I have Aperture and Shutter Priority but where’s my ISO Priority?

Digital SLR manufacturers, and for that matter DSLR buyers, are trapped in a mental strait-jacket. They can’t break clear of the design conventions, and ways of thinking about photography, that they grew up with in the days of film.

The advent of digital offers up so many possibilities and new approaches, but developments in camera design conventions are slow to catch up. That’s not to say we don’t have rapid advancement in features and capabilities. We’re in the middle of a mini revolution right now with new features emerging all the time: ever more megapixels, live view, ability to shoot at high ISO, video capability … but I’m thinking about something quite different.

The first digital SLRs looked like the manufacturers took out the film transport of an existing film camera, screwed in a digital sensor and made the minimum other changes to make the whole device work. But it still looked like a film camera and had very similar controls. Even nowadays, that hasn’t really changed. Today’s DSLRs look very closely related to their film-based predecessors and the key elements of the “user interface” (eg viewfinder, shutter, exposure controls, etc) are unchanged. We have more ancillary options now, and an LCD screen with menus to navigate through them, but the basic controls are exactly as they were.

Let’s think about exposure modes. Typically on a film SLR you would have Program Mode, Aperture Priority, Shutter Priority and Manual. On a typical current DSLR you still have those same basic modes. But does that make sense in the digital age? We now have sensor sensitivity (the ISO setting) in place of film speed, and we can change it from exposure to exposure, but we still treat it the way we did in the film era when we were constrained by film speed for a whole roll at a time.

In principle, aperture, shutter speed and ISO (or film speed) are all equally valid ways of controlling whether an image is correctly exposed. They are the three variables at our disposal for exposure control. But we are used to thinking in terms of just two, aperture and shutter speed, because we have always been able to vary these shot by shot but in the days of film the ISO was determined by the speed of the film you put in the camera. You could hardly be swapping rolls of film every other shot, so we have come to think of ISO as something pre-set, and very much the poor relation of aperture and shutter when it comes to active exposure control.

And we continue to think that way, even though in the digital era we have as much freedom to change ISO between shots as we do aperture and shutter speed. In particular, ISO is something you generally have to set manually. It is not something you could choose to be set automatically by the metering system, for each exposure individually, leaving you to set one or both of aperture and shutter speed. Nikon do now offer something called “auto ISO” with some of their cameras. It works a bit like this but only in combination with shutter priority, and it is presented as a special mode. ISO is not treated properly on a par with aperture and shutter speed.

Does this then mean we should have three modes: Aperture Priority, Shutter Priority and ISO Priority? Actually, no. Under our new paradigm we choose 2 out of 3 variables to set explicitly, the other being left to “float” and ultimately determined for us by the light level. But it gets long-winded to refer to say Aperture+ISO priority or Aperture+Shutter priority. Easier to label the mode by the one variable which is NOT getting priority, so Aperture+ISO priority equates to “floating Shutter”, Aperture+Shutter priority equates to “floating ISO” and Shutter+ISO equates to “floating Aperture”.

There we are. In the camera of the future we get Program, Floating Aperture, Floating Shutter, Floating ISO and Manual.

Or do we? Maybe we want to set the Aperture but leave the camera electronics to set both the Shutter and ISO in a kind of semi-Program mode. This leads me to a better suggestion. We have just two buttons to control exposure mode. One is labelled “Clear Settings” and the other toggles between “auto exposure” and “manual”.

It works like this. You ensure the camera is on “auto exposure” and click “C” to ensure you have no explicit exposure settings. The camera is then effectively in what we would now think of as “Program Mode” and therefore controls all exposure settings for you. At any time you can override any of the aperture, shutter and ISO settings. The camera remembers and uses as many settings as you give it but for up to two variables only. If you had last exercised control over say ISO and then aperture this would cause the shutter speed to be determined automatically in accordance with subject light levels. But if you then explicitly also set a shutter speed the camera would use both it and the chosen aperture (being the last two variables you explicitly took control of) but discard your earlier ISO choice, leaving the latter to be determined by the light level, and so on.

So at any time, your last two explicit choices out of aperture, shutter and ISO would be used, and the other would float. Simple and intuitive. Change the things you most care about and the camera will worry about whatever you least care about. If you want to go back to explicit control of only one variable, you just press the C button to clear your explicit variable settings, then start again.

This leaves you free to fiddle around with aperture if you are bothered about sharpness, depth of field and controlling lens aberrations, with shutter speed if you are bothered about freezing action or eliminating camera shake, and ISO if you are bothered about colour saturation and controlling noise. You can concentrate on those aspects which are important to you for that shot and the camera takes care of the exposure. It’s a dream.

Selecting manual, as opposed to auto exposure, would work the same way except that you could set all three variables, without the oldest being discarded. The corresponding exposure might then be out of line with the one the camera’s metering system deemed to be correct, but the relevant number of stops over or under exposure would be displayed.

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One response to “I have Aperture and Shutter Priority but where’s my ISO Priority?

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