Yes it is. Look at the Zeiss spec sheets. You need f/8 or so to reduce vignetting to negligible levels -- though quite honestly, exposure needs to be pretty minimal, with transparency (not colour neg, where you can afford generous exposure) for the vignetting to matter much. I've just sent back the Distagon (and 120 Makro-Planar and 180 Sonnar) after borrowing for a 'road test'. My review will appear in due course in Shutterbug; Frances's will appear in Black and White.