|Fig. 1. Belgian Air Force F16 displaying at Duxford, May 2012. (Left) Straight from the camera. (Right) Blue sky intensified as described in the text.|
When I'm photographing aircraft against blue skies, I like the blue to be really intense blue, a bit like the sky in high mountains (or, more prosaically, the effect given by a polarising filter). British skies are usually some pallid shade of blue (Fig. 1, left) rather than the intense blue I like (Fig. 1, right). So, for the last couple of years, I've been using a very simple way in Camera Raw and now Lightroom to intensify them. There are lots of other ways to do this, but this has the virtue of being simple and quick.
The blue of the sky sits conveniently within the colour range covered by the Blue slider in the HSL panel. The sky can be darkened simply by sliding the Luminance Blue slider to the left. Sometimes skies can have a bit of Aqua in as well, so that slider might have come down as well. The best way to do this is to use the targeted adjustment tool for Luminance and then drag downwards on the sky.
What usually happens when doing this, though, is that the apparent saturation of the sky increases to the point where it is not realistic. So now use the targeted adjustment tool to drag the Saturation on the sky down (maybe just a little) to the point where you get a fairly realistic looking intense sky.
At this point, all the manipulation to the sky can bring out a lot of noise. So it is necessary to add a bit of noise reduction from the Detail panel. It may be necessary to use both Luminance and Colour noise reduction.
For simplicity, I've set up a Lightroom preset that drops Blue and Aqua Luminance and Saturation and adds some noise reduction. This makes a good starting point for intensified skies, and typically does not require big moves on the sliders from there to get a satisfactory result. Fig. 2 shows the HSL settings for this.
|Fig. 2. HSL settings for simple blue sky preset (Lightroom 4.1)|