Ah, a few thoughts about ISO-less

Started Jul 10, 2013 | Discussions thread
Jeff
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Re: Looking for the gain knob ...
In reply to bobn2, Jul 11, 2013

Bobn2 wrote:

Jeff wrote:

Bobn2 wrote:

Part 1, unlearning what you know about ISO and exposure.

The exposure index (EI) as defined by the International Organisation for Standardization (ISO) defines how the exposure at the sensor maps to the brightness of the output file. You can read about it here . ISO does not measure the sensitivity of the sensor, nor does it measure the gain applied to the sensor. All it says is if you shine so much light on the sensor, it results in an output image with given tone, and which amount of light produces which tone depends on the ISO setting. Thus, since it is about mapping an amount of light to an output shade it is fundamentally about how you (or the camera) process the captured latent image to produce your output image. If you are using in-camera processing, you need to per-select the ISO to one of a limited number of processing options and then match your exposure to that. If you process from raw files you can tailor the processing precisely to the captured exposure, and therefore do not need to preselect the ISO. One caveat about that is that some cameras have limited capture electronics, and require to be set up differently for different amounts of light. The manufacturers link that circuitry (which changes the analog gain in front of the analog to digital converter) to the ISO control, and on those cameras you do have to pre-select the ISO to match the exposure, within a broad range. You need to know how your camera operates to know how much leeway it will give you.So ISO doesn't measure sensitivity, nor does it measure gain. But it does specify how exposure is translated into a final image. I see the semantic difference, and understand the underlying mathematics and definitions.

Let's see. ISO doesn't measure sensitivity, nor does it measure gain. But it does specify how exposure is translated into a final image. I see the semantic difference, and understand the underlying definitions.

So in practice, what does the ISO knob on my camera do?

Or differently, can you tell me where to find the gain knob on my camera?

What the ISO knob (generally there isn't one, but never mind) does depends on the camera you own. What all ISO knobs do for in-camera processing is adjust the processing to suit the nominal exposure for that ISO.

So wouldn't you say that the ISO setting is determining the gain?

What most do for raw file is scale the raw file according to the set ISO (which is a pretty silly thing to do, because it just results in wasting file space to capture noise, but nonetheless they do it). What some do, generally only over part of the ISO range, is adjust internal analog gain to optimise the capture chain for the expected exposure. They generally limit the variable gain to the lower end of the ISO range and stop increasing gain when the sensor noise floor is amplified over the level of the ADC noise floor. If they had better ADC's they wouldn't need to change the analog gain at all.

So some manufacturers program analog gain as a function of ISO to minimize overall noise. That's a good thing, right?

Look, I understand and appreciate the what you're trying to do by being careful and pinning the discussion to sound fundamentals. I fully agree that the 'exposure triangle' is misleading and actually harmful to good practice.

But I just don't see 'ISOless' as an appropriate response. It would be if we could neglect noise and DR considerations over a wide enough range of shooting conditions such that aperture and shutter were meaningful independent variables. In bad old days of film, these were coupled because of film's limited dynamic range. Dynamic range has improved, but not to the extent that I can go into a poorly lit hockey arena and dial in arbitrary dof and motion capture. There are tradeoffs due to limited sensor sensitivity and noise, and the implied ISO is one way of monitoring and making those tradeoffs. Stops of underexposure would be another, but in practice that's just a different measure for the same thing, even if we're abusing the strict definition of ISO.

In that regard, 'ISOless' can be misleading, too. If I'm working in smaller formats, there really isn't any operating range where I can neglect the impact of noise, so shutter and aperture are not independent variables, and ISO is a soft but active constraint. It's definitely not ISOless.

If you have 15 stops of dynamic range maybe you think differently. When that class of sensors is available in a $200 p&s or cell phones then ISOless becomes a useful conceptualization the great unwashed. Until then, I'm just not sure I see the practical utility for mainstream users.

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Jeff
http://www.flickr.com/photos/jck_photos/sets/
You don't make a photograph just with a camera. You bring to the act of photography all the pictures you have seen, the books you have read, the music you have heard, the people you have loved.” -- Ansel Adams

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