We present the results of an experiment aimed at measuring the formation depths of the Fe I line pair at 630 nm in the solar photosphere. We use images of the granulation obtained at different levels in the lines, from line center up to the continuum level. When we observe out of disc center their difference in formation depths is projected into a radial shift of the images by the perspective effect. We measure this shift by implementing a cross-spectral method similar to a technique previously developed for stellar applications (Aime et al. 1984). The signal-to-noise ratio is increased by averaging the cross spectra over a large number of images. This technique allows us to measure very small displacements, below the telescope resolution.

We show results we obtained on HINODE/SP observations and compare them to numerical simulations. The difference of formation depths between the two line cores is determined with a very high accuracy and compares quite well to LTE model calculation using either 1D solar model or full 3D calculations in snapshots of the granulation. However it shows significant variations in magnetic regions. The difference between line core and continuum formation depths is more difficult to measure precisely because line core and continuum images are not well correlated, due to contrast inversion and depth-dependence of granular structures. We solve this problem by measuring the perspective shifts between similar enough images taken at successive steps along the line profile, and by integrating the shifts from the continuum level to the line center forming layer.