Blue skies are beautiful, but sometimes they’re just not blue enough. That why polarising filters are so popular for landscape and travel photographers.
The light in a blue sky is partially polarised, and as you rotate a polarising filter over the lens the blue becomes more and less intense. You have to find exactly the right angle of rotation to get the blue at maximum intensity.
This sounds like a simple thing to do digitally, and it is. All you need is a photo editor with a Hue/Saturation/Lightness adjustment panel or a comparable colour editor tool.
The first step is to select the specific colour range of that blue sky. In some programs you do that with an eyedropper tool, in others you click a colour swatch closest to that colour.
Then all you need to do is reduce the Lightness value to make those blue tones darker, increase the Saturation to make the blue stronger and, if the colour starts to look a little cyan, move the Hue slider slightly to the right to add a dash of purple/violet.
The trick is to balance these three adjustments to get a realistic-looking picture without going too far. You can save your polarising effect as a preset effect, though you’ll often need to adjust the settings differently for different pictures.
Watch out for artefacts
The problem with any image manipulation of colour data is that you are artificially separating tones and gradations and leaning more heavily on some colour channels more than others. This can produce edge effects or banding where these gradations become too coarse between modified and unmodified colours. You can also get more noise, typically a ‘blotching’ effect in skies.
There are a couple of things you can do to avoid this. First, don’t get too greedy with your adjustments. The stronger you make them, the greater the risk of artefacts. Second, work from RAW files rather than JPEGs. JPEGs don’t have the colour depth to withstand heavy colour manipulation and are much more likely to produce banding.
Why this is not like a ‘real’ polariser
Real-world polarising filters don’t just make blue skies darker. They also suppress reflections, especially off polished surfaces like paintwork, still water and shiny vegetation.
It’s not possible to reproduce this effect digitally. Darker blue skies are easy, but the suppression of reflections is an optical effect that can only be applied to ‘raw’ light by an optical filter.
So even though you can make blue skies darker digitally, you still need a polarising filter to take the shine off leaves or water in landscapes, or to reduce glare in architectural, travel, automotive and products shots.