Digital cameras: spatial image quality matters

Abstract

Difficulty in understanding the relative performance and appreciating the numerous features of the ever-increasing range of digital cameras has led to the need for a simple objective test of camera quality. A method of measuring the modulation transfer function is described but, in practice, only a sector star pattern is needed to measure the spatial frequency bandwidth (SFB), which is related to resolution and the optimum print width (OPW) achievable with a particular camera, whether digital or film. The SFB of a camera divided by that of the eye in the plane of a print at arm's length and multiplied by the image format width gives the OPW. A value index (VI) is derived by dividing this number into the camera price. A pixel packing index (PPI) is the ratio of the true information content, obtained from the number of effective pixels present divided by the nominal information content, given by the stated number of pixels. It quantifies the proportion of pixels contributing to spatial image quality. Provisional results obtained using nine different cameras in the 1–4 MP range suggest that the SFB, and therefore the effective pixel size and spatial resolution, vary relatively little between cameras. A doubling of the OPW requires a fourfold increase in the number of pixels. The PPI over the full sensor area can be as little as 8%. The OPW is a useful parameter for optimising image quality settings and for comparing the quality of different cameras. The advantages are that users should be able to understand it, and it embodies both effective pixel size influencing spatial image quality and the number of pixels influencing acceptable print size. The VI, since it reflects the present highly volatile high-street prices, will only be of temporary value.