Screen Capture is free built in feature on a Windows Phone 8 Smartphone. That means you can easily Capture or take a screen shot on your windows 8 Smartphone.
How to to take a screen shot/ capture Nokia Lumia Windows Phone 8 Smartphone:
you didn't have to navigate to anywhere. Just click windows button (start button) and power on off button together to take screen shot.
The captured photos will be stored at photos function of the phone. as exact Photos> Albums> Captured.
This method was successfully tested on Nokia Lumia 920, 820. And may also work on other Windows phone 8 Smartphones like Lumia 620, 520, 505, 720 and other too.