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Hacking DX encoding

My main shooter for a while was a Pentax P3n, a mass-market plasticky 35mm camera. It was light and easy to use, but it set ISO from the DX encoding on the 35mm cartridge. There was no way to override, which was a bit of a problem because I usually shoot Tri-X at 800 or 1000. I ended up shooting in manual using a handheld meter.

I was cleaning out some old magazines and found an article in the 1993 Camera & Darkroom about how to hack DX encoding by exposing or covering up some of the squares which make it up. Here is the key chart. An X indicates a covered or non-conductive spot, no X indicates bare metal.

Expose squares by carefully removing the paint - the article suggests using "a dull jackknife blade so as not to scratch the bare metal underneath." Cover exposed squares using stickers or tape.

An example: To trick your camera into thinking Tri-X is ISO 1600, scrape the paint off position 3.

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