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The spatial resolution attributed to digital cameras is usually based on the number or size of the pixels in the sensor. On closer examination it can often be shown that the real limit to the level of detail recorded by a camera will in practice be due to the performance of the optical system. An example of how this happens is given with a comparison between two satellite camera systems.The significant influence on image quality set by the optical system of a camera has been recognised from the earliest days of photography (Kingslake, 1987). The smallest possible image patch achievable with an aberration-free and diffraction-limited optical system is an ''Airy disc'', where the central disc contains 83AE9% of the energy in the Airy disc pattern.

The smallest sized detail on the ground actually recorded by an airborne or satellite camera system can be different to that expected by an applications scientist or image analyst. When this misconception occurs, it may be due to the method used in determining the spatial resolution of the camera system concerned. The role of an optical system is of primary importance in setting a limit on the spatial resolution achieved by either a photographic or optoelectronic/digital camera system. This is because it is the first component of the imaging system to intercept radiation coming from an object on the ground. In circumstances where the smallest image patch produced at the focal plane by an optical system is comparable or larger in size than the pixel size/ spacing of a CCD then spatial resolution will become limited by the optics, not the CCD. When this occurs, the radiometric response of the camera system to small features on the ground will correspond to the small optical image patch, and not the size of pixels in the CCD. The spatial resolution characteristics of a camera system will usually be given in terms of the ground sampled distance (GSD), or as a ground resolved distance (GRD). The results obtained by the two methods of measurement these values represent are different, and the difference can be significant because of the sampling principles that are the basis of each method.

The introduction of digital cameras for aerial survey photography and use of scanners with processed aerial film present a number of technical questions concerning the way photography is to be acquired in the future. Among the questions to be addressed are the comparative metric properties of images acquired by each system and the associated image quality characteristics. The new digital camera systems will require very careful appraisal, using well-established methods of assessing metric quality already available to survey organisations. The questions concerning image quality are more difficult to answer, as they require very specialised instrumentation if measurements are to be made directly in practice. Over the course of the past several decades a more analytical approach to assessing image quality has emerged, and one that may be used by individuals. The method is particularly effective where information on the micro-image characteristics of products is made available by the manufacturers concerned. An outline is given of some considerations that need to be taken into account when using an analytical approach, with references given for further reading.