Well, this happened to a rangefinder of mine, but since it pertains to all 35mm gear, I thought I'd post it here. I loaded a roll of Agfa APX 100 into my Contax G2 which had been set to "DX coding". I was shooting with an external viewfinder and a 21mm lens and had not noticed anything amiss until I switched lenses and noticed that the shutter speed reading was out of whack in the camera viewfinder. When I checked the ISO setting, I realised it had been set to 2500 and not 100. The "DX" was lit on the LCD display though, which means the camera had somehow misread the DX code on the film canister. The batteries are new-ish in the camera, unlikely that they need replacement so soon. And there are no marks or scratches on the film canister that could possibly have caused confusion. In fact, from the DX code, for ISO 100 the topmost row should read silver-black-silver-black-silver-black. For ISO 2500, the row should read silver-black-silver-silver-silver-silver. I have no idea how the camera sensors would have read "silver" when there were blacks. At any rate, I don't know if my entire roll of film was misrated at 2500 because I'd nearly finished the roll by the time I made the discovery. Does the camera pick up the ISO every single time the camera is switched on, or only when the film is loaded, and then it holds it in memory? If so, is there a possibility that the DX coding only started to screw up midway through the roll, or...? I took out the APX roll and put in a fresh roll of Neopan 100, and the camera DX sensor seemed to be working OK again. Anyone know how to salvage a roll of APX 100 shot at ISO 2500? Won't the results be horribly grainy?
Most cameras read DX codes with conduction sensors (i.e. pins that make contact and carry a small current), not optically. I have one I routinely fool into exposing bulk-loaded Tri-X at EI 1600 using a piece of aluminum foil. If the black coating on the cassette's DX area was thin, worn, or damaged, it's possible the sensor pins might have made contact where they were meant to read "open", causing the camera to misread the code. If you see this happening with a new roll, it's possible (assuming you can rewind the leader without pulling it back inside the cassette, or have a tool to retrieve the leader afterward) to unload and put tape over the black areas of the cassette and get a correct reading; it's also possible to *change* the DX code by scraping off black or putting tape over silver, in case you want/need to change the way your camera exposed a particular film. Salvaging the ISO 100 film shot at 2500 is problematic -- that's 4 2/3 stops, and I'd be amazed if it was even possible to push APX 100 that far. Best you can do, if it's not possible to reshoot, is develop in the hottest soup you can find for as long as won't cause too much fog, and then try to save the images with high contrast printing or scanning. Yes, it'll be grainy, and there won't be anything in the shadows, and any bright highlights will be completely blocked up. Nothing's likely to change that... FWIW, I've pushed Tri-X almost this far (TXT 320, accidentally exposed through the base -- developed to EI 5000, to compensate for a real speed when backward in the holder of about EI 16, later reproduced with 35 mm Tri-X 400 as well) using a home-concocted developer made up from HC-110, Dektol, and a couple common household chemicals, but I'd be amazed if you can get APX 100 to push more than about three stops even with this Super Soup.
Just recently I had a roll of 400TX read as ISO 20! Fortunately, the extremely long exposures were obvious with the first frame, so I was able to correct it without losing much of anything. It pays to verify and reverify the setting if using auto exposure.
