Uneven polarisation effect due to changing angles with respect to the sun when using an ultra wide angle lens. The strongest effect is at 90 degrees to the sun. Image courtesy th3_n3wb13

Foliage taken with a Canon EF-S 10-22 at 10mm. Upper image is without a CPL filter, lower image is with a Hoya HD CPL. Two crops from the left and right side of the frame are shown. Due to the various orientations of the leaves, as well as surface variations of the leaves, effects can be seen across the whole field of view (unlike the effect on the blue sky). The reduction of glare (reflection) renders the foliage in a more saturated green. The polarised light stems from specular reflections off the leaf surface (see also:  V. C. Vanderbilt, L. Grant, and C. S. T. Daughtry (1985) Polarisation of Light Scattered by Vegetation. PROCEEDINGS OF THE IEEE, VOL. 73, NO. 6, JUNE)