In the Field: This image was made at Sterling Nature Center in Cayuga County, NY when the warm sunset light had accentuated the reddish clay bluffs along the southern Lake Ontario shoreline. To add viewer interest, I took advantage of the complementary colors of blue and orange between the sky and bluffs. In order to emphasize the dynamic cloud formation I placed the horizon line below center, and I used a 10-22 mm wide angle zoom lens on my Canon 40D set at 10 mm. This is equivalent to a 16 mm wide angle on a full-frame 35 mm camera. I decided to use a polarizing filter to darken the blue sky and increase contrast between the clouds and sky. However, this created a problem; because the extreme wide angle lens covered such a large angle of view, the sky was unevenly polarized, being much darker in the upper right hand part of the image. I made a second exposure two stops lighter to lighten that area of the sky so that it matched the tone of the sky in the upper left in the first exposure.
In the Computer: The task at hand was to combine the two exposures, using the left-hand portion of the sky in the first exposure, and the right-hand part of the sky in the second exposure. In Photoshop both files were opened, and the first exposure was copied on top of the second one. Using a layer mask and a soft-edged brush, I then masked over the dark portion of the sky in the first exposure to reveal the lighter sky in the second exposure underneath. The colors were optimized using a Hue/Saturation Adjustment Layer, giving the Reds and Yellows channels +10, and the Blues channel +18. Finally, a Curves Adjustment Layer was added to increase the contrast between the sky and clouds.