A method of generating an image for display by a head-mounted display device, HMD, the image being a stereoscopic three-dimensional image, the method comprising: deriving an expected depth of field for the image for display by the HMD, with respect to a point of focus of a viewer in the image for display, in response to lighting conditions of a scene represented by the image for display, such that a greater expected depth of field is associated with an image representing a scene having lighting conditions with a greater apparent brightness, the apparent brightness being an estimate of the brightness of the image for display if it were viewed as a real scene rather than on a display; generating a blurring function to be applied to the image for display, wherein the blurring function applies blurring as a function of apparent spatial depth within the image for display, the blurring function being defined such that the degree of blurring applied to a depth region which is closer, in an apparent depth direction, to the point of focus is lower than the degree of blurring applied to a depth region which is further, in an apparent depth direction, from the point of focus, and the degree of blurring with respect to the apparent spatial depth separation from the point of focus depends upon the expected depth of field; and applying the generated blurring function to the image for display by the HMD, wherein the apparent brightness of the image for display by the HMD is detected from one or more of: (i) metadata associated with the image indicating apparent brightness data; (ii) data defining simulated lighting used for rendering the image; (iii) colour and/or luminance properties of the image; and (iv) a detection of the presence of particular characteristic image features in the image, and wherein the apparent spatial depth is a measure of how far, in a radial direction, from the viewer a part of the image appears to be, and the apparent depth direction is the radial direction from the user. deriving an expected depth of field for the image for display by the HMD, with respect to a point of focus of a viewer in the image for display, in response to lighting conditions of a scene represented by the image for display, such that a greater expected depth of field is associated with an image representing a scene having lighting conditions with a greater apparent brightness, the apparent brightness being an estimate of the brightness of the image for display if it were viewed as a real scene rather than on a display; generating a blurring function to be applied to the image for display, wherein the blurring function applies blurring as a function of apparent spatial depth within the image for display, the blurring function being defined such that the degree of blurring applied to a depth region which is closer, in an apparent depth direction, to the point of focus is lower than the degree of blurring applied to a depth region which is further, in an apparent depth direction, from the point of focus, and the degree of blurring with respect to the apparent spatial depth separation from the point of focus depends upon the expected depth of field; and applying the generated blurring function to the image for display by the HMD, wherein the apparent brightness of the image for display by the HMD is detected from one or more of: (i) metadata associated with the image indicating apparent brightness data; (ii) data defining simulated lighting used for rendering the image; (iii) colour and/or luminance properties of the image; and (iv) a detection of the presence of particular characteristic image features in the image, and (i) metadata associated with the image indicating apparent brightness data; (ii) data defining simulated lighting used for rendering the image; (iii) colour and/or luminance properties of the image; and (iv) a detection of the presence of particular characteristic image features in the image, and wherein the apparent spatial depth is a measure of how far, in a radial direction, from the viewer a part of the image appears to be, and the apparent depth direction is the radial direction from the user.