Two types of filter attached to the front of your lens can be used to bring out the detail in skies.
Reflection, © Mazen Abdulmalek 2011, Canon EOS 60D
A polarising filter is useful as it can make clouds stand out from blue sky. The filter blocks some of the light from the blue areas, preventing the overexposure that can turn blue skies white in photographs.
Attach the polarising filter to the camera lens and look through the viewfinder of your EOS, or at the Live View image on the LCD screen. Rotate the filter until the saturation of the blue is at the level you want.
A polarising filter is most effective when you are shooting an area of sky that is at right angles to the sun. At midday, when the sun is overhead, this will be the sky at the horizon in any direction. At sunrise and sunset, with the sun on the eastern or western horizons, it will only be the sky on the northern and southern horizons that will be affected.
Another way to balance sky and ground exposures is the graduated neutral density filter. As the name suggests, this has a grey density at one end, fading to clear at the other. The filter is positioned over the camera lens with the grey density covering the area of the sky. This reduces the amount of light from the sky reaching the camera sensor, balancing the exposure with that of the ground.
The exposure should be determined before the filter is attached, taking a meter reading from the ground. Make a note of the shutter speed and aperture selected and then set these values manually on the camera. This will prevent the readings changing when the filter is in position over the lens.
The most useful type of graduated filter is rectangular. This fits a holder attached to the front of the lens. The filter slides up and down so that the grey area can be positioned over the sky even when the horizon is not in the centre of the frame.
High Dynamic Range (HDR)
HDR captures a number of shots, often three, of exactly the same scene at different exposures - under, correct and over exposed. The underexposed image will actually have good exposure for the sky. The correct and overexposed images will show detail in the ground. These images are combined in the camera to create a single image that brings out the detail in the highlight and shadow areas at the same time.
Some of the latest EOS cameras offer an HDR setting. But if your EOS doesn't you can still create the same effect. First, shoot three images using Auto Exposure Bracketing (AEB) to get the range of exposures. Transfer these images to your computer and use the HDR tool in the Digital Photo Professional (DPP) software supplied with your camera to merge the images into a single photo that captures detail across the range.
Make sure you have the latest version of the software by clicking here.