  An image captured with a wide-aperture camera exhibits a finite
depth-of-field, with focused and defocused pixels. A compact and robust
representation of focus and defocus helps analyze and manipulate such images.
In this work, we study the fine characteristics of images with a shallow
depth-of-field in the context of focal stacks. We present a composite measure
for focus that is a combination of existing measures. We identify in-focus
pixels, dual-focus pixels, pixels that exhibit bokeh and spatially-varying blur
kernels between focal slices. We use these to build a novel representation that
facilitates easy manipulation of focal stacks. We present a comprehensive
algorithm for post-capture refocusing in a geometrically correct manner. Our
approach can refocus the scene at high fidelity while preserving fine aspects
of focus and defocus blur.
