>
			<distortion model="ptlens" focal="15" a="0.059"  b="-0.152" c="0.070" />
			<distortion model="ptlens" focal="24" a="0.027"  b="-0.082" c="0.077" />
			<distortion model="ptlens" focal="35" a="0.019"  b="-0.055" c="0.052" />
			<distortion model="ptlens" focal="50" a="-0.013" b="0.033"  c="-0.022" />
			<distortion model="ptlens" focal="70" a="-0.029" b="0.090"  c="-0.081" />
			<distortion model="ptlens" focal="85" a="0.033"  b="-0.086" c="0.073" />
<!--
			<tca        model="poly3"  focal="15" br="-0.0001175" vr="1.0003860" bb="0.0002647" vb="1.0002513" />
			<tca        model="poly3"  focal="24" br="-0.0000432" vr="1.0000464" bb="0.0003439" vb="1.0003965" />
			<tca        model="poly3"  focal="35" br="-0.0000053" vr="0.9998168" bb="0.0000552" vb="1.0004647" />
			<tca        model="poly3"  focal="50" br="-0.0000262" vr="0.9995610" bb="-0.0000306" vb="1.0006026" />
			<tca        model="poly3"  focal="70" br="-0.0000630" vr="0.9992608" bb="0.0000502" vb="1.0006476" />
			<tca        model="poly3"  focal="85" br="-0.0000554" vr="0.9990389" bb="0.0000656" vb="1.0006373" />
-->
		</calibration>
	</lens>
</lensdatabase>

Until now it only deals with distortion and chromatic aberration. Calculation of vignetting depends on focal length, aperture and distance to the object, so a lot of test shots (approx. 180) are needed.

Update 2012-11-30: I noticed strong chromatic aberration after applying my calibration results. Therefore I changed the above snippet by commenting the tca elements. Stay tuned for a thorough calibration in 2013!

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