This paper presents an indicator of image quality. This index is based on contrast and edges, which are techniques used in measurements of atmospheric visibility during the day from fixed cameras. This indicator is applied to urban images with many details acquired for a single event but different sensor exposure conditions and at different times of the day sunny and cloudy day. Experimental results show that the index reaches a maximum value that is used to set the conditions for capture in terms of the exposure time and diaphragm aperture. The results also show that the maximum values of the index are in the lower half of the sensor response curve.