1. A method of determining misalignment between a first image and a second image, the first and second images being viewable stereoscopically, the method comprising: determining a feature position within the first image and a corresponding feature position within the second image; defining, within the first image and the second image, an optical axis of cameras capturing said respective images; calculating the misalignment between at least one of scale, roll or vertical translation of the feature position within the first image and the corresponding feature position within the second image, the misalignment being determined in dependence upon the location of the feature position of the first image and the corresponding feature position of the second image relative to the defined optical axis of the respective images; and defining within the first image at least one further feature position, and at least one further corresponding feature position within the second image, wherein the vertical translation is calculated by: selecting two feature positions in the first image and a corresponding two feature positions within the second image; and determining the vertical translation in accordance with a given roll misalignment between the first and second images, wherein the feature position in the first image and the corresponding feature position in the second image are located above the defined optical axis and the at least one further feature position in the first image and the at least one further corresponding feature position in the second image are located below the defined optical axis, and wherein the vertical translation is determined using the equation T = ( R 1 · y ⁢ ⁢ 2 l - R 2 · y ⁢ ⁢ 1 l ) ( y ⁢ ⁢ 1 l + y ⁢ ⁢ 2 l ) where T is the value of the translation, y 1 1 is the y co-ordinate of the feature position in the first image, y 2 1 is the y co-ordinate of the at least one further feature position in the first image; and R 1 =x 1 r ·sin(θ)+y 1 r ·cos(θ) where x 1 r is the x co-ordinate of the corresponding feature position in the second image, y 1 r is the y co-ordinate of the corresponding feature position in the second image and R 2 =x 2 r ·sin(−θ)+y 2 r ·cos(−θ) where x 2 r , is the x co-ordinate of the at least one further corresponding feature position in the second image, y 2 r is the y co-ordinate of the at least one further corresponding feature position in the second image and θ is the given roll misalignment.