I send my 4x5 Portra to a lab for developing and am getting a persistent problem that looks like a light leak but doesn't make a lot of sense. I am beginning to wonder if the problem is with the lab. I am getting patches of odd colour and/or higher density around the corners of the film, especially on the top edge of the image. I use the Fuji Quickchange cartridges (NB: not Quickload) which other people have found leak prone, so at first I suspected them, but consider the following: 1. E6 film in the same holders shows no patches. 2. B+W film in the same holders shows no such patches. I also have B+W and colour neg pairs taken at the same time, in the same light, with the same lens. The colour neg has the patches, the B+W not. 3. No patches on a sheet that was exposed to the inside of the camera but the shutter left closed. I.E. the sheet was unexposed, but was open to any light leaks. 4. Patches on a sheet that was exposed to ambient light outside the camera for a couple of minutes. A very dense sheet, but there are still patches of odd-density at the corners. 5. The patches are clearest on shots with high density near the corners: i.e. skies or snow. Dark areas do not seem to be affected as much. 6. The odd patches are always 3-4 mm in size and form a lozenge shape around the clip holes from processing. I am reluctant to blame my processor for a fault that may be mine, but it is beginning to look to me as if development byproducts are being retained by the clips used to hold the sheet while processing. These are thenaccellerating development. Is this a reasonable thing to suspect, and can it happen in a dip-and-dunk C41 processing line? In B+W I would suspect insufficient or too-unidirectional agitation, but I am very much a paint-by-numbers person when it comes to colour processing so before I start complaining to the lab I would appreciate comments from people here.