This paper has been prompted by observations of some anomalies in the performance of the standard imaging models (pin-hole, thin-lens and Gaussian thick- lens), in the context of composing omnifocus images and estimating depth maps from a sequence of images. A closer examination of the models revealed that they assume a position of the aperture that conflicts with the designs of many available lenses. We have shown in this paper that the imaging geometry and photometric properties of an image are significantly influenced by the position of the aperture. This is confirmed by the discrepancies between observed mappings and those predicted by the models. We have therefore concluded that the current imaging models do not adequately represent practical imaging systems. We have proposed a new imaging model which overcomes these deficiencies and have given the associated mappings. The impact of this model on some common imaging scenarios is described, along with experimental verification of the better performance of the model on three real lenses.