The combination of a polarimeter with real-time frame selection and differential speckle imaging results in diffraction-limited magnetograms that provide new insight into the morphology of solar small-scale magnetic fields. The method to record diffraction limited narrow-band filtergrams of solar features is based on two cameras taking simultaneous short exposure images through a broad-band and a narrow-band filter, respectively. Speckle imaging reconstructs the image in the broad-band channel. This reconstruction determines the instantaneous optical transfer function (OTF) for each individual broad-band exposure. Each simultaneously recorded image in the narrow-band channel is then corrected for the instantaneous OTE To recover all spatial frequencies in the narrow-band channel the so corrected single images are averaged. We have applied the method to polarimetric observations of a solar active region by tuning the narrow-band filter to the wing of a Zeeman sensitive spectral line. The most active part of the region shows no more normal granules but features with a diameter of 0.3 to 0.5 arcsec. The smallest magnetic fields are concentrated in regions with sizes at or below the diffraction limit of the telescope.