Polarising filters are used by landscape photographers to intensify blue skies, and you can achieve a similar result in image-editing software, though by a slightly different route. To be honest, it’s not really possible to properly duplicate the effects of a polarising filter in sofware because they use an optical phenomenon you can’t reproduce later – polarisers also subdue reflections in water and glossy surfaces. But you can intensify blue skies simply by adjusting that particular colour in the image.
It sounds like it might involve complicated selections, but it’s actually a lot simpler than that. Aperture can isolate and change individual colours (or colour ranges) within an image without affecting the rest. It’s not unique to Aperture – you can do the same thing in Lightroom and other programs.
Once you’ve created an effect like this that you might want to use again in the future, you can save it as an Aperture preset which can then be applied to any other image via a drop-down menu.
To demonstrate how it’s done I’ll start with this picture of a statue against a blue sky.
01 Add a Color adjustment
This is the same picture opened in Aperture with the Adjustment panel selected. Depending on how you’ve got it set up, Aperture may not display the Color panel by default. If not, you can add a Color adjustment by clicking the Add Adjustment button.
02 Preset colours
The Color panel displays a row of six different colours. You can click on one of these (the blue button, in this instance) to select that colour range in order to make adjustments. This isn’t very accurate, though, because it’s not exactly the same blue as most skies.