We're in the peak of summer. This means perfect light for high contrast exposure, deep colours and bright highlights. It also (generally) means blue skies with a diversity of cloud formations from high altitude jetsream streaks to huge, billowing cumulus clouds. Many professional photographers, and proficient amateurs alike will tell you this is a key time to use a polarising filter. There's a lot of light out there being reflected, that can be manipulated with such a filter. Many who will talk of this are generally on a digital SLR which has the option to screw on filters that polarise the light in the image.
For those who maybe do not fully realise what a polariser is, essentially it affects the light passing through the lens, before it is interpreted on the digital sensor or film surface. In simple terms it deepens colour tone and increases contrast in skies. There are other applications such as removing reflections from surfaces like glass and water, but this would require an actual polariser to do this and while many of us are now using mobile devices as our key method of taking images, the choice is somewhat more limited. There are polarisers emerging for the likes of iPhone, along with a range of add on lenses, and in time the technology itself will develop to allow this to be incorporated into the lenses generally found in smartphone cameras. If you want to find out more on polarising and its application in photography have a look at this entry in Wikipedia.
While these choices still develop, we can use the advantage of image processing to simulate some aspects of polarising. A key visual cue to polarising is the deepening of sky colour, and one of the main uses when this technique is applied. In this tutorial we are using Geló to simulate this deepening sky effect.
The first example is shown in the image above which features the tail fin of Concorde G-BOAD on display at USS Intrepid in New York City. This is an image that demonstrates the perfect situation to use a polariser to enhance an image. Bright light with significant colour definition in the red ribbon design, the gleaming white fuselage and the blue sky overhead. And we can further subtly enhance this with an overlay that deepens the contrast between the sky and aircraft, and bring out definition in the clouds.
The original image shows good definition in the sky, yet the polarised version adds impact with a deeper blue to pull out the white fuselage. You'll also notice is deepens the red and gives more form to the fuselage by enhancing the tones in shadow, whilst keeping the highlights bright. Overall it gives the subject more form, more depth and a sense of shape in the design of the aircraft.
To achieve this, simply define a deep blue colour, and use a solid filter blended with the tint blend mode to further deepen the sky colour, colour tones and shading. The settings are shown to demonstrate this.
Of course, you don't need access to a white supersonic passenger jet to use this technique. And you could use a gradient to affect only certain parts of the image if you wished.
Here is another example which shows the Staten Island Ferry, again on a bright sunny day. Using the same blend mode and colour settings as above, yet with a gradient you can selectively enhance only the top of the sky in the image, and retain the colour quality with the rest of the image. Of course, adjust the grad to suit your image and personal tastes. You'll notice between the two, the sky colour and cloud formations are more defined, and take on a stronger blue colour. You'll also notice the vivid orange colour of the ferry is enhanced.
There is a previous tutorial on simulating polarising which touches on the same idea, and gives another example of its application.