Exposure Indices, Redux

Five months ago I wrote here about exposure indices and the apparent differences between ASA film ratings of yesteryear and the ISO film ratings of today. It was, in part, an exercise in thinking out loud, and reached no definitive conclusion to the problem of why exposure recommendations for the films of yesteryear seem to have revolved around “Sunny f/11″ instead of “Sunny f/16″.

Happily, I’ve found the answer. I enjoy reading old photography handbooks; besides looking at the cool old pictures, there are a lot of neat tips and tricks that have largely been forgotten today, most of which are still very useful. I recently got a copy of the 8th edition of Aaron Sussman’s

The Amateur Photographer’s Handbook

, and in it, Sussman explains that up until the 1960s or so, most film ratings, and most exposure guides, had an “insurance stop” added in; with negative films, overexposure is of course better than underexposure - though a perfect exposure is naturally best! In a time when exposure meters were far from common and flashbulb exposure a tricky subject to master, it was seen as beneficial to give photographers better than even chances of getting an usable negative. The result was that it was de rigeur to unknowingly “correctly” overexpose film by between one and two full stops, compared to what today’s idea of “correct” exposure would be, and extraordinarily dense negatives (and somewhat poorer grain) were the result.

Most film speeds didn’t really change - a few did, when the method used to measure them did, but only slightly - but how the film was exposed, did.

I believe that this is an important tidbit of information to have if you’re shooting film and trying to re-create any sort of “classic” look. If you expose “correctly”, you’re in essence underexposing, compared to the “classic” exposure.

I’m not sure how well this would work with modern t-grain films, but I’d think that Plus-X, FP4, or Adox 100 might give something of that elusive vintage look when overexposed a stop and developed normally. It’s something to try, anyway, now that winter is upon us. In medium-format, the quality loss for overexposure is comparatively less than in 35mm, and a roll of Arista 100 is cheap enough to experiment with…

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