In microscopy, high magnifications are achievable for investigating micro-objects but the paradigm is that higher the required magnification, the lower the depth of focus. In this paper we show that it is possible to construct an extended focused image (EFI) image by a coherent optical microscope without any mechanical movement but by using the 3D imaging capability of digital holography (DH). In fact, DH has the unique property of allowing direct calculation and management of an amplitude and phase map along the longitudinal direction in front of the digital camera. That constitutes a fundamental feature of DH to construct an EFI image of an object or systems experiencing dynamic evolution since the recording of only one image is needed instead of performing a mechanical scanning and to record several images at different focus planes. In other words, by means of this approach it is possible to obtain an EFI image for studying dynamic objects, such as biological objects, dynamic MEMS.