[frank]

The focal length does not change. Uncorrected lenses or under corrected lenses just show a wider view (one can see uncorrected fish eye lenses as lenses with a LOT of barrel distortion). So, uncorrected it just will give a wider view than a corrected lens with the same focal length.

 

I already wrote "(or angle of view, more precisely)".

 

Then, if the labeled 12mm FL for the wide end of the 12-40mm lens is the true FL at the wide end, then after the distortion correction the diagonal view angle of the image would be that of FL like 13mm? The native distortion at 12mm is very large, about 8% according to the test. On the other hand, it seems that the labeled FL at the two ends of a zoom lens is often not quite accurate.

 

My Nikor 16-35mm also has a high distortion at 16mm, about 5% I guess. Each time after I correct the distortion afterwards I clearly see the change in angle of view which I don't like. So I only correct the distortion when it is necessary.