Archive for June 18th, 2006

Exposure Indices, Vintage Photography, and the Written Word

There’s a dangerous trap the photographer can fall into when using vintage photography equipment – or, more often, reading vintage books on photography. The problem is this: things change.

Today, Kodak’s traditional B&W films are Plus-X (ISO 125) and Tri-X (ISO 320 and 400, depending on variety). A very few years ago there was Verichrome Pan at ISO 100, as well. All these films have been around since the 1950’s, at least, and Verichrome – the last major orthocromatic B&W film made – predated WWII. Millions of photos have been taken on them, and hundreds of photos have been published with exposure information given – e.g. 1/25 second at f/8.

There are two catches here, though. Films used to be slower – in 1955, Tri-X was an ASA 200 film, Plus-X was 50-speed, and Verichrome was ASA 64; more or less a stop slower than they’re reckoned to be today, given the conventional wisdom that ASA == ISO. Simple, right?

Nope. Remember the “Sunny f/16 rule”? The sunny f/16 rule is pretty much a constant – bright sunlight, average subject, expose at f/16 for 1/film speed second.

The 1955 Kodak Master Photoguide has a handy-dandy exposure guide in it – the “daylight exposure computer”. You’d think it would show a setting for Plus-X of 1/50th second at f/16, right, Plus-X having been an ASA 50 film then?

You’d be wrong. They show the exposure for an average subject in bright sun with clear sky on Plus-X to be 1/25th second at f/16.

I’m not 100% sure why this is.

The obvious conclusion – which I believe to be correct, incidentally – is that ASA and ISO really aren’t equivelant; the latter is based on strictly-controlled laboratory tests, and the former seems to have been more empirical and abritrary. An alternative explanation, if you want to retain the belief that ASA and ISO speeds are comparable, is that more density was considered beneficial than is the case today.

The latter definately bears investigation as a possible part of the equation in producing B&W photos with a vintage look to them; if habitual one-stop overexposure was the norm in the 1950’s, then trying to get “that look” with a “correctly”-exposed modern film is understandably difficult.

Anyway, the point is, reading about photography – or trying to learn from exposure details written, say, a half-century ago by Ansel Adams – is not as straightforward as it could be.

Published in: Geekiness, General, History | on June 18th, 2006 | Comments Off on Exposure Indices, Vintage Photography, and the Written Word