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The Third Variable

Since the invention of photography more than 150 years ago photographers have essentially had two variables to work which allow control over exposure – aperture and shutter speed. Yes, film speed was a third variable, but not one that lent itself to ready alteration. With the advent of digital cameras this has now changed!


Mountain Winds, New Mexico. December, 2002
Canon 1Ds with 500mm f/4L IS + 1.4X at ISO 320

Film Speed

For much of the history of photography film speed was essentially “the best you could do”. The first Kodachrome in the 1950’s, for example, was rated at ASA 10. (We new call it ISO – but the scale remains unchanged). In the ‘60s Kodachrome’s speed was increased to ASA 25 and photographers thought that they’d died and gone to heaven.

Things were a bit better in B&W. ASA 100 was considered “normal”, while films like Tri-X at ASA 400 were very fast but grainy. Grain became an end in itself for some, though many struggled to tame it with specialized developers. In any event, as long as we were shooting film the speed or sensitivity of the emulsion being used were something decided on before one started shooting, and usually committed to until at least the end of the roll if not the end of the shoot.

With the advent of digital cameras over the past few years we have seen a major charge in approach. The ISO setting is now a third variable that the photographer is free to change on a frame-by-frame basis as the shooting situation demands. This is a capability not to be underestimated. In fact some new DSLRs, like the Canon 1Ds, actually allow automatic ISO bracketing.


Three Cranes — Bosque del Apache. December, 2002
Canon EOS 1Ds with 300mm f/2.8L IS + 1.4X @ ISO 400

Changing the Rules

Digital image capture technology has changed more than just our ability to incorporate the ISO setting as one of the active exposure variables. A number of the more recent high-end camera bodies, and again the Canon 1Ds especially stands out in the regard, have high ISO noise that is so low that there is hardly a noticeable difference between photographs made anywhere between ISO 100 and 800. And, I expect that we’ll see this capability filter down to a much broader range of cameras over the next few years.

This being the case we truly now do have a third exposure variable to work with. Of course a lower ISO will always produce a higher signal to noise ratio (AKA less grain) than a faster one, but nevertheless our exposure control ability has been significantly enhanced.

This was brought home to me recently on a wildlife shoot in southern New Mexico. Using a 500mm lens, and panning while following flights of birds at both sunrise and sunset, I found that I was struggling to work with a high enough shutter speed to freeze the birds yet being able to stop down enough to cover slight focusing inaccuracies during focus tracking. Bumping up the ISO to 400, 500 or even 800 for a number of frames really saved the day, and it allowed me to use a shutter speed and aperture combination appropriate for the shooting conditions.

Not to put too fine a point on it, but these photographs would literally have not been possible a few years ago. Film? Ya, I remember film.


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Concepts: Photography, Exposure, Film speed, Photographic film, F-number, Science of photography, Shutter speed, Light meter

Entities: Canon, ISO, ASA, Michael Reichmann, New Mexico

Tags: shutter speed, Canon 1Ds, film speed, cameras, digital cameras, ISO setting, photography film speed, slight focusing inaccuracies, high iso noise, shooting situation demands, active exposure variables, high-end camera bodies, image capture technology, southern new mexico, exposure control ability, ready alteration, automatic iso, lower iso, major charge, noticeable difference, frame-by-frame basis, capability filter, broader range, shooting conditions, aperture combination, new dslrs, higher signal, Kodachrome, wildlife shoot, focus tracking, photographs, The ISO, underestimated, bracketing, tame, emulsion, Tri-X, sunrise, photographer, flights