**3.1.4 Refocusing at different reconstruction distances**

With the analysis of the intensity image reconstruction we demonstrate the ability of DPA to refocus at different distances *d'*.

Figure 13 shows a comparison between DPA (row A) and ASA (row B) in the reconstruction of several holograms where the best focus of the image is: *d'=0 mm*, *d'=10 mm*, *d'=30 mm* and *d'=70 mm*.

Fig. 13. Comparison between DPA (row A) and ASA (row B) in the intensity image reconstruction for different distances *d'*.

According to figure 13, as the reconstruction distance *d'* increases, the size of the reconstructed intensity image is constant for both algorithm until *d' = 10 mm*. For values of *d'* bigger than 20 mm, the size of the reconstructed intensity image is reduced when the ASA is used, whereas it remain constant when is used the DPA. The effect is most obvious for *d' = 70 mm*.

Summarizing, two main advantages can be attributed to the DPA algorithm: capability to maintain the size of a reconstructed image, independent of the reconstruction distance and wavelength for objects larger than a CCD and phase reconstruction with curvature compensation without the necessity of either a reference hologram or parameter adjustment. For a hologram of 1024 x 768 pixels using a standard PC computer (Pentium IV, 3.2 GHz) the required time for the calculation of the phase image, the intensity image and the distance *D* is 2.3, 1.2 and 2.6 seconds respectively. The limitations are related to the two manual filtering stages that exist in the reconstruction process. After the first propagation, the manual selection of Fourier transform components at the back focal plane can introduce others element that disturb the reconstructed plane.
