1. An image analysis apparatus for processing an image to generate an image focus indicator, the apparatus comprising: a focus detector configured to detect respective local focus estimates for a plurality of blocks of the image, and to combine the local focus estimates to generate the image focus indicator for that image, wherein the focus detector is configured to detect the local focus estimates by detecting image contrast in a group of pixels around the respective block, the group of pixels comprises a square array of pixels centred around the respective block, the focus detector is configured to detect image contrast in a group by detecting the difference between the maximum pixel brightness and the minimum pixel brightness in that group, the image is an image of a 3D pair of images representing different respective views of a scene, the focus detector is configured to detect the image focus indicator in respect of each image of the pair, the image analysis apparatus is configured to compare the image focus indicators in respect of the pair of images, to detect whether the pair of images have a similar degree of focus, and the focus detector is configured to detect differences between local focus estimates of corresponding areas of the pair of images.