When you make a rst-generation (H1) hologram in the usual way, the image will always be virtual. If your reference beam was collimated, the u-distance is innite, and the focal length of the hologram is the distance between the hologram and the subject. As the image is virtual, this distance is negative. However, when the hologram is ipped to produce a real image, its focal length becomes positive. So when you illuminate your hologram with a collimated (conjugate) beam (u = ∞), the image is formed at the principal focus (v = f ).