The invention relates to a polychromatic imaging method, using a picture-taking instrument, which is placed on board an aircraft or satellite, which enables a reduction of the size of the instrument without reducing resolution of a final obtained image. To this end, a pupil of the instrument is sized for intermediate images that are limited to a first spectral sub-band without any image information being completely transmitted by the instrument for at least one wavelength that is located beyond said first spectral sub-band. Merging the intermediate image, inputted for the first spectral sub-band, with at least one other intermediate image, inputted for another spectral sub-band, recreates a polychromatic visual rendering that is satisfactory for the final image. Said other intermediate image includes the wavelength located beyond the first sub-band.