an imaging apparatus with an autofocus mechanism for obtaining focused images . the apparatus comprises : an objective lens , a focus controller for altering a distance between the objective lens and a sample , an object finder for finding objects of interest within the sample , for example cells , and a light intensity measurement unit which measures light intensity levels of the thus identified objects of interest . the focus control alters the sample - objective distance to maximize the light intensity levels being measured , thereby to arrive at a focus position . objects of interest may be identified by filtering out large objects and optionally by masking out background regions . the apparatus is useful for microscopy and particularly for fluorescent imaging in which light levels are low and noise is often high .